Supertaste me!

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.

Flavour terroirism

It is difficult enough attempting to define the key influences underlying any flavour (see, for example, TasteMatters, July 2012: Driving a better tomato), but wine seems to be in a category all its own. Wine is chemically complex, but so are many foods and beverages we consume each day. What really sets wine apart (although some products such as cheese do come close) is its immersion in an artisanal mystique. The craft of winemaking resists scientific analysis, and this is particularly true of Old-World wines. After all, if winemaking was ‘just’ science, then any wine course graduate could plant vines in a paddock somewhere and 3 years later – hey presto! – Chateau Cheval Blanc ’47!

What really sets wine making apart is, of course, the land. In attempting to understand what makes a good cornflake, debates about the role of the soil in which the corn was grown are relatively uncommon. But the idea of terroir is now so entrenched in our appreciation of wine that its influence is taken for granted. The characteristic qualities of the land – the amount of clay or minerals in the soil or the local micro-climate – are seen as crucial to understanding not just the ripening and health of the grapes used in the wine, but the flavour of the wine itself. In essence, the wine flavor becomes an expression of the soil. This is sometimes taken to extremes, with the flinty character of a white wine being derived from the flinty soil, for example.

Beyond the romance of the idea of terroir, there are some practical reasons for linking the land to the flavour of the wines it produces. Thus, accepting this link means accepting too that wines from one region will in most cases taste quite different from those of another region, even if all other factors – grape variety, wine maker, storage type and so on – are kept constant. This is important, both for marketing purposes and for establishing that certain flavour characteristics are typical of a region and hence deserving of protected status. In turn, the idea of terroir underpins the rationale for using organic, or even the slightly whacky biodynamic, practices. It means too that the final say in what a wine tastes like cannot be left to consumer demands or market forces since the winemaker operates under constraints of the soil, and the soil determines what the wine ought to taste like.

Some of the attempts to study terroir have provided little support that this is an especially large contributor to flavour. In a descriptive sensory analysis of German Riesling wines from a number of wine estates, Fischer et al. [1] noted the huge variation in sensory properties among wines from the same vineyard, suggesting that terroir was a relatively minor influence on flavour, compared to the major influence caused by vintage and wine estate. They cite another study published in German by Wahl & Patzwald (1997) in which the researchers went to the effort of transplanting seven different soil types to the same vineyard to study the impact of soil type on wine composition and sensory quality of Silvaner wines. They reported no significant impact on wine ̄flavour of the soil type, beyond different grape yields.

In fact, a scientific analysis of the impact of terroir has proven difficult. Not only do soil types and climate vary with geography, but of course so do other factors including such things as the location of the vines relative to drainage and sunshine. Even when two locations have a winemaker in common, the entrenched belief by the winemaker in the influence of terroir may be an explicit or implicit source of handling the grapes or the wine in different ways.

Recently, Cadot et al. [2] surveyed wine producers from the Anjou region of the Loire in France to determine their concept of wine (flavour) typicality and how they thought that it related to terroir. Not surprisingly, for these producers, the main characteristic that explained both typicality of the wine and its flavour was the terroir. Soil and climate characteristics were important for 93% of the wine producers (compared to 65% for wine-making practices and 5% for harvest quality). The winemakers’ judgments of what constituted a typical wine of this region (and hence what was the main influence of terroir) were sensory attributes such as colour intensity, red fruits, and soft tannins.

In contrast, a descriptive sensory evaluation of the region’s wine produced a profile of the ‘perceptual typicality’ of the wines. Here, visual descriptors, spiciness and astringency, but not red fruits or soft tannins, were important. Moreover, when it came to those factors that distinguished the more prestigious style (Anjou-Villages Brissac) from a more quaffing variety (Anjou Rouge), only those technical factors under direct control of the winemaker - maturation time, vatting time, harvest date, and proportion of the Cabernet Franc grape – were influential.

Similar conclusions were drawn from a study undertaken some years ago by one of my students at the University of Otago, Sara Springhall [3]. Sara asked 27 experts/semi-expert wine tasters (all either teaching enology, undertaking enology courses, or members of wine clubs) to taste 13 Chardonnay wines sourced from three distinct regions in New Zealand - Hawkes Bay, Marlborough and Central Otago. According to http://www.winesofnz.com, these regions are characterized respectively as having (a) high sunshine hours and variety of soil types; (b) Lots of sun, cool nights, low autumn rains and free draining alluvial soils; and (c) hot, dry summers, snowy winters, and soil structures that are very different to those of New Zealand's other regions, with heavy mineral deposits in silt loams.

