Although we commonly use the terms “flavor” and “taste” interchangeably, they mean two very different things. Taste refers to sensations that arise from taste buds in the mouth, which are the standard sweet, salty, sour, bitter and umani.
“Different people have different ways of defining flavor. Our flavor chemists define it as olfactory [or volatile compounds],” says Zata Vickers, a professor in the department of food science and nutrition at the University of Minnesota (St. Paul, Minn.). “Most sensory people define flavor as being a combination of taste plus odor or aroma and the trigeminal sensation.”
Flavors or food aromas are chemical combinations perceived behind the nasal passage by retronasal olfaction through the mouth or nose. Chicken, chocolate and butterscotch all are smells. There are no taste receptors on the tongue for those aromas, says Linda Bartoshuk, a professor in otolaryngology (diseases of the ear, nose and throat) at Yale's (New Haven, Conn.) department of surgery. During chewing and swallowing, the air in the mouth is forced up into the retronasal area, where olfactory receptors signal the brain to identify the smell.
No Taste without Smell“I see the most important development in flavor research as the linkage between smell and taste, and the discovery that smells can enhance taste intensity,” states Barry Green, a professor in otolaryngology in the department of surgery at Yale and a fellow at the John B. Pierce Laboratory (New Haven, Conn.). Traditionally, smell and taste were thought of as separate senses that interacted very little. “I think it is going to become more evident that we can modulate taste by the smell route,” infers Green.
Historically, scientists believed odors coming in through the nose or the mouth registered in the same way. Research by clinical psychologist Dana Small (Northwestern University, Chicago) has determined that odors are processed in different parts of the brain, depending on their entry point. “The brain is cued as to which way the odor came in,” explains Bartoshuk. For example, “If you are sniffing something from the outside world, it's not going to change--no matter what you've got in your mouth.”
Retronasal olfaction enhancement for taste only occurs for appropriate, congruent combinations. For example, sweet and pear juice would be fine. However, sweet will not enhance green grass because most people do not eat grass. This stems from the knowledge that humans are not born recognizing odors. Odors are learned by association. “You are not born liking chocolate. You have to learn that,” notes Bartoshuk.
“The most profound part of sensory learning occurs prior to four to six months of age,” informs Gary Beauchamp, director of the Monell Chemical Senses Center (Philadelphia). “Babies are exposed to what the mother consumes via flavors in her milk, which influences what they will like later.” Beauchamp suggests that one of the major factors that differentiates the ethnic groups in terms of their food preferences is due to these early experiences associated with very important emotional and nutritive states that the mother and infant enjoy together.
However, over the last 50 years, the culture has shifted a proportion of our population away from breast feeding to bottle feeding. “What the baby is getting is an extremely bland and absolutely constant set of flavors in [formula]. From a sensory point of view they are missing something,” says Beauchamp.
Vickers counts this as important for the food industry because our perceptions of liking foods are not based solely on chemical factors, but also on psychological and environmental influences. For many different reasons, people do not perceive smells the same and, as a result, they do not inhabit the same flavor worlds. Some people are anosmic (exhibiting the inability to detect odors) or have specific anosmias to certain odors. Bartoshuk also suggests that people who have had a lot of ear infections have taste damage, which means the world of taste and aroma is quite different for them.
Flavor in the GenesOn the other hand, “People inherently like sweet and hate bitter, although experience can serve to modulate and overcome some of this,” says Beauchamp.
Differences in the central nervous system serve to differentiate taste preferences even more. “The biggest event that has happened in the taste field in the last five years has been the discovery of some of the receptors for bitter, sweet and umami,” informs Beauchamp.
Bartoshuk finds a percentage of the population has the ability to taste the bitter compound PROP very strongly. She labels this ability as “supertasting,” meaning those individuals have an abundance of a particular receptor in the tongue that is sensitive to PROP and other bitter substances. People who cannot taste PROP or non-tasters, she hypothesizes, are missing a gene for that receptor. Tasters or people who taste PROP--but not strongly--are homozygous for the gene.
According to Bartoshuk, Asians, in general, have fewer non-tasters. “They experience taste (on average) more intensely than do Caucasians. This is probably true for Hispanic and African Americans as well.”
