Tuesday, 27 October 2015

Food for thought

According to a recent article in The Atlantic MonthlyTasting a Flavor That Doesn't Exist”, food companies are now implementing the findings of recent research on “phantom aromas” that somehow trick the brain into “manufacturing a taste”. While we can’t really expect non-technical magazines to necessarily convey the complexity of most research, this type of article nevertheless annoyingly perpetuates the idea that somehow the brain is an organ that “we” can easily confuse by feeding into it contradictory or incomplete information. This idea has a long history in the realm of visual illusions. Lines can seem longer than they are, rooms smaller, and the moon larger if we provide the brain with visual information in the right way or under certain constraints. What a silly old brain! 

But of course, our brains (that is, us) do not process sensory information out of context. Since the context may tell us something quite important about what we are sensing, why would they? The context may be environmental (Why is the moon so big? Well, things near the horizon tend to be further away) or it may come from the different sources of sensory information that arrive at the same time. This latter type of context provides the basis for the recent fascination with the multisensory nature of food, especially the findings that one sense can influence another, e.g. [1]. 

But this shouldn’t be surprising – the brain takes in and integrates whatever sources of information are needed to help us survive. In the food realm, this means identifying those things that are edible. Animal studies have even identified single nerve cells in the brain that receive information from touch, hearing and vision [2], because together such information might reliably tell us where something edible might be found.

But why do brains sometimes get it wrong? The answer is straightforward: if the information is incomplete or perhaps different senses contradict one another, the brain takes its best guess. A lot of the time, this guess relies on searching for, and identification of, patterns that are statistically more commonly experienced. Sometimes the best guess allows one sense to dominate others - visual dominance responsible for the illusion of the speaking ventriloquist’s dummy – because it is usually a more reliable source of information. At other times, such as when it’s easier to understand what someone is saying in a noisy environment if you can see their lips move, information from one sense enhances that from another. 

This is exactly the same phenomenon that we see in foods when we find that an aroma can enhance the intensity of a taste. Beginning in the late 1980s, Frank and colleagues [3] explored the phenomenon of sweet-smelling odours. From these studies, two things stood out. The first of these was that such odours seemed to be mostly those that were repeatedly experience together with sweet tastes in foods. Secondly, a sweet smell would only enhance a sweet taste and not, for example, a salty taste. They noted that the odour and taste needed to share a common property – the sweetness – whether tasted or smelled.

A smell with a taste? A phantom aroma indeed! Except …. we now need to consider the one other source of information that the brain uses. This comes from …. wait for it .... Inside our heads! The fact that sweet aromas are those experienced previously with sweet tastes (and salty aromas with salty tastes, and so on) is the key. In integrating information about food, the brain only really cares about the fact that together the tastes, aromas and tactile qualities uniquely identify something useful to us. Ah, this flavour is sweet, say you (and your brain), and hence the food is good to eat. Because we respond to the overall flavour, and not which sense provides what information, the odour is not distinguished from the sweetness of the food and this is encoded in memory – subsequently sniffing the odour can then activate this memory and … hey presto! … a phantom aroma (except what The Atlantic Monthly presumably meant was a phantom taste).*

What this all means is that flavours are assembled from information that comes from the mouth and nose, together with other information resulting from our experiences with foods and extracted, consciously or not, from memory. In other words, flavours are at least partly cognitive. Some knowledge of this cognitive landscape, traditionally largely ignored by food scientists and flavour chemists, is crucial to understand perception, whether of foods or of anything else. Cognitive psychology – or as it is known now by those who want to ask the same questions using large, expensive machines, cognitive neuroscience – has provided models and techniques for studying memory, attention, decision making and, increasingly, emotions.

The crucial role that cognitive processes play helping us understand food perceptions and preferences extends beyond explanations of flavour, and odour/taste interactions. Consider the activity of wine tasting. In describing the wine, the taster selectively attends to some qualities (berry-like, tobacco notes) by somehow mentally extracting them from the complex mixture that is wine flavour. These notes can be weak or intense, but the only way of knowing this is by comparing in memory what is perceived to what is typical. Then there is the decision that might have to be made: is this a good wine or a wine typical of the region.