The tasters were asked to sort and group the wines based on similarity of flavour and then provide descriptors for the most prominent sensory characteristics. The sorting data were analyzed by using the number of times that wines were grouped together as a measure of their “distance” from one another. This allowed the data to be represented as a multidimensional map, which showed both substantial overlap between regions (Central Otago and Marlborough) as well as clear separation (Central Otago and Hawkes Bay). The axes of the map (essentially, North-South vs. East-West) were shown to be related strongly to the common descriptors for the wines. The major axis, the one along which the wines differed most, was found to be associated positively with the woody and caramel attributes of the wines, and negatively with the wines’ sourness. Variations in wine citrus flavours were associated with the secondary axis.

These axes were also strongly associated with the chemical characteristics of the wines. In particular, alcohol and sugar content varied positively with the main axis of the map, while pH correlated negatively and volatile acidity positively with the minor axis.

What all these data mean is that both sensory attributes and chemical characteristics underpinned the ways in which the wines were sorted. However, since the wines grouped according to geographical regions were not strongly aligned to either the sensory or chemical dimensions on which these wine experts sorted the wines, the data suggest that the most important influences on the flavor of these wines occurred during winemaking. Woody and caramel qualities, for example, generally originate as a result of contact with oak during fermentation and maturation, while variations in citrus flavours, alcohol and sugar content are linked closely to when the grapes are harvested.

A failure to reveal the impact of terroir does not mean that there aren’t better or worse soils or microclimates in which to grow grapes that make good wines. But it does mean that we ought – for the moment – to be sceptical about wine producers’ claims that wine flavours are to any great extent a product of the soil. Alternatively, you can accept the claims, because of the romance of the idea …. but to be consistent, it’s probably a good idea to start asking your bartender about the soil in which the hops were grown next time you order a beer.

                                                                                                                       

1.         Fischer, U., D. Roth, and M. Christmann, The impact of geographic origin, vintage and wine estate on sensory properties of Vitis vinifera cv. Riesling wines. Food Qual Pref, 1999. 10: p. 281-288.

2.         Cadot, Y., et al., Characterisation of typicality for wines related to terroir by conceptual and by perceptual representations. An application to red wines from the Loire Valley. Food Qual Pref, 2012. 24: p. 48-58.

3.         Springhall, S., et al. Multidimensional sorting applied to understanding flavour variations in Chardonnay wines in 5th Australasian Association of Chemosensory Science Annual Scientific Meeting. 2002. Heron Island.

How sweet it is ... or is it?

As a perception that is universally positive, sweetness is unique. Moreover, sweetness is typically liked outside of its usual context of a food or drink. This is in contrast to all other sensory signals. Salt is great when attached to chips, red is terrific when painted onto a Ferrari, and Gb peerless when part of your favourite tune. But out of context, they loose much of their appeal. Not so, sweet taste. In an earlier discussion (see March, 2012: The genetics of cat food), I talked about the ability of sweet tastes to elicit smiles from human and animal newborns reflecting the adaptive priority given to our ability to sense carbohydrates. 

Even if preference is always shown for sweetness, it, like every other sensory quality, are liked to greater and lesser extents, in part depending on the type of food or beverage. The role of learning here is obvious – even if we have taken sugar in our tea or coffee for decades, we can quickly learn to like less or even no sugar through repeated exposure. Hence, given different histories of food consumption, we would expect individuals to vary in how sweet foods or beverages should ideally be.

But what is less considered is whether there might be consistent patterns of individual differences in sweetness preferences that either can’t easily be explained by, or may even precede, food choices. Such patterns in bitterness perception and preference have become familiar in recent years through research on the compound 6-n-propylthiouracil or PROP (see for example [1]). Moreover, the idea of subsets of consumers similarly showing reliable differences in their liking for sweet tastes isn’t new. In 1970 [2], Rose-Marie Pangborn described patterns of variations in both sweet and salty taste preferences. Many of us would identify with the idea of having a “sweet tooth” and research on ‘sweet liking’ has indicated that this is around two thirds of us. Meanwhile, the remaining 30% are known as ‘sweet dislikers’, but this is a misnomer, since it really reflects a preference for sweetness per se that peaks at a lower optimal than it does for the sweet likers – not an actual dislike.

