Meet the new taste ... same as the old taste?

Apart from being able to collect on drinks owed from years earlier, a major benefit of attending the same scientific conference over a prolonged period (in my case, AChemS) is to be able to observe the ebb and flow of controversies within the field. Two decades ago, taste scientists were still debating the validity of umami as a distinct primary/basic taste quality in the same way that sweetness, sourness, saltiness and bitterness had been accepted for generations. 

Nowadays, of course, chefs in newspapers and magazines use the term as if it has always been part of their repertoire. Importantly, apart from a few recalcitrant researchers who remain skeptical, the scientific issue has now been put to rest. This has a lot to do with the discovery of a specific taste receptor for glutamate – that is, a protein in taste buds that binds to this prototypical umami tastant and initiates the nerve signal to the brain that we interpret as umami (see also: Full of MSG).

To some extent, initial reluctance to accept umami as a primary taste quality was understandable. We had known about the four basic tastes forever – who was this taste come lately? A new candidate must fill all sorts of criteria, including a unique quality, a unique means of transduction (the receptor - see above), and a distinct adaptive reason why we would have evolved to respond to umami substances. To greater and lesser degrees, all of these criteria have been met. 

However, when debates over umami were still in full force (ok, that sounds dramatic – there were no actual fights in the conference bar), the issue of whether there were actually taste primaries at all was still relevant. One argument was that our language for taste, restricted as it was to a small number of qualities, essentially forced use to categorise qualities as one taste or another, when perhaps they actually fell into an intermediate state [1]. What, for example, if umami was simply a quality “midway” between salty and sweet?

It is not clear what happened to this line of argument; it seemed to just fade away with the years. Like most scientists, those working on tastes are practical and it’s hard to study basic taste qualities if you can’t even agree that such things exist. Add to this the discovery of receptor mechanisms and a clear adaptive argument and these days we are all believers in basic tastes. And recent years have seen an absolute plethora of potential candidates – fat, calcium, starch, and the newest kid on the block, kokumi. 

The case for fat being a primary taste is reasonably strong. Sensory scientist and nutritionist Rick Mattes showed almost 20 years ago that we respond to fat – but not fat substitutes - in the mouth with a rise in fats (triglycerides) in the blood, even in the absence of being able to tell the real and fake fats apart when in the mouth [2]. In the intervening period, the evidence for fat taste has grown, and Keast and Costanzo [3] assemble this evidence in a just published review in the journal Flavour.

In the same issue of this journal, there are two papers that demonstrate the impact of particular peptides (molecules made up of amino acids, like proteins) on the flavour properties of different foods (reduced-fat peanut butter and chicken consommé). In this case, the (desperately in-need of a rename) peptide was γ-glutamyl-valyl-glycine, the effect of which was in both foods to enhance certain sensory properties. For peanut butter, it was a rise in thick flavour, aftertaste, and oiliness, and for the consommé, umami, mouth-filling sensation and mouth-coating were increased. Both of these papers argued that these effects resulted from an increase in a putative new quality, kokumi.

Actually, kokumi is not very new but, just like umami, it is taking its time to filter out of scientific interest based primarily in Japan. Research to date has hedged its bets about what to call kokumi. It has been referred to as a taste quality, a flavour and a flavour enhancer. This is also very reminiscent of early discussions about umami, before the weight of evidence came down on the side of a distinct primary taste. Earlier studies of kokumi showed that another peptide, glutathione, increased perceptions of continuity (duration), mouthfulness and thickness in foods. It did not affect the intensity of other tastes except umami, with which it seemed to synergise [4]. Like glutamate, too, kokumi peptides are naturally present in foods, which suggests their importance in flavour.

It is already very well known that other molecules called nucleotides combine with glutamate to increase umami taste. Are kokumi peptides simply boosting umami taste as well? And does it matter whether we call kokumi a taste or a flavour or a flavour enhancer? From a science point of view this is, of course, crucial – the taste system appears based on primary qualities, as noted above, and we should be cautious before adding new tastes to the list. At the very least, a new taste quality means an evolutionary path that has produced specific in-built responses in us and perhaps many other mammals to a quality to help us survive. That is, kokumi might be a primary taste, but we really need to know why?

It is just as tricky if we start to use kokumi as a description of a perception. We have taken on umami with enthusiasm, but there is an argument that in English, the term savoury served the same purpose. We can accept the new term however given that we recognize its fundamental taste status. But without this for kokumi, we need to be wary that this term is not agglomerating a number of quite distinct perceptions and merging them under one umbrella. To take an example, if kokumi is defined as a quality of continuity, mouthfulness and thickness, then if thickness is increased or decreased independently of the other two properties, is kokumi affected? In essence, the risk is that we loose information – we start talking about one property, when we should be talking about three.

None of these issues will determine whether or not future research helps us define kokumi more precisely. But in the meantime, it is worth paying attention to the message advanced regarding umami - that our language could be instrumental in influencing how we actually perceived the qualities that we taste in foods.

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1. O'Mahony, M. and R. Ishii, The Umami taste concept: Implications for the dogma of four basic tastes, in Umami: A Basic Taste, Y. Kawamura and M.R. Kare, Editors. 1987, Marcel Dekker, Inc.: New York. p. 75-93.

2. Mattes, R.D., Oral fat exposure alters postprandial lipid metabolism in humans. Am. J. Clin. Nutr., 1996. 63: p. 911-917.

