Even if you are not one of them, it is a very appealing notion that some people have extra special sensory abilities. In discussing smell and taste, we tend to think that wine judges, expert food tasters, and perfumery ‘noses’ must have sensitivity far beyond our own in order to detect the incredible subtleties present in these products. This is certainly the case, at least to some extent. No one becomes a perfumer without a excellent sense of smell. But often the role of training, practice (as with any expertise), and motivation are not given enough credit in the creation of such super tasters and smellers.
By contrast, though, the existence of more general super-tasters has generated much interest recently to a great extent because they live among us, seemingly without effort on their part. Up to 25% of the population (although it does vary across cultures) can be shown to be highly sensitive to a range of food qualities, including basic tastes, texture and perhaps even overall flavour.
That taste sensitivity had a genetic basis first received wide acceptance with the discovery in 1930 that a substantial proportion of the population were taste-blind to the bitter compound phenylthiocarbamide (PTC)[1]. Later research using a related compound, 6-n-propylthiouracil (PROP), identified not only bitter non-tasters and tasters, but also a subgroup of the tasters who were exquisitely responsive to its (for them) traumatic bitterness [2].
Two things made these discoveries of interest to taste and food scientists. The first of these was research showing that PROP tasters, and especially super-tasters, found other tastes – whether in solution or in foods or beverages - also much more intense. Their experiences with chilli were hotter, their cream was creamier, their coffee more bitter, their cheese sharper, and their textures grittier, thicker and more viscous. They were more sensitive to differences or changes within foods [3]. An imminent finding of x-ray vision couldn’t be ruled out. Inevitably these differences in perception translated to differences in food preferences. And this piqued the interest of nutritionists because if you avoid green leafy vegetables such as spinach because of its bitterness, you then potentially increase your risk of cancers. But it became complex, because you probably drank less alcohol, thus reducing your risks.
The second issue of interest, particularly to taste scientists, was the identification of the TAS2R38 receptor, the gene that expressed it, T2R38, and structural variations within the gene that corresponded to the phenotype of variations in sensitivity to PROP and PTC. This is exciting because for the first time, the effect of substituting one amino acid for another in a taste receptor could be shown to have consequences in terms of health, via perception to preferences and then food choices.
The excitement over these potential links has generated a vast number of studies into PROP sensitivity and its perceptual and preference consequences. Many theses were completed. In part, the volume of research was due to the fact that PROP responses have been seductively easy to generate, typically using either an impregnated filter paper or a 10 ml solution, and a rating scale (although which rating scale has turned out to be crucial – see [4]). However, the kryptonite in the ointment is in plain sight. TAS2R38 is a receptor for PROP/PTC, but not for other bitter compounds, sweetness, sourness, texture or the burn of the capsaicin in chillies. To account for these relationships, something other mechanism is required. Fortunately, it was earlier shown that PROP intensity is highly correlated with the density of fungiform papillae on the tongue (FP; the bumps of the front surface of the tongue), and the taste buds that they contain. More FP means more intense tastes and, via the anatomical links that the trigeminal nerve has with taste buds, more intense burn and other mouthfeel sensations. At least, this is how it is assumed to work.
While this makes perfect sense – PROP intensity reflects other tastes/sensations via a common underlying cause, FP density – it does start to make the formerly simple measurement of PROP intensity relationships much more complex. Added to this complex mix, too, was the description of the genetics of other bitter taste receptors, including those that underlay the bitterness of coffee and grapefruit juice. And, of course, there is the as yet unknown genetics of variations in FP density.
At the recent 2013 Association for Chemoreception Sciences meeting in Huntington Beach, USA, John Hayes and colleagues [5] attempted to disentangle some of these relationships to provide a clearer picture of what PROP genetics does, and importantly does not, account for. By measuring the bitterness of quinine, the sweetness of sucrose, and the burn of capsaicin in addition to PROP bitterness and PROP genetics, Hayes was able to independently relate PROP phenotype and genotype to these taste and burn sensations.
As expected if a different underlying mechanism mediates the relationship between PROP and taste and oral sensations in general, the intensity of quinine, sucrose and capsaicin did vary with PROP intensity but did not vary with the genetic variations in receptor structure. In contrast, PROP bitterness directly reflected the genotype. Interestingly, though, how strong you rated capsaicin burn was a better predictor of quinine and sucrose intensity than was PROP bitterness. This is easy to understand when considering that the range of responses to PROP includes 20-25% who taste little or no bitterness – PROP non-tasters – as well as those who are medium- and super-tasters. In contrast, no one appears to be such a strict non-taster of quinine, sucrose, and capsaicin.
To return to the theme, where do we stand with the notion of super-tasting? PROP supertasters exist because they carry two alleles for tasting. But this is overlaid on the completely independent issue of whether or not an individual also has a high density of FP, and if they do, PROP will be even more intense. But this fact is of little interest as a way of predicting responses to foods, which never contain PROP. However, irrespective of your PROP taste genetics, you will still have more or fewer FP on your tongue. So, some people won’t taste PROP at all but will find quinine, or sucrose or capsaicin as very strong – in effect, you can be a PROP non-taster and a supertaster for all other oral sensations.
As for predicting food preference and intake and their consequences, it looks like PROP at best will only ever be an imperfect index. That, in and of itself, is not cause for concern. While it does make the job more complex, there is clearly a way forward to finding an even better index of food perceptions and preferences.
A sensory system turns out to be complex. Who knew?
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1. Fox, A.L., Six in ten "tasteblind" to bitter chemical. Science News Letter, 1931. 9: p. 249.
2. Bartoshuk, L.M., et al., PROP supertasters and the perception of sweetness and bitterness. Chem. Senses, 1992. 17: p. 594.
3. Prescott, J., et al., Responses of PROP taster groups to variations in sensory qualities within foods and beverages. Physiology & Behavior, 2004. 82(2-3): p. 459-469.
4. Bartoshuk, L.M., et al., Labelled scales (e.g. category, Likert, VAS) and invalid cross-group comparisons: what we have learned from genetic variation in taste. Food Quality and Preference, 2002. 14: p. 1125-138.
5. Hayes, J.E., et al., The primary qualities evoked by quinine, sucrose and capsaicin associate with propylthiouracil bitterness, but not TAS2R38 genotype, Paper presented at the Association for Chemoreception Sciences meeting, April, 2013: Huntington Beach, USA.
