How can we distinguish blended flavors?

How can we distinguish blended flavors?

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As far as I know, humans can distinguish between 5 basic tastes based on various molecules in food and their interactions. There's a level to all 5 so there can be an endless variety of tastes, and we can recognize them with some precision. What surprises me is that we can often distinguish the original flavors in a mix - for example in a salad or even more so in a blended juice. Shouldn't we perceive an entirely new flavor having the average of each basic taste? Not entirely sure this belongs on a biology forum, but I hope someone can explain :)

You're talking about the difference between "taste" and "flavor." In common speech we use these words synonymously, but they actually mean different things.

Taste refers to the five sensations your tongue can detect -- sweet, sour, salty, bitter and umami. There's growing evidence for a sixth taste, "fat."

Flavor refers to the combination of several factors:

1) taste, as just described,

2) the aromas of esters, flavinoids, ketones, and other compounds in your food, detected by the nose. This happens a) before you take a bite, and b) as you chew and swallow, releasing different compounds and higher concentrations into your nose through your retronasal passage,

3) physical sensations such as texture, heat and cold, visual appearance and even sound (for example, the crunch of celery or potato chips)

4) chemesthetic effects, like the astringency of tannins or the "artificial" heat of chili peppers. (The compound responsible for chili heat actually down-regulates physical heat receptors so they're more sensitive. Menthol up-regulates them to create a cooling sensation.)

5) Emotional and social factors, like the memory of your grandmother's homebaked bread, or many people's aversion to raw fish.

All of this put together is why blackberries and strawberries are both sweet and tart, yet blackberries taste like blackberries and strawberries taste like strawberries.

Hope that helps!


-- The collected works of Karen Page & Andrew Dornenburg

-- "Tastebuds And Molecules," Francois Chartier

-- LOTS of personal empirical evidence (I'm a chef)

Do you have a favorite food? Can you explain what exactly makes you like this food so much more than others? Is it its yummy taste, its wonderful smell, or perhaps its perfect texture? It might not be so easy to pick just one reason&mdashit is most likely a combination of all of these! There is more to flavor than just taste. In fact, the tongue is not the only part responsible for sensing the unique flavor for every food we eat. The experience of flavor is the combination of taste and smell (figure 1).

Two diagrams showing the taste and smell receptors located on the tongue and inner nose respectively. Four images of the tongue show the location of taste buds for four different tastes: salty, bitter, sour and sweet. The top-left diagram shows the location of salty taste receptors -- grouped closely around the tip and sides of the tongue and sparse around the center and back of the tongue. The bottom-left diagram for sour taste receptors has a similar distribution of receptors to salty taste receptors. The top-right diagram shows bitter taste receptors grouped closely at the back of the tongue and are very sparse at the center, sides and tip of the tongue. The bottom-right diagram shows sweet taste receptors that are densly packed in the tip and sides of the tongue (with a few receptors scattered across the surface of the tongue). The smell diagram shows how odorants enter the body through the nose and mouth and travel upwards to the top of the nasal cavity where the smell receptors are located.

Figure 1. Tasting with your tongue and smelling with your nose. (Image credits: tongue images by Rainer Zenz, via Wikimedia Commons olfaction image (modified) by LC Uni Hohenheim, via Wikimedia Commons).

When you put food in your mouth, your tongue picks up the five basic tastes: sweetness, saltiness, sourness, bitterness, and umami, which is a savory taste (think of meat or mushrooms). If you look at your tongue closely, you might see tiny bumps on its surface. These are called papillae and each of them harbors many taste buds. On average, each person's tongue has about 2,000&ndash8,000 taste buds, and each of them contains about 50&ndash100 taste receptors. Each receptor is best at sensing a single sensation and sends signals to the brain, which then identifies sweet, salty, bitter, sour, or umami. The total sum of these sensations is the "taste" of the food.

So what about smell? The five basic tastes listed above only partially contribute to the perception of flavor in your mouth. For the rest of the flavor, the scent of the food and your personal sense of smell&mdashalso called olfaction&mdashplay important roles. Once you put food into your mouth, odorants, which are specific chemicals that are responsible for its smell, enter your nostrils. Inside your nose there are millions of smell receptors. Once the receptor binds to a specific odorant, it sends a signal to your brain and you are able to identify the specific scent. But did you know that your nostrils are not the only way smells can get into your nose? Your mouth and your nasal cavity are connected in your throat! You might have seen a great demonstration of this connection if you have ever seen someone shoot water out of their nose while laughing after they have taken a drink! Smelling through the "back door" of your nose&mdashalso known as retronasal olfaction&mdashalso contributes to flavor perception when you are eating as you can see in figure 1.

So how does all the information about smell and taste come together? There is a specific part of the brain that is able to pick up and respond to signals from both smell receptors and taste receptors. This area of the brain, called the orbitofrontal cortex, is where researchers think that the information about smell and taste converge to produce the flavor we perceive. This link between smell and taste is so strong that people learn to associate certain smells with a specific taste. For instance, do you feel a sour taste on your tongue when you smell a strong lemon scent? The food industry already makes use of this by introducing phantom aromas into food. By adding barely detectable amounts of odorants into foods, our brain can perceive flavors without realizing that they are not actually there. Some research even claims that the smell of a food accounts for up to 80 percent of how we perceive flavor, compared to 20 percent for taste.

Considering this linkage between smell and taste, it should be easy to change a food's flavor by just changing its smell. So, do you think you can make an apple taste like a banana? Well, head into the kitchen, gather some food, scents, and volunteers, and find out!

Roast degree

Roast degree is one of the most important indicators with the roast. It can be measured by a color meter or by tasting. Roasters usually want to enhance coffee’s own flavours and decide the roast degree. Typically light roasted coffees are more acidic, and dark roasted coffees are more bitter. Also fruity flavours are more common on light roasts, and roasty and burnt flavours are more common on dark roasted coffee. Light roasted coffee is more fruity due to high amounts of an organic compound, 5-hydroxymethylfurfural. When roasting goes further, this compound breaks down to less fruity compounds. The amount of sulfuric compounds increases, which produces roasty and burnt flavours. As a role of thumb, we can assume that light roasted coffee brings the character of the raw coffee out better. It is easier to discriminate light roasted coffee from each other than dark roasted coffee.

'In 10 years, easily 30% of the flavors market could be made from biotech products…’ Synthetic biology and the future of flavor

FoodNavigator-USA's Elaine Watson (left) with Casey Lippmeier from Conagen (center), and Kathy Oglesby from Blue California (right)

But does it always make sense to extract flavors from plants if you can produce them more efficiently - and more sustainably – via microbial fermentation, and what might flavor production look like by the end of the 21st century?

What the flavor industry will look like in 2099 is anyone’s guess, “But if you look at the next 10 years, easily 30% of the flavor market could be made from biotech products," ​said Blue California’s ​​head of flavors and fragrances Katy Oglesby during a panel moderated by FoodNavigator-USA at the virtual SynBioBeta food & agriculture conference​ ​this week.

"The time for biotechnology really is right now.”

Ultimately, “more than 100% of today’s market could be served by synthetic biology,” ​added fellow panelist Casey Lippmeier, VP innovation at synthetic biology specialist Conagen​​, which​ provides the science underpinning products it sells directly or via affiliates such as Blue California, Sweegen, and BASF.

To those wondering if Lippmeier’s math is a little off, he clarified: “Let me explain what I mean by that… The thing about synthetic biology, is that in addition to making all the wonderful molecules that exist in nature, you can make brand new ones too… and of course you do all the safety testing, but at the end of the day they could provide some very interesting flavor - and especially fragrance - solutions that currently don’t exist today, so that’s where the ‘more than 100%’ comes from.”

The sweet spot for synthetic biology in flavor production​

In practice of course, said Lippmeier, it wouldn’t make sense to produce every​ flavor in a fermentation tank today, even though the technology is already there to create a large number of flavors this way: “To be clear, I wasn’t trying to suggest that we’ll displace all​ botanical extracts, I don’t see that​.”

Taste good? Senses inform the brain — but don’t tell everyone the same thing

All our senses inform what we taste.

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Food Network host Andrew Zimmern once asked Taria Camerino: “What do I taste like?” It’s a strange question. But not for Camerino. This Atlanta-based chef is one of many people who sense their world in special ways.

Camerino was born with synesthesia (Sin-uhs-THEE-zhah). That means her senses are tangled up with one another. (The term comes from the Greek words syn, meaning together, and aesthesis, meaning sensation.)

Explainer: Taste and flavor are not the same

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Some synesthetes see each letter of the alphabet in a particular color. Others see shapes when they hear music. Camerino’s brain links things she sees and hears to flavors. That’s how she could answer Zimmern’s unusual question.

“I watched the way he moved,” she recalls, and discovered that he tasted like toasted prawn shells and bay-leaf syrup. Her unusual sensory ability lets her understand people through food. Sometimes, she uses this gift to aid people. Some illnesses dull their victims’ senses. With Camerino’s help, some who had lost much of their sense of taste regained “more connection to [it].”

Most people are not synesthetes. But everyone’s senses work together to process taste. Scientists are discovering how these connections affect what we eat. They’re also learning how tastes’ hidden meanings might lead to behaviors that might make people sick — or do the opposite: encourage healthier eating.

Tastes carry hidden meanings

For the most part, people process different sensations — taste, smell, hearing, touch and sight — in different parts of the brain’s cerebral (Seh-REE-brul) cortex. That’s its outermost layer. But all the senses talk to each other through neural networks. These are groups of interconnected brain cells, called neurons.

The signaling between these sensory cells lets us hear the crispy crunch of a potato chip and know it’s fresh. Or see a plump peach and think, “Yum, that looks sweet!” The information we get from this chatter helps us decide what to eat.

What and how we make those decisions interests Julie Mennella. She works at the Monell Chemical Senses Center in Philadelphia, Pa. As a biopsychologist (BY-oh-sy-KOL-oh-gist), she studies how behaviors can be affected by taste preferences that develop during infancy and early childhood.

Early in human history, the ability to recognize and respond to different tastes without thinking helped people survive. A sweet taste, explains Mennella, isn’t just pleasing. It also tells someone that a food is rich in calories. That’s important because calories provide the energy needed to fuel our bodies. A salty taste indicates that food contains salt — sodium chloride. It’s a mineral needed for bone health, especially when bones are growing during childhood.

Sour and bitter tastes also carry messages — negative ones that might cause you to spit a food out. A sour taste told our ancestors a food might be spoiled. A bitter taste might have meant a meal was poisonous.

You might enjoy eating sour fruits such as lemons or grapefruit, or bitter vegetables such as turnips or Brussels sprouts. But you probably learned to like their tastes over time.

Today, most of us don’t have to hunt or forage for food as our ancient ancestors did. We can find plenty of meat and fruit — and chips and cookies — at the supermarket. What scientists call our food environment has changed.

Our brains still give us cues that guide our food choices, though. And that can be a problem — especially for kids. Mennella found in a 2016 study that kids and teens prefer far sweeter tastes than do adults.

This preference can set kids on a lifelong path to unhealthy eating. A sweet tooth can lead to overeating — and obesity. It also ups the risk someone will develop diseases such as diabetes. A strong preference for salty foods can foster overeating, too. It’s also been linked to health problems such as high blood pressure. Such issues have been growing throughout the world’s wealthier nations. By studying taste, Mennella and other scientists are searching for clues about how we sense foods in hopes of learning how to turn those risks around.

Tricking taste

Some people are drawn to really sweet things. Others, not so much. Scientists would like to know whether we’re born with these preferences.

In her own work, Mennella has found that what parents feed their infants can affect a child’s likes and dislikes later in life. One 2011 study by her group showed that infants fed sweetened water grew up preferring sweet drinks. Babies given unsweetened water did not. This suggests, she says, that cutting out sugary drinks might help infants develop into kids without an intense hankering for sweet treats.

Her goal is not to deprive children of all sweet treats. What fun would that be? Instead, she says, “We’re trying to figure out if we can shift their preference for added sugars.” (Added sugars are those that manufacturers — or diners — add to those naturally present in foods.)

Can kids be convinced to choose a banana instead of a cookie, for example? “We want to uncover some of the mysteries of how we can get children to like healthy foods,” says Mennella — such as, how many exposures to a fruit will they need before they actively reach for the fruit.

Danielle Reed is a behavioral geneticist at Monell who works with Mennella. Her lab conducts research on taste, too. But Reed is trying to find out if there is a genetic reason for taste preferences. Are we born liking or disliking super-sweet or salty tastes? For example, she says, do people tend to inherit a strong preference for certain tastes because of their genetics — or just learn to like them?