Upon this discovery, implications have associated supertasting with health. Supertasters tend to be thinner than nontasters. “I think (for supertasters) the world often tastes too intense. It's just too strong. There are a lot of things that they don't like,” says Bartoshuk.
There is an anatomical relationship between taste and the perception of fat in the mouth and the perception of burn in the mouth, she explains. The taste buds are contained in structures called fungiform papillae. Each taste bud is surrounded by pain fibers. Supertasters have many more fungiform papillae than medium tasters and non-tasters. Therefore, not only do they taste more intensely, they feel more pain and have a stronger oral touch sensation. This explains their ability to detect gradations of lipids in food.
The discovery of bitter receptors has created a hub of excitement in sensory circles. Subsequently, there have been advances in bitter blockers that many anticipate will lead to more effective ways of inhibiting bitterness. There are about 20-25 bitter receptors. A bitter compound may influence only one bitter receptor, whereas there may be other compounds to which many receptors respond.
Many chefs and food formulation scientists have known for years that sodium is a common compound that reduces bitterness and enhances flavor by knocking out many bitter receptors at once. Beauchamp has written several papers with Paul Breslin (Monell Center, Philadelphia) on the subject. They have found that all types of sodium salts work well particularly with bitter vegetables and, though it works for many bitter compounds, it does not inhibit all of them. “There are ways to add sodium salts to foods and not make them salty,” states Beauchamp. However, if saltiness is a concern, sodium gluconate is a less salient alternative.
Many bitter inhibitors interfere with transduction by inhibiting the taste compound from properly stimulating the receptor. Sodium's bitter-blocking tendencies suggest there may be other compounds that would act similarly. “We think sodium acts at the receptor level. Using the new tools of molecular biology, hopefully, somebody may discover [more] bitter blockers,” says Beauchamp.
One U.S. biotechnology company founded by Robert Margolskee, a pioneering researcher, got a jump ahead of the pack. His was the first company to use molecular biology to successfully identify compounds that decrease the perception of bitterness caused by bitter-tasting molecules. Their patent for the bitter-blocker was made available by the U.S. Patent and Trademark Office in January 2003.
Sweetness and Sugar“It's much easier to block sweetness because there appear to be only one or two receptor systems involved,” says Green. Eventually, scientists would like to make sweeteners by figuring out how to intensify the response receptors have to a sweetener.
Sugar is a universal sweetener. Our sensory system is tuned very specifically to only the sugars that are useful to us. So, our sweet receptors respond to sucrose, glucose and fructose, and are less sensitive to other low-calorie/less-digestible sugars such as the polyols. “It's almost like artificial sweeteners are cardboard versions of sugar,” explains Bartoshuk. “It may well be that some of the sweeteners don't stimulate all of the sweet receptors.” Sugar molecules, however, have several places in their configuration that can stimulate receptors. Artificial sweeteners usually have only one. This explains, for example, why substituting an artificial sweetener for sugar will not taste the same for everyone. “There are genetic variations in taste, so you can't please everybody with the same product,” explains Bartoshuk.
Some speculate that one relatively new ingredient produced as a food grade substance could be used in weight control (the ingredient is known as 2-4 methoxy-phenoxy propionic acid, also known as lactisole). “It is a most remarkable compound that seems to inhibit almost every sweetener, whether saccharin, aspartame, sucrose or protein sweeteners. It knocks them all out,” informs Beauchamp. Commercially, the product is co-crystallized with sucrose and thus results in a product that is 99% sugar. “It looks like sugar, but when you put it in your mouth it tastes like sand.” It remains to be seen if it will really work in weight control, he adds.
The sweet-inhibiting compound means the co-crystallized product can be used as a bulking agent in savory products like soups or sauces when sweetness is not needed. “From a sensory point of view, prior to our discovery of a specific sweet receptor, this product (because it blocks all sweeteners) was amongst the best evidence that there was probably only a single [receptor],” says Beauchamp.
Sensory TestingIn past and present sensory testing, it was assumed that a group of trained judges would rate and respond to tastes and flavors similarly. “PROP shows that there is no reason to expect that everybody in a group is going to agree on the way something tastes,” says Vickers.