In fact, we don’t have to rely on expert wine tasters to illustrate our reliance on cognitive processes in tasting. Everyday evaluations of food carried out within the food industry or in food research environments illustrate the point equally well. Sensory scientists routinely administer tests to trained panelists or consumers to evaluate foods perceptions or preferences – what sensory qualities does a food possess, how strong are these qualities, are two products different, or perhaps which product is liked more. Alternatively, we can see the administration of sensory tests to panelists or preference tests to consumers as requests to pay attention to product attributes, make decisions about differences or preferences, to analyse complex flavours into their elements, remember previous product experiences for comparison, or to describe emotions arising from product experiences. 

Even the way in which sensory tests are carried out is a function of what we know about cognitive biases. If you want people to attend to the overall flavour, don't ask them about different aspects of the flavour. In a group of products varying in intensity on an attribute, the first sample will be held in memory to act as the point of comparison for the other samples. Asking about the intensity of a single product will activate long-term memory for the intensity of what they usually consume. The answer to whether two products are the same or different will depend on the cognitive criterion for difference that the person adopts. And so on.

Cogito ergo … er, gustavero? **


*  Those wishing a more detailed account of this process should see one of the following reviews: [4-6]
** Even if you don’t know Latin, you should know Descartes – look it up!

1. Piqueras-Fiszman, B. and C. Spence, Sensory expectations based on product-extrinsic food cues: An interdisciplinary review of the empirical evidence and theoretical accounts. Food Qual Prefer, 2015. 40: 165-179.
2. Meredith, A. and B.E. Stein, Interactions among converging sensory inputs in the superior colliculus. Science, 1983. 221: 389-391.
3. Frank, R.A. and J. Byram, Taste-smell interactions are tastant and odorant dependent. Chemical Senses, 1988. 13(3): 445-455.
4. Prescott, J., Chemosensory learning and flavour: Perception, preference and intake. Physiol Behav, 2012. 107: 553-559.
5. Prescott, J. and R.J. Stevenson, Chemosensory Integration and the Perception of Flavor, in Handbook of Olfaction & Gustation: Modern Perspectives, R.L. Doty, Editor. 2015, John Wiley & Sons. 1008-1028.
6.  Stevenson, R.J. and R.A. Boakes, Sweet and sour smells: the acquisition of taste-like qualities by odors, in Handbook of Multisensory Processes, G. Calvert, C.B. Spence, and B. Stein, Editors. 2004, MIT Press: Cambridge. 69-83.

Wednesday, 23 September 2015

Complex odours and simple smells

I have just finished reading the introduction to a thesis. It was well-written and well-argued, a pleasure to read. Yet, what struck me was the fact that almost all of the literature referenced was published within the past 20 years. Now, for a student, 20 years ago may seem like ancient history, and of course there’s little point in going back too far if your field is molecular genetics. But the literature relevant to food or taste or odour preferences and perceptions is not like that: there’s a lot of psychology from mid-20th century (and sometimes earlier) onwards that is highly relevant.

In 1968, Zajonc published the initial paper on the phenomenon of mere exposure (ME) [1], the idea that liking for something increases as we are repeatedly exposed to it. Notice that I didn’t say “as we become more familiar with it”, and this is because Zajonc found that, strangely, familiarity did not increase for his rapidly-presented visual stimuli even though liking did. This is just one of the many interesting aspects of this phenomenon. Indeed, his research participants weren’t even aware of the nature of the stimulus to which they were exposed since the exposure time was typically about 0.004 seconds.

Fast-forward forty years, and my colleagues and I studied ME for odours, which turned out to be different form the ME seen with simple visual stimuli [2]. We were interested in effects of attention on development of odour liking following studies that showed if you ignored a novel visual stimulus, it became less liked. We were familiar too with research showing that emotions direct attention – you’ll pay increased attention to something that is either appealing or threatening, for example. In fact, we managed to replicate the findings of the studies using visual stimuli, showing that you can be repeatedly exposed to an odour without getting to like it if your attention is directed elsewhere. It is as if, by allocating attention away from a stimulus, you are also sending a message to the emotion processing bits of your brain about its lack of emotional relevance.