Reed has been studying identical twins. People who grow up in the same household, eating the same foods, may “learn” to prefer the tastes that their parents have been feeding them. To test whether they instead inherited that preference, she compares siblings. Identical twins share the same genes — and usually the same household, as kids — so one would expect their taste preferences to be similar. But are taste preferences equally similar in fraternal (non-identical) twins and in siblings who aren’t twins?

In one 2015 study, her team confirmed that identical twins indeed prefer similar levels of sweetness most of the time. In contrast, sweet preferences could vary a lot among fraternal twins and non-twins. Reed concludes that some aspects of how foods taste to us are inherited.

And this isn’t true only for sweetness, she notes, but for bitterness as well. Some people may be biologically wired to find turnips too bitter to eat. Others might not find them bitter at all.

Reed says that people born to prefer sweets may have to work harder to avoid overeating them. But kids who find turnips too bitter to eat don’t have to give up on all vegetables. Their parents can simply seek alternatives that taste good to them — maybe carrots or peas.

More than meets the tongue

Dana Small is a clinical psychologist at Yale University in New Haven, Conn. She studies how people’s brains respond to taste and smell, and to the two senses together. People tend to pick up flavor preferences, she says, depending on what food is available where they live or on what was served to them as children.

For example, American children “might find a plate of deep-fried bugs highly repulsive,” she notes. “But if you grew up in Indonesia eating them as a snack, you would have learned to find them delicious.”

Like Mennella and Reed, Small is trying to understand eating behaviors that can lead to obesity and diabetes. Her studies look inside people’s brains to see how tastes and smells affect their flavor perceptions.

Volunteers lie in a type of brain scanner known as an fMRI machine. (The initials stand for functional magnetic resonance imaging.) As they lie there, another machine drops bits of liquid into their mouths. These liquids contain different flavorings. Yet another device delivers specific odors across their noses. The fMRI scanner shows where brain activity spikes in response to those tastes and smells. Those brain sites show how we’re responding to the tastes and smells.

In one 2015 study, Small’s team studied how being sated — no longer hungry — affects the brain’s response to food cues. The researchers pumped the smells and flavors of a milkshake, then mac and cheese, into the noses and mouths of 32 volunteers. This triggered activity in a brain area called the amygdala (Ah-MIG-duh-lah). It’s a brain area that responds to food cues when people are hungry. Small says this is usually reduced after a meal. Yet, even though all of her volunteers had just eaten, some of them still showed a big response to the food cues. And these people, it turned out, also were the ones most likely to have gained weight over the next year.

Small is not sure yet how this research may one day be used to tackle unhealthy eating patterns. For now, she says, “We are just trying to understand how unhealthy eating works.” Later, researchers might work on changing unhealthy patterns.

The role of foods’ appealing smells

Timothy McClintock also is studying the role of smells in helping to define a food’s flavor. As a physiologist (Fiz-ee-OL-oh-gist) at the University of Kentucky in Lexington, he studies how different parts of the body work. He has been working to “map” cells in the brain that respond to scents.

To do this, he replaces a gene in certain nerve cells of mice. These olfactory (Oal-FAK-tuh-ree) cells respond to scents by emitting light — fluorescing. By looking for that telltale light, he can spot which nerve cell is related to which odor receptor — and therefore, to which smell. So far, McClintock has “mapped” receptors for 10 different smells.

If McClintock can learn which receptors and smells link up, this might help researchers “develop flavors that do a better job of appealing to us,” he says. Using his research, maybe scientists will one day be able to “develop chemicals that block receptors we don’t want to smell.” That could be good news for cancer patients who undergo chemical treatment — chemotherapy — to fight their disease.

“Chemo” can change the way foods smell and taste to these patients. It could, for instance, give a peach a salty taste when someone had been expecting it would be sweet. That distorted sense of taste, he notes, “can really throw you off to the point where you don’t like a food.” It may even prompt some patients to stop eating — even though their bodies need the energy to heal.

Smell also influences taste and flavor, McClintock notes. When you bite down on that peach, its juices release odor molecules. These travel through a passage that runs from your mouth to a large open space, known as the naval cavity, at the back of the nose. Those scented molecules then dissolve into mucus, before meeting up with scent receptors that are attached to sensory nerve cells.

Each kind of odor fits into a special slot in receptors found high up in the nasal cavity. When the scent molecules reach a receptor, the nerve cells in the nose fire off electrical signals. These signals travel to the olfactory bulb, which is the brain’s first processing station for odor information. From there, the signal moves on to the brain system that deals with emotions. Other nerves relay that information to the brain’s olfactory cortex, a brain region where more odor processing takes place.

The rest of our senses also are involved in flavor. When you look at a slice of pizza, the optic nerve at the back of your eye transmits a message about it to your brain’s visual cortex. This gives you your first clue about what to expect from its taste, says McClintock. When you put the pizza in your mouth, it’s probably still warm from the oven. That’s temperature, which is a part of touch. It’s important to flavor, as well, says McClintock. Compare your warm pizza to a cold leftover slice. The warm one has more flavor — in part because taste receptors are sending stronger messages to our brains.

Taste to the rescue

Recently, Camerino, the synesthetic chef, helped tackle some of McClintock’s concerns about the health of chemo patients. At a conference at the University of Kentucky, chefs and scientists like McClintock discussed how taste plays out in our brains.

Classroom questions

Later, Camerino made a dessert that she hoped would taste good to cancer patients on chemotherapy. The dish was a cake made with oranges and topped with a sauce made of ground-up basil and pistachios. Camerino thought the citrus flavor of the oranges would cut through the metallic taste that she knew some chemo patients experience. And she thought they might be able to somehow “feel” the basil on the tops of their mouths, even if they couldn’t taste it.

The result was promising: The cancer patients rated her creation “delightful and unexpected!”

Power Words

amygdala An area deep within the brain and near the temporal lobe. Among other things, the amygdala plays a role in emotions. The term comes from the Greek word for an almond, which this region resembles in shape.

behavior The way something, often a person or other organism, acts towards others, or conducts itself.

blood pressure The force exerted against vessel walls by blood moving through the body. Usually this pressure refers to blood moving specifically through the body&rsquos arteries. That pressure allows blood to circulate to our heads and keeps the fluid moving so that it can deliver oxygen to all tissues. Blood pressure can vary based on physical activity and the body&rsquos position. High blood pressure can put someone at risk for heart attacks or stroke. Low blood pressure may leave people dizzy, or faint, as the pressure becomes too low to supply enough blood to the brain.

bug The slang term for an insect. Sometimes it&rsquos even used to refer to a germ.

calorie The amount of energy needed to raise the temperature of 1 gram of water by 1 degree Celsius. It is typically used as a measurement of the energy contained in some defined amount of food.

cancer Any of more than 100 different diseases, each characterized by the rapid, uncontrolled growth of abnormal cells. The development and growth of cancers, also known as malignancies, can lead to tumors, pain and death.

cavity (in geology or physics) A large rigid pocketlike structure. (in biology) An open region pocketlike structure surrounded by tissues. Or (in dentistry) a tiny hole in a tooth that develops over time. Dental cavities are more likely to happen when a person eats a lot of sugar or does not brush and floss regularly. Dentists refer to these as caries.

cell The smallest structural and functional unit of an organism. Typically too small to see with the unaided eye, it consists of a watery fluid surrounded by a membrane or wall.

cerebral cortex (pl. cerebral cortices) The outermost layer of neural tissue covering the front part of a vertebrate animal&rsquos brain.

chemical A substance formed from two or more atoms that unite (bond) in a fixed proportion and structure. For example, water is a chemical made when two hydrogen atoms bond to one oxygen atom. Its chemical formula is H2O. Chemical also can be an adjective to describe properties of materials that are the result of various reactions between different compounds.

chemotherapy A chemical treatment that&rsquos most often used to kill cancer cells in the body. Chemotherapy can have many unpleasant side effects as it kills not only cancer cells but many healthy cells as well.

citrus A genus of flowering trees that tend to produce fruits with a juicy edible flesh. There are several main categories: the oranges, mandarins, pummelos, grapefruits, lemons, citrons and limes.

cortex The outermost layer of neural tissue of the brain.

diabetes A disease where the body either makes too little of the hormone insulin (known as type 1 disease) or ignores the presence of too much insulin when it is present (known as type 2 diabetes).

diet The foods and liquids ingested by an animal to provide the nutrition it needs to grow and maintain health. (verb) To adopt a specific food-intake plan for the purpose of controlling body weight.

dissolve To turn a solid into a liquid and disperse it into that starting liquid. (For instance, sugar or salt crystals, which are solids, will dissolve into water. Now the crystals are gone and the solution is a fully dispersed mix of the liquid form of the sugar or salt in water.)

environment The sum of all of the things that exist around some organism or the process and the condition those things create. Environment may refer to the weather and ecosystem in which some animal lives, or, perhaps, the temperature and humidity (or even the placement of components in some electronics system or product).

flavor The particular mix of sensations that help people recognize something that has passed through the mouth. This is based largely on how a food or drink is sensed by cells in the mouth. It also can be influenced, to some extent, by its smell, look or texture.

fluorescent (v. fluoresce) Adjective for something that is capable of absorbing and reemitting light. That reemitted light is known as fluorescence.

food environment A term used for types and the availability of foods to which people have access. For instance, people in low-income, urban areas may not have ready access to free produce much of the year. People in rural areas may have limited access to seafood or exotic foods.

forage To search for something, especially food. It&rsquos also a term for the food eaten by grazing animals, such as cattle and horses.

fraternal (in genetics) The term for a type of twin birth where each baby comes from a separate fertilized egg. This is in contrast to identical twins, which result from a single fertilized egg (creating two separate but nearly identical babies).

functional magnetic resonance imaging (fMRI) A special type of medical scanning technology for studying brain activity. It uses a strong magnetic field to monitor blood flow in the brain as an individual is performing some task (from reading or viewing pictures to thinking about various spoken words). Tracking areas of elevated blood flow can tell researchers which brain regions are especially active during those activities.

gene (adj. genetic) A segment of DNA that codes, or holds instructions, for a cell&rsquos production of a protein. Offspring inherit genes from their parents. Genes influence how an organism looks and behaves.

genetic Having to do with chromosomes, DNA and the genes contained within DNA. The field of science dealing with these biological instructions is known as genetics. People who work in this field are geneticists.

high blood pressure The common term for a medical condition known as hypertension. It puts a strain on blood vessels and the heart.

mineral Crystal-forming substances that make up rock, such as quartz, apatite or various carbonates. Most rocks contain several different minerals mish-mashed together. A mineral usually is solid and stable at room temperatures and has a specific formula, or recipe (with atoms occurring in certain proportions) and a specific crystalline structure (meaning that its atoms are organized in regular three-dimensional patterns). (in physiology) The same chemicals that are needed by the body to make and feed tissues to maintain health.

molecule An electrically neutral group of atoms that represents the smallest possible amount of a chemical compound. Molecules can be made of single types of atoms or of different types. For example, the oxygen in the air is made of two oxygen atoms (O2), but water is made of two hydrogen atoms and one oxygen atom (H2O).

mucus A slimy substance produced in the lungs, nose, digestive system and other parts of the body to protect against infection. Mucus is made mainly of water but also includes salt and proteins such as mucins. Some animals use mucus for other purposes, such as to move across the ground or to defend themselves against predators.

nasal Having to do with the nose.

nasal cavity The big open space inside the nose. It sits between the bottom of the skull and roof of the mouth. A mucus membrane lines its walls. This region helps to warm and filter the air we breathe in. And sensory cells within it also can pick up scents and relay that information to the brain.

nerve A long, delicate fiber that transmits signals across the body of an animal. An animal&rsquos backbone contains many nerves, some of which control the movement of its legs or fins, and some of which convey sensations such as hot, cold or pain.

network A group of interconnected people or things. (v.) The act of connecting with other people who work in a given area or do similar thing (such as artists, business leaders or medical-support groups), often by going to gatherings where such people would be expected, and then chatting them up. (n. networking)

neural network A computer program designed to work in a way similar to the human brain. The programs can &ldquolearn&rdquo from examples, just as the brain does.

neuron An impulse-conducting cell. Such cells are found in the brain, spinal column and nervous system.

neuroscience The field of science that deals with the structure or function of the brain and other parts of the nervous system. Researchers in this field are known as neuroscientists.

obesity (adj. obese) Extreme overweight. Obesity is associated with a wide range of health problems, including type 2 diabetes and high blood pressure.

olfaction (adj. olfactory) The sense of smell.

olfactory bulb A region in the front of the brain that receives information from smell-receptor nerves in the nose (and nasal cavity).

optic nerve A nerve that carries information, as electrical impulses, to the brain from the retina of the eye. The brain then translates those signals to images.

