Humans use several kinds of information to decide what to eat, and the combination of experience and sensory evaluation helps us to choose whether to consume a particular food. If the sight, smell, and taste of the food are acceptable, and we see others enjoying it, we finish chewing and swallow it. Several senses combine to create the idea of food flavor in the brain. For example, a raw chili pepper has a crisp texture, an odor, a bitter and sour taste, and a chemesthetic ‘burn.’ Each of these sensory modalities is associated with a particular group of receptors: at least three subtypes of somatosensory receptors (touch, pain, and temperature), about 400 human odor receptors, which respond either singly or in combination at least five types of taste receptors [bitter, sour, sweet, salty, and umami (the savory experience associated with monosodium glutamate)], and several families of other receptors tuned to the irritating chemicals in foods, especially herbs and spices (for example, Eugenol found in cloves or Allicin found in garlic). The information from all these receptors are transmitted to the brain, where it is processed and integrated. Experience is a potent modifier of chemosensory perception, and persistent exposure to an odorant is enough to change sensitivity.
The combined senses of taste, smell and the common chemical sense merge to form what we call ‘flavor.’ People show marked differences in their ability to detect many flavors. Most of the genes identified to date encode receptors responsible for detecting tastes or odorants. Although the repertoire of receptors involved in taste perception is relatively small, with 25 bitter and only a few sweet and umami receptors, the number of odorant receptors is much larger, with about 400 functional receptors and another 600 potential odorant receptors predicted to be non-functional. Despite this, to date, there are only a few cases of odorant receptor variants that encode differences in the perception of odors: receptors for androstenone (musky), isovaleric acid (cheesy), cis-3-hexen-1-ol (grassy), and the urinary metabolites of asparagus. A genome-wide study also implicates genes other than olfactory receptors for some individual differences in perception. Although there are only a small number of examples reported to date, there may be many more genetic variants in odor and taste genes yet to be discovered.
Each person lives in a unique flavor world, and part of this difference lies in our genetic composition, especially within our sensory receptors. This idea is illustrated by bitter perception and bitter receptors. The bitter receptor family, TAS2, has approximately 25 receptors, found at three locations in the human genome. We say ‘approximately’ because bitter receptors have copy number variants, and it is currently unclear at what point a recently duplicated gene should be assigned a distinct name. This conundrum is more than a mere matter of record-keeping; the bitter receptor gene copy number is a source of biological variation and may affect perception, although this prospect has not yet been demonstrated empirically.
The first demonstration that genetic variants contribute to person-to-person differences in human taste perception was for the bitter receptor TAS2R38. It has been known since 1931 that some people are insensitive to the bitter compound phenylthiocarbamide (PTC), a chemical that was synthesized by Arthur Fox for making dyes. While he was working in his laboratory, Fox accidentally tasted the compound and found it bland, yet when his benchmate also accidentally tasted the compound, he found it very bitter. This observation contributed to the formation of a hypothesis, now widely accepted, that there is a family of bitter receptors, at least one of which is sensitive to this compound, but that is inactive in some people. In 2003, this hypothesis was tested using genetic linkage analysis. Relatives such as parents and children were assessed for their ability to taste PTC and for their pattern of DNA sharing. The genomic region most often shared by relatives with similar tasting ability was near the TAS2R38 gene, but this evidence in itself was insufficient to conclude that the TAS2R38 gene was responsible for this sensory trait. Genes encoding bitter taste receptors are physically clustered on chromosomes, and nearby DNA regions tend to be inherited together, so it was not clear whether TAS2R38 or a neighboring receptor was the responsible gene. This issue was resolved later, when individual bitter receptors were introduced into cells without taste receptors. Only those cells that contained the TAS2R38 gene responded to PTC. Moreover, cells containing naturally occurring genetic variants of the TAS2R38 gene from people who could not taste PTC were also unresponsive to this bitter compound. Together, these data showed that TAS2R38 and its variants explained the inability of some people to taste PTC at concentrations at which it is readily detectable to others. The inability to taste PTC as bitter can be considered a categorical trait (either people can taste it or they cannot), and can also be considered a quantitative trait, that is, as a continuum, but with most people falling at either end.
Although the genetics of taste perception has been dominated by the study of PTC and its effects, evidence is gradually accumulating that the ability (or inability) to perceive other bitter tastes is heritable. For example, identical twins, who have identical genetics, are more similar in their perception of bitter compounds (other than PTC) than are fraternal twins, who are no more similar genetically than siblings. A variant in a cluster of bitter receptors on chromosome 12 is associated with quinine perception, and the bitterness of some high-intensity sweeteners is associated with alleles within a cluster of bitter receptors on chromosome 12. These observations suggest that individual differences in bitter perception may be common, and related to genotype.
Bitterness is a part of human life in two ways, in food and in medicine. In general, humans tend to avoid bitter foods; in a study by Mattes, nearly half of people surveyed ate no bitter foods at all. When these subjects were asked to consume a bitter solution, they diluted it with water until the bitterness could no longer be detected. Other common methods to reduce bitterness include cooking, or the addition of salt or flavors, but bitterness is not an inevitable part of life for everyone. To illustrate this point, when we asked 8 people to rate 23 vegetables for bitterness intensity, we found that some people were insensitive to even the most bitter vegetables. Of course, people who are sensitive to the bitterness of a particular vegetable or other food can avoid eating it.
Beyond the receptor
Most of the known gene variations relating to perceptual differences in taste and smell are specific to a single receptor. It may be that receptor variation affects only the perception of its ligand or it may have broader effects due to brain rewiring (in response to missing input) or to the clustering of receptor variants (LD). Thus, more characterization of human perceptual differences in conjunction with genotype studies is needed. The reduced ability to detect a single compound (such as PTC) might be associated with a reduced ability to detect structurally unrelated bitter compounds or even other taste qualities.
Food-tasting business (Getting paid to eat)
The only thing that matters here is the food. Behind a door marked "Sensory Staff Only," a dozen or so professional tasters spend their days testing ingredients that end up in thousands of products around the world. They sniff, taste, feel and spit. All day long.
Painstakingly trained, these tasters can describe the difference between a "vine-y green" and a "fresh-cut grassy green," a roasted peanut note from an over-roasted one. They are 'sensory specialists" — and an increasingly mechanized, sophisticated and complex food industry rests on their taste buds. "You get paid to eat stuff," said Chaquita Johnson-Moore, a chef-turned-sensory panelist, during a recent break from a session tasting a nutritional drink. "People can't believe there are people who do this."
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