Now, a recent paper by Delplanque and colleagues has shown that the ME effect with odours seems to depend on just how pleasant the odour is judged prior to exposure [3]. Importantly, the effect is not linear – that is, if an odour is either very pleasant or very unpleasant to begin with, you get little impact of being repeatedly exposed to it. This is not difficult to explain with very pleasant odours as they may be reaching a sort of hedonic asymptote. Also, since these are odours with which we are likely to be familiar, they may be more easily identified – in essence, we’ve already made up our mind about how pleasant they are.

Initially very unpleasant odours remain unpleasant despite exposure quite probably because their “hedonic tone” is a signal of potential danger – a signal that would be diluted if ME could change liking for them. As an example, odours of rotting, faecal odours, or highly pungent odours may provide us with good adaptive information because of their ability to repel us.

So, odours can be seen to be quite different to meaningless visual stimuli, of the type used in Zajonc’s studies, because unless they are highly novel, there is already some degree of pleasantness built in. Moreover, complete odour novelty is hard to find since the notes in complex odours may be reminiscent of other, better known smells. It is telling, for example, that the odour that showed the greatest change in the Delplanque study was the allegedly “no odour” control.

Familiarity is well known to be associated with increasing pleasantness in odours, but only if we exclude those odour that are highly unpleasant, for which the association breaks down. If for a moment, we do exclude these, does that mean that we like odours because they are more familiar? Is this what ME is achieving for odours?

These questions bring me to the older paper that I really wanted to discuss, namely Berlyne’s studies of novelty, liking and stimulus complexity [4]. Like Zajonc, Berlyne used repeatedly presented visual stimuli and he initially found that lower familiarity was associated with high pleasantness. He realized, though, that he was failing to consider the complexity of that stimulus. To take account of this, he proposed that increased complexity – and/or increased novelty – produces high arousal, which is unpleasant. So, if we reduce the complexity and/or novelty of a stimulus, it becomes paired with lower arousal, and hence produces increased feelings of pleasure/reward.

So how does this apply to odours? Surely, we get bored with simple food or other odours, while complex odours are interesting? Well, maybe. But what constitutes complexity as far as odours are concerned? Let’s take the example of wines, mainly because we are used to talking about them in complex terms. It is evident that highly complex wines – those with many different facets or odour notes – are valued highly. You don’t get 95 on the “100 point” scale (see: Overrated wines) by having a simple flavour. But ….. such points aren’t awarded for liking; they are awarded for appreciation, which is not the same thing at all. You and I can like a complex Burgundy an awful lot, even if the only term that we can apply to it is “wine flavour”.

In fact, and perhaps surprisingly, as we become more familiar with odours, we not only like them more, we also judge them as less complex [5]. But interestingly, familiarity is not associated with the number of notes that an odour might have – something that we might ordinarily think of (as in the case of wine, above) as a proxy for complexity.

So, attention, arousal, novelty, liking and complexity are all linked somehow, and if your brain is hurting, I can’t blame you – it’s all too complex! Try this. Zajonc proposed that exposed stimuli become like because of increased ease with which the brain can process them. Complexity may be synonymous with novelty simply because the brain takes longer or expends more effort in this processing. In terms of perception and behavior, something that is novel or surprising is, by definition, a violation of expectations that will involve increased (unpleasant) arousal and a direction of attention to stimulus features to assess if some action needs to be taken – perhaps there is danger, for example.

Consider this sequence next time you encounter something wrong or unexpected with, say, a drink. Arousal increases as you pay close attention to your drink: you sniff, repeatedly, attempting to identify the problem …. this process is, in a way, making features stand out and the drink odour seems more complex. You sniff again – hmmm, cat’s pee. That’s ok, then. You asked for a Riesling, but you were given a Sauvignon blanc. Arousal decreases, as familiarity and liking both increase, and you can pay attention to something else, like your meal.