perception The state of being aware of something &mdash or the process of becoming aware of something &mdash through use of the senses.

physiologist A scientist who studies the branch of biology that deals with how the bodies of healthy organisms function under normal circumstances.

prawn A large marine crustacean that resembles a shrimp. The term is also applied to some large species of shrimp.

pressure Force applied uniformly over a surface, measured as force per unit of area.

receptor (in biology) A molecule in cells that serves as a docking station for another molecule. That second molecule can turn on some special activity by the cell.

risk The chance or mathematical likelihood that some bad thing might happen. For instance, exposure to radiation poses a risk of cancer. Or the hazard &mdash or peril &mdash itself. (For instance: Among cancer risks that the people faced were radiation and drinking water tainted with arsenic.)

salt A compound made by combining an acid with a base (in a reaction that also creates water). The ocean contains many different salts &mdash collectively called &ldquosea salt.&rdquo Common table salt is a made of sodium and chlorine (sodium chloride).

scanner A machine that runs some sort of light (which includes anything from X-rays to infrared energy) over a person or object to get a succession of images. When a computer brings these images together, they can provide a motion picture of something or can offer a three-dimensional view through the target. Such systems are often used to see inside the human body or solid objects without breaching their surface.

sibling An offspring that shares the same parents (with its brother or sister).

sodium A soft, silvery metallic element that will interact explosively when added to water. It is also a basic building block of table salt (a molecule of which consists of one atom of sodium and one atom of chlorine: NaCl). It is also found in sea salt.

synesthesia A brain condition in which a person connects a sensory experience to an unassociated symbol, as a letter or number. People who have this trait are known as synesthetes.

taste One of the basic properties the body uses to sense its environment, especially foods, using receptors (taste buds) on the tongue (and some other organs).

transmit (n. transmission) To send or pass along.

visual cortex A part of the cerebral cortex (the outermost nerve tissue covering the front part of the brain) that receives visual information from the eye that will be transformed into images.


Journal: J. A. Mennella and N.K. Bobowski. Psychophysical tracking method to measure taste preferences in children and adults. Journal of Visualized Experiments, Issue 113, July 2016, p. e54163. doi: 10.3791/54163.

Journal: L. Huang et al. A common genetic influence on human intensity ratings of sugar and high-potency sweeteners. Twin Research and Human Genetics. Vol. 18, August 2015, p. 361. doi: 10.1017/thg.2015.42.

Journal: X. Sun et al. Basolateral amygdala response to food cues in the absence of hunger is associated with weight gain susceptibility. The Journal of Neuroscience. Vol. 35, May 20, 2015, p. 7964. doi:10.1523/jneurosci.3884-14.2015.

Journal: T. McClintock et al. In Vivo identification of eugenol-responsive and muscone- responsive mouse odorant receptors. The Journal of Neuroscience. Vol. 34, November 19, 2014, p. 15669. doi:10.1523/jneurosci.3625-14.2014.

Journal: A. K. Ventura and J. A. Mennella. Innate and learned preferences for sweet taste during childhood. Current Opinion in Clinical Nutrition and Metabolic Care. Vol. 14, July 2011, p. 379. doi: 10.1097/MCO.0b013e328346df65.

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The word coffee entered the English language in 1582 via the Dutch koffie, borrowed from the Ottoman Turkish kahve ( قهوه ), borrowed in turn from the Arabic qahwah ( قَهْوَة ). [11] The Arabic word qahwah was traditionally held to refer to a type of wine whose etymology is given by Arab lexicographers as deriving from the verb قَهِيَ qahiya, 'to lack hunger', in reference to the drink's reputation as an appetite suppressant.

The term coffee pot dates from 1705. [12] The expression coffee break was first attested in 1952. [12]

Legendary accounts

According to one legend, ancestors of today's Oromo people in a region of Kaffa in Ethiopia were the first to recognize the energizing effect of the coffee plant. [2] However, no direct evidence that has been found earlier than the 15th century indicating who among the African populations used it as a stimulant, or where coffee was first cultivated. [2] The story of Kaldi, the 9th-century Ethiopian goatherd who discovered coffee when he noticed how excited his goats became after eating the beans from a coffee plant, did not appear in writing until 1671 and is probably apocryphal. [2]

Another legend attributes the discovery of coffee to a Sheikh Omar. According to an old chronicle (preserved in the Abd-Al-Kadir manuscript), Omar, who was known for his ability to cure the sick through prayer, was once exiled from Mocha in Yemen to a desert cave near Ousab (modern-day Wusab, about 90 kilometres (56 mi) east of Zabid). [13] Starving, Omar chewed berries from nearby shrubbery but found them to be too bitter. He tried roasting the seeds to improve the flavor, but they became hard. He then tried boiling them to soften the seed, which resulted in a fragrant brown liquid. Upon drinking the liquid Omar was revitalized and sustained for days. As stories of this "miracle drug" reached Mocha, Omar was asked to return and was made a saint. [14]

Historical transmission

The earliest credible evidence of coffee-drinking or knowledge of the coffee tree appears in the middle of the 15th century in the accounts of Ahmed al-Ghaffar in Yemen. [2] It was here in Arabia that coffee seeds were first roasted and brewed, in a similar way to how it is prepared now. Coffee was used by Sufi circles to stay awake for their religious rituals. [15] Accounts differ on the origin of the coffee plant prior to its appearance in Yemen. From Ethiopia, coffee could have been introduced to Yemen via trade across the Red Sea. [16] One account credits Muhammad Ibn Sa'd for bringing the beverage to Aden from the African coast. [17] Other early accounts say Ali ben Omar of the Shadhili Sufi order was the first to introduce coffee to Arabia. [18] According to al Shardi, Ali ben Omar may have encountered coffee during his stay with the Adal king Sadadin's companions in 1401. Famous 16th-century Islamic scholar Ibn Hajar al-Haytami notes in his writings of a beverage called qahwa developed from a tree in the Zeila region. [15] Coffee was first exported out of Ethiopia to Yemen by Somali merchants from Berbera and Zeila in modern-day Somaliland, which was procured form Harar and the Abyssinian interior. According to Captain Haines, who was the colonial administrator of Aden (1839–1854), Mocha historically imported up to two-thirds of their coffee from Berbera-based merchants before the coffee trade of Mocha was captured by British-controlled Aden in the 19th century. Thereafter, much of the Ethiopian coffee was exported to Aden via Berbera. [19]

Berbera not only supplies Aden with horned cattle and sheep to a very large extent, but the trade between Africa and Aden is steadily increasing greatly every year. In the article of coffee alone there is considerable export, and ' Berbera' coffee stands in the Bombay market now before Mocha. The coffee shipped at Berbera comes from far in the interior from Hurrar, Abyssinia, and Kaffa. It will be to the advantage of all that the trade should come to Aden through one port, and Berbera is the only place on the coast there that has a protected port, where vessels can lie in smooth water. [20]

By the 16th century, coffee had reached the rest of the Middle East, Persia, Turkey, and northern Africa. The first coffee seeds were smuggled out of the Middle East by Sufi Baba Budan from Yemen to the Indian subcontinent during the time. Before then, all exported coffee was boiled or otherwise sterilised. Portraits of Baba Budan depict him as having smuggled seven coffee seeds by strapping them to his chest. The first plants grown from these smuggled seeds were planted in Mysore.

Coffee had spread to Italy by 1600, and then to the rest of Europe, Indonesia, and the Americas. [21] [ better source needed ]

In 1583, Leonhard Rauwolf, a German physician, gave this description of coffee after returning from a ten-year trip to the Near East:

A beverage as black as ink, useful against numerous illnesses, particularly those of the stomach. Its consumers take it in the morning, quite frankly, in a porcelain cup that is passed around and from which each one drinks a cupful. It is composed of water and the fruit from a bush called bunnu.

The thriving trade between Venice and North Africa, Egypt, and the Middle East (back then Ottoman Empire), brought many goods, including coffee, to the Venetian port. From Venice, it was introduced to the rest of Europe. Coffee became more widely accepted after it was deemed a Christian beverage by Pope Clement VIII in 1600, despite appeals to ban the "Muslim drink". The first European coffee house opened in Rome in 1645. [21]

The Dutch East India Company was the first to import coffee on a large scale. [22] The Dutch later grew the crop in Java and Ceylon. [23] The first exports of Indonesian coffee from Java to the Netherlands occurred in 1711. [24]

Through the efforts of the British East India Company, coffee became popular in England as well. John Evelyn recorded tasting the drink at Oxford in England in a diary entry of May 1637 to where it had been brought by a student of Balliol College from Crete named Nathaniel Conopios of Crete. [25] [26] Oxford's Queen's Lane Coffee House, established in 1654, is still in existence today. Coffee was introduced in France in 1657, and in Austria and Poland after the 1683 Battle of Vienna, when coffee was captured from supplies of the defeated Turks. [27]

When coffee reached North America during the Colonial period, it was initially not as successful as it had been in Europe as alcoholic beverages remained more popular. During the Revolutionary War, the demand for coffee increased so much that dealers had to hoard their scarce supplies and raise prices dramatically this was also due to the reduced availability of tea from British merchants, [28] and a general resolution among many Americans to avoid drinking tea following the 1773 Boston Tea Party. [29] After the War of 1812, during which Britain temporarily cut off access to tea imports, the Americans' taste for coffee grew.

During the 18th century, coffee consumption declined in England, giving way to tea-drinking. The latter beverage was simpler to make, and had become cheaper with the British conquest of India and the tea industry there. [30] During the Age of Sail, seamen aboard ships of the British Royal Navy made substitute coffee by dissolving burnt bread in hot water. [31]

The Frenchman Gabriel de Clieu took a coffee plant to the French territory of Martinique in the Caribbean in the 1720s, [32] from which much of the world's cultivated arabica coffee is descended. Coffee thrived in the climate and was conveyed across the Americas. [33] Coffee was cultivated in Saint-Domingue (now Haiti) from 1734, and by 1788 it supplied half the world's coffee. [34] The conditions that the slaves worked in on coffee plantations were a factor in the soon to follow Haitian Revolution. The coffee industry never fully recovered there. [35] It made a brief come-back in 1949 when Haiti was the world's 3rd largest coffee exporter, but declined rapidly after that.

Meanwhile, coffee had been introduced to Brazil in 1727, although its cultivation did not gather momentum until independence in 1822. [36] After this time massive tracts of rainforest were cleared for coffee plantations, first in the vicinity of Rio de Janeiro and later São Paulo. [37] Brazil went from having essentially no coffee exports in 1800, to being a significant regional producer in 1830, to being the largest producer in the world by 1852. In 1910–20, Brazil exported around 70% of the world's coffee Colombia, Guatemala, and Venezuela exported half of the remaining 30% and Old World production accounted for less than 5% of world exports. [38]

Cultivation was taken up by many countries in Central America in the latter half of the 19th century, and almost all involved the large-scale displacement and exploitation of the indigenous people. Harsh conditions led to many uprisings, coups and bloody suppression of peasants. [39] The notable exception was Costa Rica, where lack of ready labor prevented the formation of large farms. Smaller farms and more egalitarian conditions ameliorated unrest over the 19th and 20th centuries. [40]

Rapid growth in coffee production in South America during the second half of the 19th century was matched by growth in consumption in developed countries, though nowhere has this growth been as pronounced as in the United States, where a high rate of population growth was compounded by doubling of per capita consumption between 1860 and 1920. Though the United States was not the heaviest coffee-drinking nation at the time (Nordic countries, Belgium, and Netherlands all had comparable or higher levels of per capita consumption), due to its sheer size, it was already the largest consumer of coffee in the world by 1860, and, by 1920, around half of all coffee produced worldwide was consumed in the US. [38]

Coffee has become a vital cash crop for many developing countries. Over one hundred million people in developing countries have become dependent on coffee as their primary source of income. It has become the primary export and backbone for African countries like Uganda, Burundi, Rwanda, and Ethiopia, [41] as well as many Central American countries.

Several species of shrub of the genus Coffea produce the berries from which coffee is extracted. The two main species commercially cultivated are Coffea canephora (predominantly a form known as 'robusta') and C. arabica. [42] C. arabica, the most highly regarded species, is native to the southwestern highlands of Ethiopia and the Boma Plateau in southeastern Sudan and Mount Marsabit in northern Kenya. [43] C. canephora is native to western and central Subsaharan Africa, from Guinea to Uganda and southern Sudan. [44] Less popular species are C. liberica, C. stenophylla, C. mauritiana, and C. racemosa.