One approach to quantifying the sweet liker/disliker dimension has been to simply obtain the preferred sucrose concentration using a forced-choice procedure. While this is useful and reliable as a measure, it will miss differences in patterns of liking that become evident across a range of sucrose concentrations. Distinct groups of sweet likers and dislikers can also be segregated by patterns of increasing or decreasing preferences for the sweetness of sucrose concentrations. Typically, sweet likers show a monotonic increase as sweetness increases, reaching an asymptote at or close to the highest concentration. Dislikers, on the other hand, often show a steep decline that begins at relatively low concentrations. But it is also clear that some individuals fail to be especially impressed by sweetness at any concentration, instead showing a moderate liking that is insensitive to variations in concentration. These patterns, and the proportions of consumers contained in each, are yet to be definitive, and what’s missing in all the research is estimates of how people vary obtained from large population samples.

The important issue arising from categorizing consumers as sweet likers, dislikers or ‘indifferents’ is whether or not classification into groups predicts anything useful. After all, within a culture, there is more or less general agreement about optimal sweetness levels: fruit should be sweet, but beer shouldn’t.  Such ‘agreement’ can be seen as the consequence of cumulative experience of the typical products that are on the market. Nevertheless, given the prevailing sweetness levels of foods and drinks, do some consumers reject some products because they are just too sweet? In other words, does classifying consumers into sweet liking groups predict which of the available foods will be chosen? And if it does, what are the implications for public health?

As we stand, none of these questions can be answered with any certainty. But increasingly, there are research findings that point to a tentative conclusion that sweet liking might a key determinant of food preferences. Julie Mennella and her team recently evaluated their method of assessing sweet liking in a large sample of adults, adolescents and children [3]. Amongst other findings, they noted a significant positive association between preferred concentrations sucrose in solution and the sugar content of their favourite breakfast cereal.

Since our foods likes (and dislikes) are learned, the importance of the sweet liking dimension is also illustrated by findings that it can influence preference development. In this study [4], a novel odour was repeatedly paired in solution with the sweet taste of saccharin. This ought to produce a liking for the odour, since the “positive feelings” for the sweetness are transferred to the odour, a process known as evaluative conditioning. And so it did – but only for the sweet likers. Consistent with their classification as individuals who do not find sweetness especially rewarding, the sweet dislikers failed to develop a liking for the novel odour. Since evaluative conditioning is thought to be a very common mechanism for food preference development, the implications of such a finding are critical in helping us understand how food preferences – and hence food choices - can vary within populations.

Just one of the obvious next questions, given the energy provided by carbohydrate sweeteners, is whether or not sweet liking is a risk factor for obesity. Again, we need much larger samples of consumers from which ask this question, but Mennella’s study, plus another one from her lab, failed to find a relationship between body weight (BMI) and preferred sucrose concentration.

Unlike the case with PROP sensitivity, for which variations in specific genes are implicated, there is little strong evidence one way or another that sweet liking is determined by early food experiences, or genetics, or a combination of both.  However, Mennella also provides interesting data that bear on the question of the origin of the sweet liker dimension.  Her sample was analysed in terms of demographic factors including race, education, income and sex. Her black participants preferred a significantly higher sucrose concentration, as did those with a lower income. Boys preferred a higher concentration than girls, and children higher sweetness than adults. None of these findings rule out a role for taste genetics, but it is important to note that variations in sweet liking reflects hedonics - and not sensitivity to - sweetness. In other words, sweetness of equal intensity is liked differently. Mennella herself notes that black mothers often feed sugar water during infancy and suggests that this is one plausible origin of the greater liking for higher sweetness levels in this group.

If it seems premature to conclude that sweet liking will be an important predictor of food choices, the research to date nevertheless supports a view that this variable is worth further consideration. As with variations in PROP sensitivity and the recently reported thermal tasting, we don’t yet know what ultimate benefits measurement of sweet liking will bring, but understanding the impact of person-to-person differences in consumers’ patterns of sensory and hedonic responses is unlikely to be wasted effort. Sweet liking could be the factor that mediates the tortuous path between responses to products in consumer evaluations and what products consumers actually choose to eat.

                                                                                                                       

1.         Tepper, B.J., Nutritional Implications of Genetic Taste Variation: The Role of PROP Sensitivity and Other Taste Phenotypes. Annu. Rev. Nutr., 2008. 28: p. 367-388.

2.         Pangborn, R.M., Individual variation in affective responses to taste stimuli. Psychon. Sci., 1970. 21(2): p. 125-126.

3.         Mennella, J.A., et al., Evaluation of the Monell Forced-Choice, Paired-Comparison Tracking Procedure for Determining Sweet Taste Preferences across the Lifespan. Chem Senses, 2011. 36: p. 345-355.

4.         Yeomans, M.R., J. Prescott, and N.G. Gould, Individual differences in responses to tastes determine hedonic and perceptual changes to odours following odour/taste conditioning. Q. J. Exp. Psychol., 2009. 62(8): p. 1648-1664.