1. Zajonc, R.B., Attitudinal effects of mere exposure. J. Pers. Soc. Psychol., 1968. 9(2): 1-27.
2. Prescott, J., H. Kim, and K.-O. Kim, Cognitive mediation of hedonic changes to odors following exposure. Chemosens. Percept., 2008. 1: 2-8.
3. Delplanque, S., G. Coppin, L. Bloesch, I. Cayeux, and D. Sander, The mere exposure effect depends on an odor's initial pleasantness. Frontiers in psychology, 2015. 6: 911.
4. Berlyne, D.E., Novelty, complexity, and hedonic value. Percept. Psychophys., 1970. 8: 279-286.
5. Sulmont, C., S. Issanchou, and E.P. Koster, Selection of odorants for memory tests on the basis of familiarity, perceived complexity, pleasantness, similarity and identification. Chemical Senses, 2002. 27: 307-317.

Tuesday, 18 August 2015

Death of the expert?

Are you a wine expert? No? A food expert, perhaps? No ….really? If, like me, you have a tendency to eat food more than once a day, you really should have been paying attention! But, attending or not, we all have a vast implicit store of knowledge about food, or at least the food that is typical of the culture within which we live. The average consumer can spot a change in one of their regular foods or drinks without very much effort. But what they can’t do is talk very much about that difference, since most of us lack even a basic food vocabulary. And trying to quantify the degree of difference say between two foods is even harder.

For food producers this is a problem. Maintaining the quality of foods can be a challenge when ingredient supplies or storage conditions or packing materials vary. Especially for those foods that are easily influenced by geography or weather or the health of the plant or animal – dairy, wine, olive oil, tea and so on – the expert taster has been a source of information that helps eliminate those samples with faults.

In addition, much of the sensory study of food flavours has been undertaken using experts – specifically trained panellists – whose training is aimed precisely at producing individuals who respond to sensory qualities in an analytical, non-hedonic, and highly sensitive manner [1]. In other words, they are meant to respond entirely unlike the typical consumer. Panellists who are well trained tend to produce data that are reliably reproducible. In contrast, consumers’ data tend to be highly variable, both within (for example, over time) and between individuals. These distinctions are sometimes described as contrasting objective with subjective data, with the implication that the former is intrinsically more desirable for understanding products.

But the trained panel’s apparent advantages come at a cost of reduced ecological validity - that is, application to the real world of consumers. The point of sensory analysis is ultimately to understand the basis of consumers’ decisions about products, which are intimately linked to hedonics. Thus, sensitive trained panellists may describe product differences that are either unperceived by consumers or, even if perceived, then unimportant in terms of their influence on product acceptability. Conversely, minor product differences could potentially exert major effects on food preferences. 

Linking measures of product sensory attributes to consumer preferences implies that trained panellists and consumers actually perceive product attributes in the same way.  But this is not necessarily true. We know that food flavours reflect the integration – or melding together – of taste, olfactory, and tactile information, and this integration means that these individual sensory attributes are melded together. But the whole approach of experts is to analyse, to break apart this synthesis, and with it the integration that produces the overall food flavour. As one example, this is evident when we asked consumers to be analytical and it reduces their liking for the flavour [2]. So, a major issue with consumers is not just that they are inconsistent and variable in their assessments of sensory properties, but that they also ‘see’ flavour as wholes rather than a collection of parts

As recently as the 1990s, it was accepted that the way to find out about consumer preferences was to ask them about liking – which they are clearly easily able to do – and to find out about the product by using trained experts, most often former consumers who’d had the hedonics knocked out of them. The small panels of tasters were experts because of their ability to focus on the minutiae of a product’s sensory qualities in a highly reproducible way. So, after 20, 40 or 80 hours of training, panelists can understand the difference between cohesiveness and fracturability, or between kerosene notes and acetone notes. When faced with such distinctions, the consumer merely looks bemused.