All coffee plants are classified in the large family Rubiaceae. They are evergreen shrubs or trees that may grow 5 m (15 ft) tall when unpruned. The leaves are dark green and glossy, usually 10–15 cm (4–6 in) long and 6 cm (2.4 in) wide, simple, entire, and opposite. Petioles of opposite leaves fuse at the base to form interpetiolar stipules, characteristic of Rubiaceae. The flowers are axillary, and clusters of fragrant white flowers bloom simultaneously. Gynoecium consists of an inferior ovary, also characteristic of Rubiaceae. The flowers are followed by oval berries of about 1.5 cm (0.6 in). [45] When immature they are green, and they ripen to yellow, then crimson, before turning black on drying. Each berry usually contains two seeds, but 5–10% of the berries [46] have only one these are called peaberries. [47] Arabica berries ripen in six to eight months, while robusta takes nine to eleven months. [48]

Coffea arabica is predominantly self-pollinating, and as a result, the seedlings are generally uniform and vary little from their parents. In contrast, Coffea canephora, and C. liberica are self-incompatible and require outcrossing. This means that useful forms and hybrids must be propagated vegetatively. [49] Cuttings, grafting, and budding are the usual methods of vegetative propagation. [50] On the other hand, there is great scope for experimentation in search of potential new strains. [49]

In 2016, Oregon State University entomologist George Poinar, Jr. announced the discovery of a new plant species which is a 45-million-year-old relative of coffee found in amber. Named Strychnos electri, after the Greek word for amber (electron), the flowers represent the first-ever fossils of an asterid, which is a clade of flowering plants that not only later gave us coffee, but also sunflowers, peppers, potatoes, mint – and deadly poisons. [51]

The traditional method of planting coffee is to place 20 seeds in each hole at the beginning of the rainy season. This method loses about 50% of the seeds' potential, as about half fail to sprout. A more effective process of growing coffee, used in Brazil, is to raise seedlings in nurseries that are then planted outside at six to twelve months. Coffee is often intercropped with food crops, such as corn, beans, or rice during the first few years of cultivation as farmers become familiar with its requirements. [45] Coffee plants grow within a defined area between the tropics of Cancer and Capricorn, termed the bean belt or coffee belt. [52]

Of the two main species grown, arabica coffee (from C. arabica) is generally more highly regarded than robusta coffee (from C. canephora). Robusta coffee tends to be bitter and have less flavor but better body than arabica. For these reasons, about three-quarters of coffee cultivated worldwide is C. arabica. [42] Robusta strains also contain about 40–50% more caffeine than arabica. [53] Consequently, this species is used as an inexpensive substitute for arabica in many commercial coffee blends. Good quality robusta beans are used in traditional Italian espresso blends to provide a full-bodied taste and a better foam head (known as crema).

Additionally, Coffea canephora is less susceptible to disease than C. arabica and can be cultivated in lower altitudes and warmer climates where C. arabica will not thrive. [54] The robusta strain was first collected in 1890 from the Lomani River, a tributary of the Congo River, and was conveyed from the Congo Free State (now the Democratic Republic of the Congo) to Brussels to Java around 1900. From Java, further breeding resulted in the establishment of robusta plantations in many countries. [55] In particular, the spread of the devastating coffee leaf rust (Hemileia vastatrix), to which C. arabica is vulnerable, hastened the uptake of the resistant robusta. Hemileia vastatrix is a fungal pathogen [56] and results in light, rust-colored spots on the undersides of coffee plant leaves. Hemileia vastatrix grows exclusively on the leaves of coffee pants. [57] Coffee leaf rust is found in virtually all countries that produce coffee. [58]

Mycena citricolor is another threat to coffee plants, primarily in Latin America. Mycena citricolor, commonly referred to as American Leaf Spot, is a fungus that can affect the whole coffee plant. [59] It can grow on leaves, resulting in leaves with holes that often fall from the plant. [59]

Over 900 species of insect have been recorded as pests of coffee crops worldwide. Of these, over a third are beetles, and over a quarter are bugs. Some 20 species of nematodes, 9 species of mites, and several snails and slugs also attack the crop. Birds and rodents sometimes eat coffee berries, but their impact is minor compared to invertebrates. [60] In general, arabica is the more sensitive species to invertebrate predation overall. Each part of the coffee plant is assailed by different animals. Nematodes attack the roots, coffee borer beetles burrow into stems and woody material, [61] and the foliage is attacked by over 100 species of larvae (caterpillars) of butterflies and moths. [62]

Mass spraying of insecticides has often proven disastrous, as predators of the pests are more sensitive than the pests themselves. [63] Instead, integrated pest management has developed, using techniques such as targeted treatment of pest outbreaks, and managing crop environment away from conditions favouring pests. Branches infested with scale are often cut and left on the ground, which promotes scale parasites to not only attack the scale on the fallen branches but in the plant as well. [64]

The 2-mm-long coffee borer beetle (Hypothenemus hampei) is the most damaging insect pest to the world's coffee industry, destroying up to 50 percent or more of the coffee berries on plantations in most coffee-producing countries. The adult female beetle nibbles a single tiny hole in a coffee berry and lays 35 to 50 eggs. Inside, the offspring grow, mate, and then emerge from the commercially ruined berry to disperse, repeating the cycle. Pesticides are mostly ineffective because the beetle juveniles are protected inside the berry nurseries, but they are vulnerable to predation by birds when they emerge. When groves of trees are nearby, the American yellow warbler, rufous-capped warbler, and other insectivorous birds have been shown to reduce by 50 percent the number of coffee berry borers in Costa Rica coffee plantations. [65]

Beans from different countries or regions can usually be distinguished by differences in flavor, aroma, body, and acidity. [66] These taste characteristics are dependent not only on the coffee's growing region, but also on genetic subspecies (varietals) and processing. [67] Varietals are generally known by the region in which they are grown, such as Colombian, Java and Kona.

Arabica coffee beans are cultivated mainly in Latin America, eastern Africa or Asia, while robusta beans are grown in central Africa, throughout southeast Asia, and Brazil. [42]

Ecological effects

Originally, coffee farming was done in the shade of trees that provided a habitat for many animals and insects. [68] Remnant forest trees were used for this purpose, but many species have been planted as well. These include leguminous trees of the genera Acacia, Albizia, Cassia, Erythrina, Gliricidia, Inga, and Leucaena, as well as the nitrogen-fixing non-legume sheoaks of the genus Casuarina, and the silky oak Grevillea robusta. [69]

This method is commonly referred to as the traditional shaded method, or "shade-grown". Starting in the 1970s, many farmers switched their production method to sun cultivation, in which coffee is grown in rows under full sun with little or no forest canopy. This causes berries to ripen more rapidly and bushes to produce higher yields, but requires the clearing of trees and increased use of fertilizer and pesticides, which damage the environment and cause health problems. [70]

Unshaded coffee plants grown with fertilizer yield the most coffee, although unfertilized shaded crops generally yield more than unfertilized unshaded crops: the response to fertilizer is much greater in full sun. [71] While traditional coffee production causes berries to ripen more slowly and produce lower yields, the quality of the coffee is allegedly superior. [72] In addition, the traditional shaded method provides living space for many wildlife species. Proponents of shade cultivation say environmental problems such as deforestation, pesticide pollution, habitat destruction, and soil and water degradation are the side effects of the practices employed in sun cultivation. [68] [73]

The American Birding Association, Smithsonian Migratory Bird Center, [74] National Arbor Day Foundation, [75] and the Rainforest Alliance have led a campaign for 'shade-grown' and organic coffees, which can be sustainably harvested. [76] Shaded coffee cultivation systems show greater biodiversity than full-sun systems, and those more distant from continuous forest compare rather poorly to undisturbed native forest in terms of habitat value for some bird species. [77] [78]

Coffee production use a large volume of water. On average it takes about 140 liters (37 U.S. gal) of water to grow the coffee beans needed to produce one cup of coffee, producing 1 kg (2.2 lb) of roasted coffee in Africa, South America or Asia requires 26,400 liters (7,000 U.S. gal) of water. [ clarification needed ] [79] Coffee is often grown in countries where there is a water shortage, such as Ethiopia. [80]

Used coffee grounds may be used for composting or as a mulch. They are especially appreciated by worms and acid-loving plants such as blueberries. [81] Some commercial coffee shops run initiatives to make better use of these grounds, including Starbucks' "Grounds for your Garden" project, [82] and community sponsored initiatives such as "Ground to Ground". [83]

Climate change may significantly impact coffee yields during the 21st century, such as in Nicaragua and Ethiopia which could lose more than half of the farming land suitable for growing (Arabica) coffee. [84] [85] [86]

As of 2016, at least 34% of global coffee production was compliant with voluntary sustainability standards such as Fairtrade, UTZ, and 4C (The Common Code for the Coffee Community). [87]

Sustainable production

Green coffee production – 2020
Country Production (in thousand 60-kg bags)
Brazil 69,000
Vietnam 29,000
Indonesia 12,400
Colombia 14,300
Ethiopia 7,373
Honduras 6,100
India 5,700
World 175,647
Source: ICO [88]

In 2020, world production of green coffee beans was 175,647,000 60 kg bags, led by Brazil with 39% of the total (table). [88] Vietnam, Indonesia, and Colombia were other major producers.

Coffee berries and their seeds undergo several processes before they become the familiar roasted coffee. Berries have been traditionally selectively picked by hand a labor-intensive method, it involves the selection of only the berries at the peak of ripeness. More commonly, crops are strip picked, where all berries are harvested simultaneously regardless of ripeness by person or machine. After picking, green coffee is processed by one of two types of method—a dry process type of method which is often simpler and less labor-intensive, and a wet process type of method, which incorporates batch fermentation, utilizes larger amounts of water in the process, and often yields a milder coffee. [89]

Then they are sorted by ripeness and color, and most often the flesh of the berry is removed, usually by machine, and the seeds are fermented to remove the slimy layer of mucilage still present on the seed. When the fermentation is finished, the seeds are washed with large quantities of fresh water to remove the fermentation residue, which generates massive amounts of coffee wastewater. Finally, the seeds are dried. [90]

The best (but least used) method of drying coffee is using drying tables. In this method, the pulped and fermented coffee is spread thinly on raised beds, which allows the air to pass on all sides of the coffee, and then the coffee is mixed by hand. In this method the drying that takes place is more uniform, and fermentation is less likely. Most African coffee is dried in this manner and certain coffee farms around the world are starting to use this traditional method. [90]

Next, the coffee is sorted, and labeled as green coffee. Some companies use cylinders to pump in heated air to dry the coffee seeds, though this is generally in places where the humidity is very high. [90]

An Asian coffee known as kopi luwak undergoes a peculiar process made from coffee berries eaten by the Asian palm civet, passing through its digestive tract, with the beans eventually harvested from feces. Coffee brewed from this process [91] is among the most expensive in the world, with bean prices reaching $160 per pound or $30 per brewed cup. [92] Kopi luwak coffee is said to have uniquely rich, slightly smoky aroma and flavor with hints of chocolate, resulting from the action of digestive enzymes breaking down bean proteins to facilitate partial fermentation. [91] [92]

In Thailand, black ivory coffee beans are fed to elephants whose digestive enzymes reduce the bitter taste of beans collected from dung. [93] These beans sell for up to $1,100 a kilogram ($500 per lb), achieving the world's most expensive coffee, [93] three times costlier than palm civet coffee beans. [92]


The next step in the process is the roasting of the green coffee. Coffee is usually sold in a roasted state, and with rare exceptions, such as infusions from green coffee beans, [94] coffee is roasted before it is consumed. It can be sold roasted by the supplier, or it can be home roasted. [95] The roasting process influences the taste of the beverage by changing the coffee bean both physically and chemically. The bean decreases in weight as moisture is lost and increases in volume, causing it to become less dense. The density of the bean also influences the strength of the coffee and requirements for packaging.

The actual roasting begins when the temperature inside the bean reaches approximately 200 °C (392 °F), though different varieties of seeds differ in moisture and density and therefore roast at different rates. [96] During roasting, caramelization occurs as intense heat breaks down starches, changing them to simple sugars that begin to brown, which alters the color of the bean. [97]

Sucrose is rapidly lost during the roasting process, and may disappear entirely in darker roasts. During roasting, aromatic oils and acids weaken, changing the flavor at 205 °C (401 °F), other oils start to develop. [96] One of these oils, caffeol, is created at about 200 °C (392 °F), which is largely responsible for coffee's aroma and flavor. [23]

Roasting is the last step of processing the beans in their intact state. During this last treatment, while still in the bean state, more caffeine breaks down above 235 °C (455 °F). Dark roasting is the utmost step in bean processing removing the most caffeine. Although, dark roasting is not to be confused with the decaffeination process.