3. Keast, R. and A. Costanzo, Is fat the sixth taste primary? Evidence and implications. Flavour, 2015. 4: p. 5.

4.     Ueda, Y., et al., Flavour characteristics of glutathione in raw and cooked foodstuffs. Biosci. Biotech. Biochem., 1997. 61(12): p. 1977-1980.

Unaware eating

When was your last memorable meal: last anniversary, birthday, or other special occasion? What about dinner last night at home? While you may actually be able to remember what you ate last night, the details about quantities and how much time you spent eating are likely to be sketchy at best. To a large extent, this is due to the fact that we seldom pay much attention to eating, except perhaps when in a special restaurant, when awareness is part of both the enjoyment and our motivation to maintain a vivid picture of where all that money went. The term “mindless eating” is often used to convey the way in which we consume foods without monitoring the amounts, and has been linked to overconsumption of snack foods and, of course, obesity.

Going back a century or so, Ivan Pavlov was inducing his canine “research participants” to salivate in response to sounds that they had learnt meant that the evening meal was probably on the way. He termed these responses “psychic secretions” to indicate the role of a mental – that is, psychological – process linking the new signal (the sound) to the original stimulus for salivation (the food), rather than an automatic, in-built reflex. At least in humans, it is generally considered that a such learned connections must be conscious – we are not going to salivate to the sound of a bell unless we know that it signals dinner. However, awareness is not an “all or none” phenomenon and it is often the case that, once learning has taken place, we do not pay attention to those cues that induce a desire to eat or that influence what we want to eat or how much we eat.

Two adjacent papers in a recent issue of the journal Appetite show, in quite different ways, how eating can come under the control of cues outside of our immediate awareness. Feel like you are a free agent in your food choices? That you make these choices according to your appetites, or values, or needs? Gaillet-Torrent and colleagues [1] show that pre-exposure to an ambient pear odour induced their participants to more frequently select a fruit-based dessert for their lunch than those who were not exposed to this odour. These researchers argue that the pear odour ‘primed’ a later food choice that was consistent with the odour quality, namely fruit. Consistent with this, there was no impact of the odour on other, non-fruity lunch courses.

Is this a surprise? After all, the smell of chicken roasting obviously influences our desire to consume that chicken rather than, say, a tuna salad. However, the key here is awareness. The odour exposed group showed no indication on questioning that they were consciously aware of the pear odour that had been present in the room in which they waited for the experiment to commence. And the effect was not merely a slight bias towards the fruit dessert. The control group, not exposed to any odour and hence able to provide a measure of the relative attractiveness of the fruit dessert, choose the alternative dessert – a brownie – by a margin of 3 to 1. So the priming odour not only pushed choice towards one dessert but substantially away from one that might have been chosen otherwise.

A tendency to overeat while paying attention to television is well-known as a prototypical example of mindless eating. But it has not been clear why this occurs. Does an engaging TV program simply distract us from actively monitoring what we eat? Lucy Braude and Dick Stevenson [2] studied this phenomenon, asking whether the increased intake was a function of TV interfering with either (or both) the hedonic changes that occur during eating (a decline in liking known as sensory-specific satiety) or the ability to pay attention to our internal cues signaling reduction of hunger or increasing fullness.

The study asked participants to consume either a single snack food or a variety of different snack foods while either watching TV or not. Replicating the already established findings that both watching TV and food variety produced increased energy intake, this study also showed that liking decreased for the food or foods eaten – the effect of sensory-specific satiety (SSS). However, the most interesting aspect of the results was that while, as expected, eating a single food results in a decrease in liking for that food, this decrease only occurred in the no TV condition. In other words, watching TV eliminated the SSS that we would expect to occur. Eating a variety of snack foods, which produces less SSS in any case, was, in contrast, unaffected by TV watching.

While hunger and fullness ratings did not change due to watching TV, intake (as mentioned above) did. Essentially, this means that greater amounts of snack food were consumed while watching TV to produce the same ratings of fullness and hunger as those who did not watch TV. Both this finding and the effect of TV on SSS are interpreted by these authors as reflecting a disruption of our largely automatic monitoring of both sensory pleasure and cues for hunger. Thus, those bits of the mind that watch what we eat are largely absent: true mindless eating.

Both of these studies shed light on the hidden influences that shape what we eat. Overconsumption is a major concern among many populations and the failure of weight-loss diets to work in the long term is well established. It is recognized that part of the problem is that we are constantly exposed to cues (odours, sights, and even sounds) that signal foods and drinks, especially those high in fat or sugar, making it difficult to resist the associated conditioned impulses to eat [3]. Demonstrating that substantial influence over what and how much we consume can be exerted by cues that evade awareness only emphasizes how difficult the process of exerting control over food intake can be. Conversely, of course, active attention to eating ought to be a means of regaining control. The problem is that lifestyles in many affluent countries work against this. It is no coincidence, I think, that my colleagues in France, where for an affluent country there is relatively low levels of obesity, sit down to eat a substantial meal twice a day (see: http://prescotttastematters.blogspot.nl/2012/12/le-topic-du-jour-gout-qui-importe.html). For them, snacking on the run or while watching TV has been relatively rare.

                                                                                                                       

1.  Gaillet-Torrent, M., et al., Impact of a non-attentively perceived odour on subsequent food choices. Appetite, 2014. 76: p. 17-22.

2.  Braude, L. and R.J. Stevenson, Watching television while eating increases energy intake. Examining the mechanisms in female participants. Appetite, 2014. 76: p. 9-16.

3. Ferriday, D. and J.M. Brunstrom, How does food-cue exposure lead to larger meal sizes? Brit. J. Nutr., 2008. 100: p. 1325-1332.

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.