More recently, however, it has been seen as acceptable to ask consumers to distinguish among different products, either via sorting them into groups – which tends not to require a sensory vocabulary – or in the past few years identifying words that apply to whatever is being tasted methods do not require the development of an extensive vocabulary of relevant terms by the consumers themselves. These check-all-that-apply (CATA) techniques allow an insight into consumer perceptions of the similarities and differences between different samples or products.

The biggest change, though, appears to be emerging in the area of simply telling products apart, that is, discrimination. In an earlier discussion (see The highly discriminating consumer), I pointed out the increasing recognition being given to role of emotions in discrimination and how an emotional state might encourage sensitivity.  In turn, this derives from the different ways in which experts and consumers ‘see’ products.

The ability to discriminate between different tastes, flavours or foods is typically seen as a function of perceptual sensitivity derived from detailed analysis of their sensory properties. Hence, trained panels are recommended for such tasks. Conversely, consumers are thought to be relatively insensitive to the presence of, or variations in, sensory qualities, at least when compared to trained panelists, and there has been a reluctance to use consumers to undertake discrimination tests. This practice, however, relies on two assumptions that are increasingly being questioned. 

The first of these is that high levels of sensitivity to variations in sensory qualities is always important. In fact, trained panels may over-estimate the effect of variations on consumer preferences. Thus, measuring the concentration of trichloranisol (TCA; also known as cork taint) at which wine consumers would reject wine produced a much higher value that that given by wine tasters trained to be sensitive to TCA. In effect, wines rejected as tainted by the experts would have been perfectly acceptable to the majority of wine drinkers [3]. 

So, consumers may be entirely appropriate to use in discrimination tasks when making judgments about what other consumers might perceive in products. The question is then how to find differences between products that are actually relevant to consumers? That is, would consumers actually notice the difference and, if they did, would they actually care? In a recently published study, Kim and colleagues [4] used an affective consumer discrimination method. The essence of this test was to perform a standard test of discrimination known as the duo-trio test in which consumers were asked to match one of two unknown stimuli to a reference. In this case, though, the reference was established beforehand as being preferred (affective groups) or no such pre-test was performed (analytical group). They were able to demonstrate that for those consumers who were genuinely “affective discriminators” – that is, they showed a clear preference for the product used as a reference (as compared to non-discriminators) – their ability to match the samples was higher than for an analytical group, who did not undergo this preference pre-test.

Hence, rather than encouraging these consumers to ignore their preferences, actively utilizing these preferences produced superior discrimination. Of course, the presence of sub-groups of ‘affective discriminators’ sounds like unwanted variability of the sort that does not occur with experts. But we should be used to the idea of preference segments. Variability in consumer perceptions and preferences is currently a rapidly increasing area of research (see for example How sweet it is ... or is it?). The key is not to manage-out consumer variability by turning them into experts, but rather measure, identify and interpret the sources of the variability and ask what it tells us about food preference and choices in the population. Eliminating the impact of individual differences means that important variables that have a consistent impact on product perceptions and ultimately acceptability are ignored.

1. Prescott, J., J. Hayes, and N. Byrnes, Sensory Science, in The Encyclopedia of Agriculture and Food Systems., N.K.V. Alfen, Editor. 2014, Elsevier. pp. 80 - 101.
2. Prescott, J., S.M. Lee, and K.O. Kim, Analytic approaches to evaluation modify hedonic responses. Food Qual. Pref., 2011. 22(4): p. 391-393.
3. Prescott, J., et al., An estimate of the “consumer rejection threshold” for TCA in white wine. Food Qual. Pref., 2005. 16: p. 345-349.
4. Kim, M.-A., H.-M. Sim, and H.-S. Lee, Affective discrimination methodology: Determination and use of a consumer-relevant sensory difference for food quality maintenance. Food Research International, 2015. 70: p. 47-54.

Wednesday, 15 July 2015

Remembering to like

Most of us have no trouble naming the foods we like most, or indeed saying how much we like them. We may not use a rating scale to do this, but can still convey that we like chocolate cake “a lot” or even – perhaps to confuse non-English speakers – “an awful lot”. But making a general statement about liking for foods (or anything else) carries with it a couple of important assumptions. The first of these relates to context (doesn’t everything?): chocolate cake is great in the evening as a dessert, but perhaps less enjoyable first thing in the morning. This is why we can say that we like chocolate cake more than we do cornflakes, but at the same time also say that we prefer the cornflakes if we are talking about breakfast.