Grading roasted beans

Depending on the color of the roasted beans as perceived by the human eye, they will be labeled as light, medium light, medium, medium dark, dark, or very dark. A more accurate method of discerning the degree of roast involves measuring the reflected light from roasted seeds illuminated with a light source in the near-infrared spectrum. This elaborate light meter uses a process known as spectroscopy to return a number that consistently indicates the roasted coffee's relative degree of roast or flavor development.

Roast characteristics

The degree of roast has an effect upon coffee flavor and body. Darker roasts are generally bolder because they have less fiber content and a more sugary flavor. Lighter roasts have a more complex and therefore perceived stronger flavor from aromatic oils and acids otherwise destroyed by longer roasting times. [98] Roasting does not alter the amount of caffeine in the bean, but does give less caffeine when the beans are measured by volume because the beans expand during roasting. [99]

A small amount of chaff is produced during roasting from the skin left on the seed after processing. [100] Chaff is usually removed from the seeds by air movement, though a small amount is added to dark roast coffees to soak up oils on the seeds. [96]


Decaffeination of coffee seeds is done while the seeds are still green. Many methods can remove caffeine from coffee, but all involve either soaking the green seeds in hot water (often called the "Swiss water process") [101] or steaming them, then using a solvent to dissolve caffeine-containing oils. [23] Decaffeination is often done by processing companies, and the extracted caffeine is usually sold to the pharmaceutical industry. [23]


Coffee is best stored in an airtight container made of ceramic, glass or non-reactive metal. [102] Higher quality prepackaged coffee usually has a one-way valve which prevents air from entering while allowing the coffee to release gases. [103] Coffee freshness and flavor is preserved when it is stored away from moisture, heat, and light. [102] The tendency of coffee to absorb strong smells from food means that it should be kept away from such smells. [102] Storage of coffee in refrigerators is not recommended due to the presence of moisture which can cause deterioration. [102] Exterior walls of buildings which face the sun may heat the interior of a home, and this heat may damage coffee stored near such a wall. [102] Heat from nearby ovens also harms stored coffee. [102]

In 1931, a method of packing coffee in a sealed vacuum in cans was introduced. The roasted coffee was packed and then 99% of the air was removed, allowing the coffee to be stored indefinitely until the can was opened. Today this method is in mass use for coffee in a large part of the world. [104]


Coffee beans must be ground and brewed to create a beverage. The criteria for choosing a method include flavor and economy. Almost all methods of preparing coffee require that the beans be ground and then mixed with hot water long enough to allow the flavor to emerge but not so long as to draw out bitter compounds. The liquid can be consumed after the spent grounds are removed. Brewing considerations include the fineness of grind, the way in which the water is used to extract the flavor, the ratio of coffee grounds to water (the brew ratio), additional flavorings such as sugar, milk, and spices, and the technique to be used to separate spent grounds. Optimal coffee extraction occurs between 91 and 96 °C (196 and 205 °F). [105] Ideal holding temperatures range from 85 to 88 °C (185 to 190 °F) to as high as 93 °C (199 °F) and the ideal serving temperature is 68 to 79 °C (154 to 174 °F). [106] The recommended brew ratio for non-espresso coffee is around 55 to 60 grams of grounds per litre of water, or two level tablespoons for a 150-to-180-millilitre (5 to 6 US fl oz) cup. [107]

The roasted coffee beans may be ground at a roastery, in a grocery store, or in the home. Most coffee is roasted and ground at a roastery and sold in packaged form, though roasted coffee beans can be ground at home immediately before consumption. It is also possible, though uncommon, to roast raw beans at home.

Coffee beans may be ground in various ways. A burr grinder uses revolving elements to shear the seed a blade grinder cuts the seeds with blades moving at high speed and a mortar and pestle crushes the seeds. For most brewing methods a burr grinder is deemed superior because the grind is more even and the grind size can be adjusted.

The type of grind is often named after the brewing method for which it is generally used. Turkish grind is the finest grind, while coffee percolator or French press are the coarsest grinds. The most common grinds are between these two extremes: a medium grind is used in most home coffee-brewing machines. [108]

Coffee may be brewed by several methods. It may be boiled, steeped, or pressurized.

Brewing coffee by boiling was the earliest method, and Turkish coffee is an example of this method. [109] It is prepared by grinding or pounding the seeds to a fine powder, then adding it to water and bringing it to the boil for no more than an instant in a pot called a cezve or, in Greek, a μπρίκι : bríki (from Turkish ibrik). This produces a strong coffee with a layer of foam on the surface and sediment (which is not meant for drinking) settling at the bottom of the cup. [109]

Coffee percolators and automatic coffeemakers brew coffee using gravity. In an automatic coffeemaker, hot water drips onto coffee grounds that are held in a paper, plastic, or perforated metal coffee filter, allowing the water to seep through the ground coffee while extracting its oils and essences. The liquid drips through the coffee and the filter into a carafe or pot, and the spent grounds are retained in the filter. [110]

In a percolator, boiling water is forced into a chamber above a filter by steam pressure created by boiling. The water then seeps through the grounds, and the process is repeated until terminated by removing from the heat, by an internal timer, [110] or by a thermostat that turns off the heater when the entire pot reaches a certain temperature.

Coffee may be brewed by steeping in a device such as a French press (also known as a cafetière, coffee press or coffee plunger). [111] Ground coffee and hot water are combined in a cylindrical vessel and left to brew for a few minutes. A circular filter which fits tightly in the cylinder fixed to a plunger is then pushed down from the top to force the grounds to the bottom. The filter retains the grounds at the bottom as the coffee is poured from the container. Because the coffee grounds are in direct contact with the water, all the coffee oils remain in the liquid, making it a stronger beverage. This method of brewing leaves more sediment than in coffee made by an automatic coffee machine. [111] Supporters of the French press method point out that the sediment issue can be minimized by using the right type of grinder: they claim that a rotary blade grinder cuts the coffee bean into a wide range of sizes, including a fine coffee dust that remains as sludge at the bottom of the cup, while a burr grinder uniformly grinds the beans into consistently-sized grinds, allowing the coffee to settle uniformly and be trapped by the press. [112] Within the first minute of brewing 95% of the caffeine is released from the coffee bean. [ citation needed ]

The espresso method forces hot pressurized and vaporized water through ground coffee. As a result of brewing under high pressure (typically 9 bar), [113] the espresso beverage is more concentrated (as much as 10 to 15 times the quantity of coffee to water as gravity-brewing methods can produce) and has a more complex physical and chemical constitution. [114] A well-prepared espresso has a reddish-brown foam called crema that floats on the surface. [108] Other pressurized water methods include the moka pot and vacuum coffee maker.

Cold brew coffee is made by steeping coarsely ground beans in cold water for several hours, then filtering them. [115] This results in a brew lower in acidity than most hot-brewing methods.


Brewed coffee from typical grounds prepared with tap water contains 40 mg caffeine per 100 gram and no essential nutrients in significant content. [116] In espresso, however, likely due to its higher amount of suspended solids, there are significant contents of magnesium, the B vitamins, niacin and riboflavin, and 212 mg of caffeine per 100 grams of grounds. [117]


Once brewed, coffee may be served in a variety of ways. Drip-brewed, percolated, or French-pressed/cafetière coffee may be served as white coffee with a dairy product such as milk or cream, or dairy substitute, or as black coffee with no such addition. It may be sweetened with sugar or artificial sweetener. When served cold, it is called iced coffee.

Espresso-based coffee has a variety of possible presentations. In its most basic form, an espresso is served alone as a shot or short black, or with hot water added, when it is known as Caffè Americano. A long black is made by pouring a double espresso into an equal portion of water, retaining the crema, unlike Caffè Americano. [118] Milk is added in various forms to an espresso: steamed milk makes a caffè latte, [119] equal parts steamed milk and milk froth make a cappuccino, [118] and a dollop of hot foamed milk on top creates a caffè macchiato. [120] A flat white is prepared by adding steamed hot milk (microfoam) to espresso so that the flavour is brought out and the texture is unusually velvety. [121] [122] It has less milk than a latte but both are varieties of coffee to which the milk can be added in such a way as to create a decorative surface pattern. Such effects are known as latte art.

Coffee can also be incorporated with alcohol to produce a variety of beverages: it is combined with whiskey in Irish coffee, and it forms the base of alcoholic coffee liqueurs such as Kahlúa and Tia Maria. Darker beers such as stout and porter give a chocolate or coffee-like taste due to roasted grains even though actual coffee beans are not added to it. [123] [124]

Instant coffee

A number of products are sold for the convenience of consumers who do not want to prepare their own coffee or who do not have access to coffeemaking equipment. Instant coffee is dried into soluble powder or freeze-dried into granules that can be quickly dissolved in hot water. [125] Originally invented in 1907, [126] [127] it rapidly gained in popularity in many countries in the post-war period, with Nescafé being the most popular product. [128] Many consumers determined that the convenience in preparing a cup of instant coffee more than made up for a perceived inferior taste, [129] although, since the late 1970s, instant coffee has been produced differently in such a way that is similar to the taste of freshly brewed coffee. [ citation needed ] Paralleling (and complementing) the rapid rise of instant coffee was the coffee vending machine invented in 1947 and widely distributed since the 1950s. [130]

Canned coffee has been popular in Asian countries for many years, particularly in China, Japan, South Korea, and Taiwan. Vending machines typically sell varieties of flavored canned coffee, much like brewed or percolated coffee, available both hot and cold. Japanese convenience stores and groceries also have a wide availability of bottled coffee drinks, which are typically lightly sweetened and pre-blended with milk. Bottled coffee drinks are also consumed in the United States. [131]

Liquid coffee concentrates are sometimes used in large institutional situations where coffee needs to be produced for thousands of people at the same time. It is described as having a flavor about as good as low-grade robusta coffee, and costs about 10¢ a cup to produce. The machines can process up to 500 cups an hour, or 1,000 if the water is preheated. [132]

Brazil is the largest coffee exporting nation, accounting for 15% of all world exports in 2019. [7]

Commodity market

Coffee is bought and sold as green coffee beans by roasters, investors, and price speculators as a tradable commodity in commodity markets and exchange-traded funds. Coffee futures contracts for Grade 3 washed arabicas are traded on the New York Mercantile Exchange under ticker symbol KC, with contract deliveries occurring every year in March, May, July, September, and December. [133] Coffee is an example of a product that has been susceptible to significant commodity futures price variations. [134] [135] Higher and lower grade arabica coffees are sold through other channels. Futures contracts for robusta coffee are traded on the London International Financial Futures and Options Exchange and, since 2007, on the New York Intercontinental Exchange.

Dating to the 1970s, coffee has been incorrectly described by many, including historian Mark Pendergrast, as the world's "second most legally traded commodity". [136] [137] Instead, "coffee was the second most valuable commodity exported by developing countries," from 1970 to circa 2000. [138] This fact was derived from the United Nations Conference on Trade and Development Commodity Yearbooks which show "Third World" commodity exports by value in the period 1970–1998 as being in order of crude oil in first place, coffee in second, followed by sugar, cotton, and others. Coffee continues to be an important commodity export for developing countries, but more recent figures are not readily available due to the shifting and politicized nature of the category "developing country". [136]

International Coffee Day, which is claimed to have originated in Japan in 1983 with an event organized by the All Japan Coffee Association, takes place on September 29 in several countries. [139] [140] [141]

Industry advocacy


Nordic countries are the highest coffee consuming nations consumption in Finland is the world's highest, close to or more than double that of Brazil Italy France Greece and Canada, which is the 10th-highest consumer, and close to triple coffee consumption in the United States, which ranked 25th in 2018. [145] The top 10 coffee consuming countries, measured per capita, per annum are: [146]

    – 12 kg (26 lb) – 9.9 kg (21 lb 13 oz) – 9 kg (20 lb) – 8.7 kg (19 lb 3 oz) – 8.4 kg (18 lb 8 oz) – 8.2 kg (18 lb 1 oz) – 7.9 kg (17 lb 7 oz) – 6.8 kg (15 lb 0 oz) – 6.5 kg (14 lb 5 oz) – 6.5 kg (14 lb 5 oz)

A 2017 review of clinical trials found that drinking coffee is generally safe within usual levels of intake and is more likely to improve health outcomes than to cause harm at doses of 3 or 4 cups of coffee daily. Exceptions include possible increased risk in women having bone fractures, and a possible increased risk in pregnant women of fetal loss or decreased birth weight. [5] Results were complicated by poor study quality, and differences in age, gender, health status, and serving size. [5]


A 1999 review found that coffee does not cause indigestion, but may promote gastrointestinal reflux. [147] Two reviews of clinical studies on people recovering from abdominal, colorectal, and gynecological surgery found that coffee consumption was safe and effective for enhancing postoperative gastrointestinal function. [148] [149]