Vickers has studied how taste holds up over repeated eating of certain foods. Vickers observes that foods like bread, rice and milk often are very bland but have long-term acceptability. Many manufacturers may want to learn why these staple foods, which are not addictive (like caffeine), have long-term acceptance; such foods are often repeat purchases.
“Manufacturers will have to take [genetic variations] into account in setting up sensory panels,” advises Beauchamp. This is exciting news for many who hope to obtain more objective feedback than descriptive analysis, and who want to determine how people will respond to taste tests. They believe a time will come when the reaction to foods can be predicted by a person's genotype. “To me, there is some likelihood that is going to happen in the next few years,” predicts Beauchamp. Many food companies are trying to develop methods of measuring responses based on imaging parts of the brain.
“Functional Magnetic Resonance Imaging (FMRI) of brain activity [during tasting] has a great promise of finding something interesting,” he envisions. “Many companies are keeping an eye on this as a potential way of evaluating responses--without having to worry if people are telling the truth or even know the truth.”
It would be useful for sensory labs to have on their panels particularly sensitive individuals who are able to characterize and pick up more subtle parts of a flavor than other panelists.
In the next five years, Vickers predicts that research on repeated eating patterns will draw a wider audience of investigators, as history has determined the first two to three bites in a taste test can be very misleading. “What someone thinks after a couple of bites is quite different than what he thinks after eating an entire serving of something,” she explains. “Some foods do well in a taste test but, in real life, people get tired of them fast.”
Sidebar 1: Trigeminal SensationsThe John B. Pierce Laboratory's Barry Green coined the term “chemesthesis” to describe the cooling, warming or sensory irritation we perceive when consuming foods containing compounds such as menthol, capsaicin or alcohol. Such sensations are sensed via the trigeminal nerve in the front of the mouth and the glossopharyngeal nerve in the back of the mouth.
Scientists are anxious to find novel chemesthetic compounds since many people love the burning, stinging or tingling sensations they induce. They are an important part of flavor perception. Some of them (capsaicin in particular) can provide insights into how pain is perceived in the mouth.
More recently, Green's laboratory discovered a phenomenon that he calls “thermal taste.” Thermal tasters detect sweet taste on the tip of the tongue after the tongue, which is normally at 35º to 37ºC, has been cooled to 15º to 20ºC and then warmed rapidly. About half of the individuals tested perceived thermal sweetness. Green also was able to evoke thermal salty and sour tastes by cooling rather than warming the tongue, though these qualities are perceived by a smaller number of individuals. “We found that people who perceived thermal taste were more sensitive to all chemical tastes as well.”
He believes the ability to perceive thermal taste is related to supertasting. But, unlike with the bitter compound PROP, thermal tasting cuts across all taste qualities, suggesting that it is not due to a specific receptor, or tied to any single nerve. Green found all PROP tasters are thermal tasters, though not all thermal tasters taste PROP. “That shows that these are two independent mechanisms. One having to do with peripheral receptor expression and receptor density (PROP sensitivity), and the other probably having to do with a brain mechanism (thermal tasting),” says Green. “Thermal tasters also perceive vanillin (vanilla flavor), when it is sensed in the mouth, to be much stronger than did thermal non-tasters. That really points to a difference somewhere in the brain where taste and smell are integrated. Now the challenge is going to be to figure out what that brain mechanism is, and where in the brain it occurs.” Discovering this could also help in understanding how the intensity of taste and flavor are processed in the brain.
Sidebar 2: Smelling Memories“The beauty of humanity is the more different odors that we learn, the more we can acquire pleasure,” says Bartoshuk.
Since there are so many odor molecules, they tend to share certain receptors. Each receptor or group of receptors is tuned to a different set of features. All of the molecules that are built the same way project to the same place in the brain. Our brain creates a template in our memory of every different odor combination we encounter. “If odors are paired with calories, sweetness or good experiences, they become preferred. If they are paired with nausea, they become hated,” explains Bartoshuk. “If a product has a lot of calories and sugar, a person would learn to love it.”