Secondly, though, and much less obviously, I could ask you about your love of chocolate cake in relation to past or current consumption of it. Thus, being hungry and faced with the prospect of your first bite of the slice in front of you, your answer is pretty obvious. Ok, then what about after your first bite? After your second bite? Third? Tenth? There is a well-known phenomenon called sensory-specific satiety (SSS) that kicks in once we start to eat. This refers to the fact that simple exposure to the sensory properties of a food – its taste, smell, texture – causes the food to become less liked. 

The effect of SSS isn’t enormous – we see no more than about a 20% reduction in liking if we measure it using rating scales. But it seems to be enough to motivate us to want to eat other foods that are available instead. Hence, the theory behind SSS is that it is an adaptation that helps maintain a varied diet. It is apparently also important in telling us when to stop eating. In studies that allowed participants to consume as much food as they wanted, the reason ‘I just got tired of eating that food’ was found to be just as common a reason for stopping eating as ‘feeling full’.

The distinction between these reasons is important: SSS is not about being full. Even chewing a food and not swallowing it will produce a decrease in liking for that food. Nor is it about nutrients – SSS occurs between foods that are similar in their flavours, but not between foods that are similar in nutrient content, but not flavour. So, we get bored with exposure to flavours and, indeed, pretty much any aspect of the food. Repeatedly eat red M&Ms and you’ll soon find the blue ones becoming strangely more appealing. Chew on something quite elastic for a while (say, a caramel) and you’ll start to long for a little bit of crispness. 

Becoming bored with eating the same food repeatedly is not news, of course. With the exception of breakfast, where many people trade off boredom for convenience, having the same meal over and over again on successive days leads to both loss of pleasure and reduced consumption. It is not a surprise that many diets use boredom as a way pushing the dieter to eat less and hence loose weight. Whether over the short term, within a meal (SSS) or over days or weeks (boredom/monotony), we eat more when confronted with foods that are varied in the flavours or textures.

But what if you didn’t recall what you’d recently eaten? In an earlier post (February, 2012: Remembrance of foods past), I wrote about the dramatic case of a severely amnesic man who ate three full lunches over the space of little more than an hour with apparent gusto. Clearly, in his case, the inability to recall what he’d eaten allowed him to maintain an interest in eating.

A recent study suggests that, for all of us, memory and attention play a crucial role in determining the degree of pleasure we derive from food, and hence how much we are likely to consume. Larson and colleagues [1] asked study participants to view pictures of either sweet or salty foods and rate how appetizing they thought those foods were. The idea was that in making these ratings, the participants were implicitly retrieving experience of those foods from memory. All participants were then asked to eat some salty peanuts and rate how enjoyable these were. If the participants had rated only 20 food pictures, then there was no effect on enjoyment of whether the pictures were of sweet or salty foods. However, after 60 pictures, those who had previously evaluated salty food pictures enjoyed the peanuts much less than those who evaluated sweet food pictures. What was producing this effect? In a subsequent study, the researchers included a condition in which participants were asked only to evaluate the brightness of the food picture and not how appetizing the food in the picture was, and this produced no such effect. As a result, the authors concluded that attending to the food sensory qualities (sweetness or saltiness) and retrieving information from memory was the source of this effect.

This study has a number of crucial implications. The first of these is that it provides evidence of the cross-modal nature of SSS, in that a picture, when allowed to activate prior experiences, could reduce enjoyment of an actual food flavour. We know that different aspects of foods (tastes, odours, textures) are bound together in memory as flavours, and that experiencing one aspect such as an odour can elicit the food’s taste qualities and any positive (sweet) or negative (bitter) qualities the taste may have [2]. This study points to a similar integration of visual and taste/odour food properties, also with hedonic consequences.