In 2012, the National Institutes of Health–AARP Diet and Health Study found that higher coffee consumption was associated with lower risk of death, and that those who drank any coffee lived longer than those who did not. However the authors noted, "whether this was a causal or associational finding cannot be determined from our data." [150] A 2014 meta-analysis found that coffee consumption (4 cups/day) was inversely associated with all-cause mortality (a 16% lower risk), as well as cardiovascular disease mortality specifically (a 21% lower risk from drinking 3 cups/day), but not with cancer mortality [151] with exception being oral cancer mortality. [152]

Additional meta-analyses corroborated these findings, showing that higher coffee consumption (2–4 cups per day) was associated with a reduced risk of death by all disease causes. [153] [154] An association of coffee drinking with reduced risk for death from various sources was confirmed by a widely cited prospective cohort study of ten European countries in 2017. [155]

Cardiovascular disease

Moderate coffee consumption is not a risk factor for coronary heart disease. [156] A 2012 meta-analysis concluded that people who drank moderate amounts of coffee had a lower rate of heart failure, with the biggest effect found for those who drank more than four cups a day. [157] A 2014 meta-analysis concluded that cardiovascular disease, such as coronary artery disease and stroke, is less likely with three to five cups of non-decaffeinated coffee per day, but more likely with over five cups per day. [158] A 2016 meta-analysis showed that coffee consumption was associated with a reduced risk of death in patients who have had a myocardial infarction. [159]

The effect of no or moderate daily consumption of coffee on risk for developing hypertension has been assessed in several reviews during the 21st century. A 2019 review found that one to two cups consumed per day had no effect on hypertension risk, whereas drinking three or more cups per day reduced the risk, [160] a finding in agreement with a 2017 analysis which showed a 9% lower risk of hypertension with long-term consumption of up to seven cups of coffee per day. [161] Another review in 2018 found that the risk of hypertension was reduced by 2% with each one cup per day increment of coffee consumption up to 8 cups per day, compared with people who did not consume any coffee. [162] By contrast, a 2011 review had found that drinking one to three cups of coffee per day may pose a slightly increased risk of developing hypertension. [163]

Mental health

The UK NHS advises that avoiding coffee may reduce anxiety. [164] Caffeine, the major active ingredient in coffee, is associated with anxiety. [165] [166] At high doses, typically greater than 300 mg, caffeine can both cause and worsen anxiety. [167] For some people, discontinuing caffeine use can significantly reduce anxiety. [168] Caffeine-induced anxiety disorder is a subclass of substance- or medication-induced anxiety disorder. [169] Populations that may be most impacted by caffeine consumption are adolescents and those already suffering anxiety disorders. [170] Preliminary research indicated the possibility of a beneficial relationship between coffee intake and reduced depression. [5] [171] [172] Long-term preliminary research, including assessment of symptoms for dementia and cognitive impairment, was inconclusive for coffee having an effect in the elderly, mainly due to the poor quality of the studies. [5] [173]

Parkinson's disease

Meta-analyses have consistently found that long-term coffee consumption is associated with a lower risk of Parkinson's disease. [5]

Type II diabetes

In a systematic review and meta-analysis of 28 prospective observational studies, representing over one million participants, every additional cup of caffeinated and decaffeinated coffee consumed in a day was associated, respectively, with a 9% and 6% lower risk of type 2 diabetes. [174]


The research on the effects of coffee consumption on cancer risk generally indicate that it has no effect (gastric cancer), [175] [176] or produces a lower risk of cancer (carcinoma and lung cancer). [177] [178] A 2011 review found that regular coffee consumption of up to 6 cups per day reduced the risk of several types of cancer. [179]

Liver disease

Increasing evidence has shown that coffee consumption is protective against the progression of liver disease to cirrhosis. This is associated with antioxidant and anti-fibrotic effects of coffee. [180]

One psychoactive chemical in coffee is caffeine, an adenosine receptor antagonist that is known for its stimulant effects. [181] Coffee also contains the monoamine oxidase inhibitors β-carboline and harmane, which may contribute to its psychoactivity. [182]

In a healthy liver, caffeine is mostly broken down by hepatic enzymes. The excreted metabolites are mostly paraxanthines—theobromine and theophylline—and a small amount of unchanged caffeine. Therefore, the metabolism of caffeine depends on the state of this enzymatic system of the liver. [183]

Polyphenols in coffee have been shown to affect free radicals in vitro, [184] but there is no evidence that this effect occurs in humans. Polyphenol levels vary depending on how beans are roasted as well as for how long. As interpreted by the Linus Pauling Institute and the European Food Safety Authority, dietary polyphenols, such as those ingested by consuming coffee, have little or no direct antioxidant value following ingestion. [185] [186] [187]

Depending on the type of coffee and method of preparation, the caffeine content of a single serving can vary greatly. [188] [189] [190] The caffeine content of a cup of coffee varies depending mainly on the brewing method, and also on the coffee variety. [191] According to the USDA National Nutrient Database, a 240-millilitre (8 US fl oz) cup of "coffee brewed from grounds" contains 95 mg caffeine, whereas an espresso (25 ml) contains 53 mg. [192]

According to an article in the Journal of the American Dietetic Association, coffee has the following caffeine content, depending on how it is prepared: [189]

Serving size Caffeine content
Brewed 200 mL (7 US fl oz) 80–135 mg
Drip 200 mL (7 US fl oz) 115–175 mg
Espresso 45–60 mL ( 1 + 1 ⁄ 2 –2 US fl oz) 100 mg

Caffeine remains stable up to 200 °C (392 °F) and completely decomposes around 285 °C (545 °F). [193] Given that roasting temperatures don't exceed 200 °C (392 °F) for long and rarely if ever reach 285 °C (545 °F), the caffeine content of a coffee is not likely changed much by the roasting process. [194]

Widely known as coffeehouses or cafés, establishments serving prepared coffee or other hot beverages have existed for over five hundred years. The first coffeehouse in Constantinople was opened in 1475 by traders arriving from Damascus and Aleppo. [195] Soon after, coffeehouses became part of the Ottoman culture, spreading rapidly to all regions of the Ottoman Empire.

Coffeehouses in Mecca became a concern as places for political gatherings to the imams who banned them, and the drink, for Muslims between 1512 and 1524. In 1530 the first coffeehouse was opened in Damascus. [196]

In the 17th century, coffee appeared for the first time in Europe outside the Ottoman Empire, and coffeehouses were established and quickly became popular. The first coffeehouses in Western Europe appeared in Venice, as a result of the traffic between La Serenissima and the Ottomans the very first one is recorded in 1645. The first coffeehouse in England was set up in Oxford in 1650 by a Jewish man named Jacob in the building now known as "The Grand Cafe". A plaque on the wall still commemorates this the cafe is now a cocktail bar. [198] By 1675, there were more than 3,000 coffeehouses in England. [199]

A legend says that after the second Turkish siege of Vienna in 1683, the Viennese discovered many bags of coffee in the abandoned Ottoman encampment. Using this captured stock, a Polish soldier named Kulczycki opened the first coffeehouse in Vienna. This story never happened. Nowadays it is proven that the first coffeehouse in Vienna was opened by the Armenian Johannes Theodat in 1685. [200] [201]

In 1672 an Armenian named Pascal established a coffee stall in Paris that was ultimately unsuccessful and the city had to wait until 1689 for its first coffeehouse when Procopio Cutò opened the Café Procope. This coffeehouse still exists today and was a major meeting place of the French Enlightenment Voltaire, Rousseau, and Denis Diderot frequented it, and it is arguably the birthplace of the Encyclopédie, the first modern encyclopedia. [202] America had its first coffeehouse in Boston, in 1676. [203] Coffee, tea and beer were often served together in establishments which functioned both as coffeehouses and taverns one such was the Green Dragon in Boston, where John Adams, James Otis, and Paul Revere planned rebellion. [30]

The modern steamless espresso machine was invented in Milan, Italy, in 1938 by Achille Gaggia, [204] and from there spread in coffeehouses and restaurants across Italy and the rest of Europe in the early 1950s. An Italian named Pino Riservato opened the first espresso bar, the Moka Bar, in Soho in 1952, and there were 400 such bars in London alone by 1956. Cappucino was particularly popular among English drinkers. [205] Similarly in the United States, the espresso craze spread. North Beach in San Francisco saw the opening of the Caffe Trieste in 1957, which served Beat Generation poets such as Allen Ginsberg and Bob Kaufman alongside Italian immigrants. [205] Similar such cafes existed in Greenwich Village and elsewhere. [205]

The first Peet's Coffee & Tea store opened in 1966 in Berkeley, California by Dutch native Alfred Peet. He chose to focus on roasting batches with fresher, higher quality seeds than was the norm at the time. He was a trainer and supplier to the founders of Starbucks. [206]

The American coffeehouse chain Starbucks, which began as a modest business roasting and selling coffee beans in 1971, was founded by three college students, Jerry Baldwin, Gordon Bowker, and Zev Siegl. The first store opened on March 30, 1971 at the Pike Place Market in Seattle, followed by a second and third over the next two years. [207] Entrepreneur Howard Schultz joined the company in 1982 as Director of Retail Operations and Marketing, and pushed to sell premade espresso coffee. The others were reluctant, but Schultz opened Il Giornale in Seattle in April 1986. [208] He bought the other owners out in March 1987 and pushed on with plans to expand—from 1987 to the end of 1991, the chain (rebranded from Il Giornale to Starbucks) expanded to over 100 outlets. [209] The company has 25,000 stores in over 75 countries worldwide. [210]

South Korea experienced almost 900 percent growth in the number of coffee shops in the country between 2006 and 2011. The capital city Seoul now has the highest concentration of coffee shops in the world, with more than 10,000 cafes and coffeehouses. [211]

A contemporary term for a person who makes coffee beverages, often a coffeehouse employee, is a barista. The Specialty Coffee Association of Europe and the Specialty Coffee Association of America have been influential in setting standards and providing training. [212]

Coffee is often consumed alongside (or instead of) breakfast by many at home or when eating out at diners or cafeterias. It is often served at the end of a formal meal, normally with a dessert, and at times with an after-dinner mint, especially when consumed at a restaurant or dinner party. [ citation needed ]


A coffee break in the United States and elsewhere is a short mid-morning rest period granted to employees in business and industry, corresponding with the Commonwealth terms "elevenses", "smoko" (in Australia), "morning tea", "tea break", or even just "tea". An afternoon coffee break, or afternoon tea, often occurs as well.

The coffee break originated in the late 19th century in Stoughton, Wisconsin, with the wives of Norwegian immigrants. The city celebrates this every year with the Stoughton Coffee Break Festival. [213] In 1951, Time noted that "[s]ince the war, the coffee break has been written into union contracts". [214] The term subsequently became popular through a Pan-American Coffee Bureau ad campaign of 1952 which urged consumers, "Give yourself a Coffee-Break – and Get What Coffee Gives to You." [215] John B. Watson, a behavioral psychologist who worked with Maxwell House later in his career, helped to popularize coffee breaks within the American culture. [216] Coffee breaks usually last from 10 to 20 minutes and frequently occur at the end of the first third of the work shift. In some companies and some civil service, the coffee break may be observed formally at a set hour. In some places, a cart with hot and cold beverages and cakes, breads and pastries arrives at the same time morning and afternoon, an employer may contract with an outside caterer for daily service, or coffee breaks may take place away from the actual work-area in a designated cafeteria or tea room. More generally, the phrase "coffee break" has also come to denote any break from work.

Prohibition and condemnation

Coffee was initially used for spiritual reasons [ which? ] . At least 1,100 years ago, traders brought coffee across the Red Sea into Arabia (modern-day Yemen), where Muslim dervishes began cultivating the shrub in their gardens. At first, the Arabians made wine from the pulp of the fermented coffee berries. This beverage was known as qishr (kisher in modern usage) and was used during religious ceremonies. [217]

An ulema of jurists and scholars meeting in Mecca in 1511 prohibited coffee drinking as haraam, but whether coffee was intoxicating was hotly debated over the next 30 years until the ban was finally overturned in the mid-16th century. [218] Use in religious rites among the Sufi branch of Islam led to coffee's being put on trial [ when? ] in Mecca: it was accused of being a heretical substance, and its production and consumption were briefly repressed. An edict of Sultan Murad IV ( r . 1623–1640 ) later prohibited it in Ottoman Turkey. [219]

Ethiopian Orthodox Christians prohibited coffee, regarded as a Muslim drink, until as late as 1889 as of 2019 [update] it is considered [ by whom? ] a national drink of Ethiopia for people of all faiths. [ citation needed ] In 1670 some French doctors condemned coffee as poisonous. [220] Coffee's early association in Europe with rebellious political activities led to King Charles II of England outlawing coffeehouses from January 1676 (although the subsequent uproar forced the monarch to back down two days before the ban was due to come into force). [30] King Frederick the Great banned it in Prussia in 1777 for nationalistic and economic reasons concerned about the price of imports, he sought to force the public back to consuming beer. [221] Lacking coffee-producing colonies, Prussia had to import all its coffee at a great cost. [222]

A contemporary example of religious prohibition of coffee can be found in The Church of Jesus Christ of Latter-day Saints. [223] The organization regards the consumption of coffee as both physically and spiritually unhealthy. [224] This attitude comes from the Mormon doctrine of health, published in 1833 by founder Joseph Smith in a revelation called the "Word of Wisdom". This text does not identify coffee by name, but includes the statement that "hot drinks are not for the belly", which Latter-day Saints have interpreted as forbidding both coffee and tea. [224]

Quite a number of members of the Seventh-day Adventist Church also avoid caffeinated drinks. In its teachings, the Church encourages members to avoid tea, coffee, and other stimulants. Abstinence from coffee, tobacco, and alcohol by many Adventists has afforded a near-unique opportunity for studies to be conducted within that population group on the health effects of coffee drinking, free from confounding factors. One study showed a weak but statistically significant association between coffee consumption and mortality from ischemic heart disease, other cardiovascular disease, all cardiovascular diseases combined, and all causes of death. [225]

For a time, controversy existed in the Jewish community over whether the coffee seed was a legume - and therefore prohibited for Passover. Upon petition from coffeemaker Maxwell House, orthodox Jewish rabbi Hersch Kohn in 1923 classified the coffee seed as a berry rather than as a seed, and therefore kosher for Passover. [226]

Fair trade

The concept of fair trade labeling, which guarantees coffee growers a negotiated preharvest price, began in the late 1980s with the Max Havelaar Foundation's labeling program in the Netherlands. In 2004, 24,222 metric tons (of 7,050,000 produced worldwide) were fair trade in 2005, 33,991 metric tons out of 6,685,000 were fair trade, an increase from 0.34% to 0.51%. [227] [228] A number of fair trade impact studies have shown that fair trade coffee produces a mixed impact on the communities that grow it. Many studies are skeptical about fair trade, reporting that it often worsens the bargaining power of those who are not part of it. The very first fair-trade coffee was an effort to import a Guatemalan coffee into Europe as "Indio Solidarity Coffee". [229]

Since the founding of organizations such as the European Fair Trade Association (1987), the production and consumption of fair trade coffee has grown as some local and national coffee chains started to offer fair trade alternatives. [230] [231] For example, in April 2000, after a year-long campaign by the human rights organization Global Exchange, Starbucks decided to carry fair-trade coffee in its stores. [232] Since September 2009 all Starbucks Espresso beverages in UK and Ireland are made with Fairtrade and Shared Planet certified coffee. [233]

A 2005 study done in Belgium concluded that consumers' buying behavior is not consistent with their positive attitude toward ethical products. On average 46% of European consumers claimed to be willing to pay substantially more for ethical products, including fair-trade products such as coffee. [232] The study found that the majority of respondents were unwilling to pay the actual price premium of 27% for fair trade coffee. [232]

Folklore and culture

The Oromo people would customarily plant a coffee tree on the graves of powerful sorcerers. They believed that the first coffee bush sprang up from the tears that the god of heaven shed over the corpse of a dead sorcerer. [234]

Johann Sebastian Bach was inspired to compose the humorous Coffee Cantata, about dependence on the beverage, which was controversial in the early 18th century. [235]

Economic impacts

Market volatility, and thus increased returns, during 1830 encouraged Brazilian entrepreneurs to shift their attention from gold to coffee, a crop hitherto reserved for local consumption. Concurrent with this shift was the commissioning of vital infrastructures, including approximately 7,000 km of railroads between 1860 and 1885. The creation of these railways enabled the importation of workers, in order to meet the enormous need for labor. This development primarily affected the State of Rio de Janeiro, as well as the Southern States of Brazil, most notably São Paulo, due to its favorable climate, soils, and terrain. [236]

Coffee production attracted immigrants in search of better economic opportunities in the early 1900s. Mainly, these were Portuguese, Italian, Spanish, German, and Japanese nationals. For instance, São Paulo received approximately 733,000 immigrants in the decade preceding 1900, whilst only receiving approximately 201,000 immigrants in the six years to 1890. The production yield of coffee increases. In 1880, São Paulo produced 1.2 million bags (25% of total production), in 1888 2.6 million (40%), in 1902 8 million bags (60%). [237] Coffee is then 63% of the country's exports. The gains made by this trade allow sustained economic growth in the country.

The four years between planting a coffee and the first harvest extends seasonal variations in the price of coffee. The Brazilian Government is thus forced, to some extent, to keep strong price subsidies during production periods.


Coffee competitions take place across the globe with people at the regional competing to achieve national titles and then compete on the international stage. World Coffee Events holds the largest of such events moving the location of the final competition each year. The competition includes the following events: Barista Championship, Brewers Cup, Latte Art and Cup Tasters. A World Brewer's Cup Championship takes place in Melbourne, Australia, every year that houses contestants from around the world [238] to crown the World's Coffee King. [239] [240]

Hands-on Activity A Tasty Experiment

Units serve as guides to a particular content or subject area. Nested under units are lessons (in purple) and hands-on activities (in blue).

Note that not all lessons and activities will exist under a unit, and instead may exist as "standalone" curriculum.

TE Newsletter

The nose is used for smelling, which is helpful to identify foods.


Engineering Connection

Chemical and food engineers use information about how people sense taste to develop artificial flavors that taste more like the real flavors they are designed to mimic.

Learning Objectives

After this activity, students should be able to:

  • Explain the importance of the sense of smell to the ability of humans to recognize familiar foods.
  • Explain why it is adaptive for an animal to use its senses to identify foods as being either nutritious or noxious.
  • Create bar graphs comparing quantities of different items.
  • Interpret bar graphs comparing quantities of different items.

Educational Standards

Each TeachEngineering lesson or activity is correlated to one or more K-12 science, technology, engineering or math (STEM) educational standards.

All 100,000+ K-12 STEM standards covered in TeachEngineering are collected, maintained and packaged by the Achievement Standards Network (ASN), a project of D2L (

In the ASN, standards are hierarchically structured: first by source e.g., by state within source by type e.g., science or mathematics within type by subtype, then by grade, etc.

NGSS: Next Generation Science Standards - Science

4-LS1-2. Use a model to describe that animals' receive different types of information through their senses, process the information in their brain, and respond to the information in different ways. (Grade 4)

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Common Core State Standards - Math
  • Draw a scaled picture graph and a scaled bar graph to represent a data set with several categories. Solve one- and two-step "how many more" and "how many less" problems using information presented in scaled bar graphs. (Grade 3) More Details

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International Technology and Engineering Educators Association - Technology
  • Compare, contrast, and classify collected information in order to identify patterns. (Grades 3 - 5) More Details

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State Standards
North Carolina - Math
  • Draw a scaled picture graph and a scaled bar graph to represent a data set with several categories. Solve one- and two-step "how many more" and "how many less" problems using information presented in scaled bar graphs. (Grade 3) More Details

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Materials List

  • yogurt or pudding, 4 different flavors, 8 to 12 ounces each (use the larger amount for class sizes greater than 24) if using yogurt, choose types that are "blended" and do not contain fruitbits that might provide textural clues it is not necessary to have all four flavors be either all yogurt or all pudding for example, it is fine to have two different flavors of yogurt and two different flavors of pudding see the Safety Issues section for food allergy considerations
  • 4 cup or bowls, each capable of holding at least 8 ounces

Worksheets and Attachments

More Curriculum Like This

In this lesson and activity, students conduct an experiment to determine whether or not the sense of smell is important to being able to recognize foods by taste. In an opening discussion, students explore why it might be adaptive for humans and other animals to be able to identify nutritious versus.

Pre-Req Knowledge

  • An understanding of what biological adaptation is.
  • Ability to construct a bar graph (helpful, but not essential).


Before a scientist or researcher starts an experiment, he or she must first have a prediction about the expected outcome of the experiment. We call this prediction a hypothesis. However, a hypothesis is not simply a guess. Instead, it is a prediction based on prior knowledge of and experience with the subject.

For example, if a gardener wants to find out if it is really necessary to fertilize tomato plants, he or she might grow six tomato plants, but fertilize only three of them. In this case, the hypothesis being tested might be: Fertilized tomato plants produce more tomatoes than unfertilized tomato plants. The data collected during the experiment would either support or refute the hypothesis, such as, in this case, the total number of tomatoes produced by the fertilized plants compared to the total number produced by the unfertilized plants.

In this tomato plant experiment, the experimenting gardener collects data that involves numbers. In science, this is usually the case, because numbers can easily be compared and are based on countable things that really happened—not what the experimenter thought might happen.

In the taste experiment you will conduct next, you will taste and try to identify several different foods. You will not be allowed to see the food for any of the tastings, and for some you will not be permitted to smell the food either.

For this taste experiment, how might we collect data that involves numbers, or measuring something? (See if students have any ideas. This may be a hard question for some students, but help them to realize that they can simply keep track of the correct and incorrect responses by counting them.)

What is your hypothesis for this experiment? (Listen to student ideas. Expect them to respond with ideas such as: We will be able to correctly identify the most foods if we can smell them.)

Remember that hypotheses are not random guesses, but are based on everything you already know—your prior knowledge and experiences. Tell me what your hypotheses are based on? (Listen to student explanations. Expect them to refer to the demonstration with the student volunteer witnessed in the associated lesson demonstration, or to their own personal experiences. Then, move on to conduct the activity.)


Experiment Preparation with the Students

  1. Make sure all students know how to use the watches to determine when 15 seconds have elapsed. Explain that this is the amount of time that students are permitted before they must state the flavor of the food they just tasted—or else state that they are unable to identify it. Since students take turns being the timers, everyone must be able to determine when 15 seconds have elapsed.
  2. Explain the basic procedure for the experiment, as follows:

The room is set up with four tables to serve as tasting stations, and the class is divided into four groups. Each group rotates through the four stations (and each group uses four datasheets, one at each station). At each station, two members of the group sit on one side of the table. One of these two members serves as the "feeder," because s/he feeds the food to the other students, who try to identify it. In front of the feeder is the food to be tasted it is inside a box that is turned on its side so the rest of the team cannot see the food. One at a time, the other members of the team come to the table.The feeder puts a small amount (about one-half to three-quarters of a teaspoon) of food on a clean spoon, and gently feeds the student.

Meanwhile, the other member seated at the table is the "timer." This student tells the feeder when to put the food in the taster's mouth, and then announces when time is up 15 seconds later. Sometime during that 15 seconds the taster must identify the food, or else "give up."

Mention that that the taster must state his or her answer very quietly, so that other students who have not yet tasted that food do not hear. Also mention that the taster must identify both the type of food and its flavor. For example, if the taster thinks the food is Jello®, s/he says orange Jello®, cherry Jello®, or whatever flavor s/he thinks it is. Once the taster has given his or her response, the timer and presenter record that response on the datasheet.

  1. Make sure that all students know how to fill in the datasheet, starting with identifying the tasting station number, and then listing the names of the team members that hold their noses in the left column. They enter the names of the team members that do the tasting without holding their noses into the third column, and once the taste testing begins, they enter response of each student into the appropriate "Response" column. If the taster does not give a response within the 15 seconds, the response data collected is recorded as "none."
  2. Divide the entire class into four teams. If necessary, ask for help in rearranging the classroom to set up the four tasting stations.
  3. At each station place one-quarter of the spoons, a trash receptacle for used spoons, a watch and a box containing a cup or bowl of food to be tasted, but cover the box with the fabric while it is in transit so that students are not able to see its contents. Also place a copy of the datasheet at each station, and a placard with a number between one and four to indicate the number of that particular tasting station.
  4. Have each team choose two of its members to serve as the timer and the presenter for the first station. Have the remaining team members decide which half to hold their noses and close their eyes, and which half to only close their eyes. For teams with an odd number of students, have the extra student hold his or her nose and close his or her eyes while tasting. Be sure to explain that as they rotate through the different stations, every student will have an opportunity to do both types of tastings, and most will have an opportunity to be the presenter and/or the timer.

Class Experiment

Part 1: Conducting the taste tests

  1. Once the class is clear on what is to happen, assign each team to a station and let them begin the taste tests. Watch closely to see that the instructions are being followed, and answer any procedural questions that arise.
  2. When each team has finished at its first station and filled out its datasheet completely, place a new datasheet at each station and make sure the box containing the food is covered from sight. Then have the teams rotate in one direction to the next nearest tasting station. There, they choose a new presenter and timer, and divide the remainder of the team into "smelling" and "non-smelling" halves, as they did for the first station. Then they conduct the taste tests and record their data.
  3. Repeat this procedure for the third and fourth tasting stations. Remind students to keep their voices as quiet as possible, and not share their food identifications with other teams as they rotate to new stations. If necessary, explain that giving away a food type or flavor would ruin the fun—and make the experiment invalid.
  4. While students are conducting their tests, spend some time with each team and ask them the Investigating Questions.

Part 2: Graphing and interpreting the data

  1. Once the tasting experiment has been completed, announce or write on the classroom board the foods and flavors for each of the four tasting stations. Then have each team look over its four datasheets. What do students notice about the data? Were students more successful at identifying the foods when they could smell them? If so, was there a big difference in the number of correct responses between the able-to-smell versus not-able-to-smell groups? Were the foods at some stations more difficult to identify, based on the number of incorrect responses, than the foods at other stations? If so, was this consistent across all the teams?
  2. Explain to the class that bar graphs let us see at a glance the answers to these questions. Give each person a sheet of graph paper, and show the class how to set up the axes to make a bar graph of the results for the first tasting station. Make the graph consist of two pairs of vertical bars. The first pair shows the results of the able-to-smell tasters. Within that pair, the first bar shows the number of correct responses, and second shows the number of incorrect responses. The second pair of bars shows the results of the not-able-to-smell tasters. Again, the first bar shows the number of correct responses, and second shows the number of incorrect responses. Then have students choose a crayon or marker color to fill in both of the correct-responses bars, and a second color for the incorrect response bars. The use of colors means that they need to add a legend to the graph indicating what the colors represent. Ask: Why is using two different colors in the graph a good idea? Expect students to respond that colors help to make any differences in the successes of the two different tasting groups more noticeable.
  3. If students have not already done so, make sure that their y-axie are labeled appropriately, such as "number of tasters," and the x-axes include labels beneath each pair of bars indicating whether they represent the able-to-smell responses or the not-able-to-smell responses. Also point out that all graphs need informative titles. Ask the class to come up with one. Examples that are fairly specific might be: "Results from Food Tasting Experiments" or "Food Taste Experiments With and Without Smell." Next, point out that the graphs need to indicate the station number and the data source. Include this information as part of the title or as a separate label elsewhere.
  4. Once students have completed their graphs for the first tasting station, provide more graph paper and have them construct similar graphs for each of the other three stations. As they are working, circulate through the room and ask what they think their graphs show about people's ability to taste foods under different circumstances.
  5. When teams have finished the four graphs, have each team combine the results for all four stations. In other words, ask students to determine the total number of correct responses from all the able-to-smell tastings that occurred in their teams, and the total number of correct responses from the not-able-to-smell tastings that occurred. Then have them do the same for the incorrect responses, and have them graph these results on a new sheet of data paper. Expect the bars on these graphs to be much higher than on the previous graphs.
  6. By now, each student has completed five graphs. From each team, choose (or have the team choose) one graph from each of the five types. Tape them to the classroom board or mount them on a bulletin board so that results of each station are all together in one spot, and the combined results (the last graphs made) are together. At this point, any similarities and differences between the teams' results become apparent, so ask students to point these out to you. If any noticeable differences exist between teams, ask why they think these might have occurred. Finally, ask students what they conclude about whether or not smell is important to the ability to recognize and identify foods, and ask whether or not their hypotheses were supported.

Part 3: Relating the experiment to human adaptations

  1. Ask students what they remember from the earlier discussion (in the associated lesson) about the adaptive value of being able to recognize and remember whether certain things are good to eat, that is, nutritious, or bad to eat, that is, noxious (harmful to one's health). Students may be interested to know that when babies are just starting to eat soft foods (after a few months of drinking only milk), they behave much the same way our early ancestors probably did when finding a strange berry or unknown root. They knew from experience that some things that looked like edible might make them sick, so they would only take a small sample at first. If they did not get sick after several hours, they would then eat a larger quantity —and of course, remember what it looked and tasted like for future reference. Similarly, infants often refuse to eat more that a bite or two of a food they have not tasted before, even though the parent knows that it is a safe and healthful food. The next time the infant is offered the food, it will eat a little more. After that, it will be willing to eat full portions. This cautious behavior when experiencing new foods seems to be instinctive (one we are born with) in humans.
  2. After this brief discussion of adaptive behavior, ask the class a harder question: what is the difference between the sense of taste and the sense of smell? Give them some time to share their opinions, and then draw a map of the tongue's upper surface on the classroom board, showing the regions that respond to the sweet, salty, sour and bitter aspects of food. Then ask the class how the tongue can distinguish between different flavors of pudding, which are all sweet, if it has only the ability to distinguish between, say, sweet versus salty foods? Since it cannot, explain how the sense of smell works, especially as it relates to eating. (Refer to the Lesson Background and Concepts for Teachers section of the associated lesson.) Then ask if this information about the sense of smell is consistent with the results students obtained in the experiment. Conclusion: Food identification is difficult and sometimes impossible without the sense of smell!
  3. Finally, conclude the activity and lesson by pointing out that not only is food tasting behavior adaptive, but the human body structures and senses that enable us to taste foods—which includes smells—are adaptations of the body that have helped humans survive through the years.


Example quiz or discussion questions:

  • A fellow student tells you that he is going to give you either a piece of a plain brownie or a piece of a brownie containing walnuts, but you have to close your eyes and hold your nose while you chew and swallow it. If you can correctly identify which piece he has given you, he will then give you the rest of the brownie. Do you think you will be able to correctly identify which piece he has given you? Why do you think that?
  • Another student tells you that she is going to give you either a spoonful of cherry Jello® or a spoonful of orange Jello®. However, if you want more than a spoonful, you will have to close your eyes and hold your nose while you eat it and then correctly identify the flavor. Do you think you will be able to do it? Why do you think that?
  • Why do some people think that food has less flavor when they have a stuffy nose from a bad cold?
  • Create a bar graph similar to those students created, but showing the results of a different experiment, such as the one shown in the Effects of Studying for a Spelling Test example bar graph. Then ask the following questions:
  1. What was the total number of correct answers given by students who studied for the spelling test?
  2. What was the total number of incorrect answers given by students who studied for the spelling test?
  3. What was the total number of correct answers given by students who did not study for the spelling test?
  4. What was the total number of incorrect answers given by students who did not study for the spelling test?
  5. What can you conclude from this experiment?
  6. What hypothesis do you think this experiment was testing?

Investigating Questions

As students are conducting the experiment, ask the following questions:

  • What do you notice about the responses so far?
  • Are tasters more successful at identifying the foods when they can smell them? If so, do you see a big difference in the number of correct responses between the able-to-smell versus not-able-to-smell groups, or is it only a small difference?
  • So far, does it look like your hypothesis is going to be supported?
  • Do the foods at some stations seem more difficult to identify than the foods at other stations? If so, why do you think that might be?

At the end of the concluding discussion, ask:

Safety Issues

  • Well in advance of the activity, check for food allergies. Since the amount of dairy product involved in the tasting experiment is small, this is likely not a problem for students with lactose intolerances. Nevertheless, consult with parents if you have lactose-intolerant students in the class. If you must entirely avoid dairy products, use pureed fruit baby food products instead.
  • Make sure all surfaces in the tasting areas are scrupulously clean before beginning.
  • Have all students wash their hands thoroughly before and after serving as "feeders."

Troubleshooting Tips

Make sure that students state their food identification responses before they release their noses for the not-able-to-smell components of the tastings. If responses are given after releasing their noses, do not count them as correct responses, but instead mark them on the datasheets as "none."

Demonstrate the correct amount of food that feeders should put on a spoon: about one-half to three-quarters of a teaspoon. If they use too much, it may slide off the spoon before it reaches the taster's mouth, or the food may run out before the end of the experiment.

Remind students not to share food identification information with students who have not yet had an opportunity to taste it.

Emphasize that the point of the experiment is not to get the food answers "right," but to find out if it is harder to tell what a food is if you are unable to smell it.

Activity Extensions

Many elderly people complain that food is not as flavorful to them as it was in their younger years. Have students do some library and/or online research to try to find out if indeed this happens, and if so, why. They could also survey older people, asking them if they find food less flavorful than it was when they were younger. Also, see if you can locate a dozen or more elderly volunteers (perhaps grandparents of students) willing to visit the classroom. They can serve as the tasters for the same experiment that students performed on themselves, and students can compare results from the elderly group to their own results. This time, students would be testing the hypothesis: Elderly people will not be able to identify food as well as fourth-graders.


Hebrank, M.R., 1995. "An Exercise in Good Taste," in Biology on a Shoestring, National Association of Biology Teachers, Reston, VA.



Supporting Program


This content was developed by the MUSIC (Math Understanding through Science Integrated with Curriculum) Program in the Pratt School of Engineering at Duke University under National Science Foundation GK-12 grant no. DGE 0338262. However, these contents do not necessarily represent the policies of the NSF, and you should not assume endorsement by the federal government.

Making sense of scents: Mice can identify specific odors amid complex olfactory environments

For many animals, making sense of the clutter of sensory stimuli is often a matter or literal life or death. Exactly how animals separate objects of interest, such as food sources or the scent of predators, from background information, however, remains largely unknown. Even the extent to which animals can make such distinctions, and how differences between scents might affect the process were largely a mystery -- until now.

A new study, described in an August 3 paper in Nature Neuroscience, a team of researchers led by Venkatesh Murthy, Professor of Molecular and Cellular Biology, showed that while mice can be trained to detect specific odorants embedded in random mixtures, their performance drops steadily with increasing background components. The team included Dan Rokni, Vikrant Kapoor and Vivian Hemmelder, all from Harvard University.

"There is a continuous stream of information constantly arriving at our senses, coming from many different sources," Murthy said. "The classic example would be a cocktail party -- though it may be noisy, and there may be many people talking, we are able to focus our attention on one person, while ignoring the background noise.

"Is the same also true for smells?" he continued. "We are bombarded with many smells all jumbled up. Can we pick out one smell "object" -- the smell of jasmine, for example, amidst a riot of other smells? Our experience tells us indeed we can, but how do we pick out the ones that we need to pay attention to, and what are the limitations?"

To find answers to those, and other, questions, Murthy and colleagues turned to mice.

After training mice to detect specific scents, researchers presented the animals with a combination of smells -- sometimes including the "target" scent, sometimes not. Though previous studies had suggested animals are poor at individual smells, and instead perceived the mixture as a single smell, their findings showed that mice were able to identify when a target scent was present with 85 percent accuracy or better.

"Although the mice do well overall, they perform progressively poorer when the number of background odors increases," Murthy explained.

Understanding why, however, meant first overcoming a problem particular to olfaction.

While the relationship between visual stimuli is relatively easy to understand -- differences in color can be easily described as differences in the wavelength of light -- no such system exists to describe how two odors relate to each other. Instead, the researchers sought to describe scents according to how they activated neurons in the brain.

Using fluorescent proteins, they created images that show how each of 14 different odors stimulated neurons in the olfactory bulb. What they found, Murthy said, was that the ability of mice to identify a particular smell was markedly diminished if background smells activated the same neurons as the target odor.

"Each odor gives rise to a particular spatial pattern of neural responses," Murthy said. "When the spatial pattern of the background odors overlapped with the target odor, the mice did much more poorly at detecting the target. Therefore, the difficulty of picking out a particular smell among a jumble of other odors, depends on how much the background interferes with your target smell. So, we were able to give a neural explanation for how well you can solve the cocktail party problem.

"This study is interesting because it first shows that smells are not always perceived as one whole object -- they can be broken down into their pieces," he added. "This is perhaps not a surprise -- there are in fact coffee or wine specialists that can detect faint whiffs of particular elements within the complex mixture of flavors in each coffee or wine. But by doing these studies in mice, we can now get a better understanding of how the brain does this. One can also imagine that understanding how this is done may also allow us to build artificial olfactory systems that can detect specific chemicals in the air that are buried amidst a plethora of other odors."


Sight, or vision, is the ability of the eyes to perceive images of visible light. The structure of the eye is key in how the eye works. Light enters the eye through the pupil and is focused through the lens onto the retina on the back of the eye. Two types of photoreceptors, called cones and rods, detect this light and generate nerve impulses which are sent to the brain via the optic nerve. Rods are sensitive to the brightness of light, while cones detect colors. These receptors vary the duration and intensity of impulses to relate the color, hue, and brightness of perceived light. Defects of the photoreceptors can lead to conditions such as color blindness or, in extreme cases, complete blindness.


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