Secondly, these authors suggest that such effects may be in operation when we view advertising or other cues to foods, if these cues cause us to imagine the experience of the food based on our memory. That is, rehearsing the experience of a food in response to a cue will cause us to experience less enjoyment of the food itself and potentially to reduce the amount eaten. If true, this runs contrary to what we know about the effect of environmental cues, which are known to trigger eating [3]. This apparent paradox may be due to the fact that those affected by cues most are those suffering from hedonic hunger – that is, those who have deprived themselves of highly palatable foods. In their case, relative lack of recent memory for such foods may be the issue.

One final implication of this study concerns the phenomenon of mindless eating (see February, 2012:  Unaware eating), in which attention diverted from eating – for example, by watching TV – leads to higher consumption. The Larson study suggests that it is possible that paying attention to food while we are eating is necessary for SSS to occur because it requires access to prior memories. Perhaps we therefore experience less SSS with novel foods, for which we have no memories?

1. Larson, J.S., J.P. Redden, and R.S. Elder, Satiation from sensory simulation: Evaluating foods decreases enjoyment of similar foods. Journal of Consumer Psychology, 2014. 24(2): p. 188-194.
2. Prescott, J. and R.J. Stevenson, Chemosensory Integration and the Perception of Flavor., in Handbook of Olfaction & Gustation: Modern Perspectives., R.L. Doty, Editor. 2015. p. 1008-1028.
3. Fedoroff, I., J. Polivy, and C.P. Herman, The specificity of restrained versus unrestrained eaters’ responses to food cues: general desire to eat, or craving for the cued food? Appetite, 2003. 41: p. 7-13.

Saturday, 16 May 2015

Smelling like a dog

Pretty much everyone (excluding you and I, of course) agrees that, for humans, smell and taste are the minor senses. Read a psychology or physiology text and you’ll see what I mean – huge attention paid to vision, then hearing, then touch and then …. ok, I suppose we should tack on half a page about smell and a few lines on taste, the latter sometimes accompanied by the now discredited tongue map of taste perceptions. 

There’s no doubt that the human chemosenses are minor if judged by the amount of attention we have paid to them in the past. How else could we possibly explain that something so apparently fundamental as which parts of the tongue were sensitive to which tastes was transmitted through generations in error? And as recently as twenty years ago, quite a few taste scientists firmly believed that a total of four primary tastes was it. Now we can barely open a journal without falling over evidence for some new basic quality – fat, starch, umami, calcium, rubber. Ok, only my own data, rather hastily collected late one night point to the last one.

There is really nothing very unique about the chemosenses – accepted ideas are overturned in science all the time. But given the relative paucity of research in this area compared to other sensory systems, it would seem that there is a greater risk that chemosensory scientists will spend their time looking for new receptors or ion channels or orbito-frontal Christmas lights rather than going over old ground. Not many of us want to spend our time navel-gazing in front of the fire, sherry in hand, wondering what we actually know after all. Surely that’s the job of those Cartesians in the philosophy department on the floor below.

Which makes it all the more unusual and refreshing when someone does do this. At the recent Association for Chemoreception Sciences meeting (www.achems.org), Matthias Laska of Linköping University in Sweden was motivated to question something that we all know to be ‘true’ [1]. This is the almost self-evident fact that many mammals – the dog is the foremost example – are much more sensitive to smells than are humans. You believe this, don’t you? And the reason is ….. ? If you’d have asked me a month ago, I would have agreed with you, and I would no doubt have said that I’ll be able to dig up the data – just give me a day or two.

Laska did do the necessary olfactory archaeology and the evidence is, in fact, …. missing. In other words, he did not find even moderately strong evidence for human inferiority in olfactory sensitivity. Laska examined published detection threshold values (that is, the lowest concentration of a compound that can be detected) for humans and compared them with those from a variety of other mammals. The list included dogs, rats, mice, bats, pigs, primates, seals and otters – so at least some mammals who seem to spend their days snuffling along the ground or twitching their noses. Because methods often vary, he took a quite conservative approach to the comparison, and only contrasted mean human threshold values (which would contain high as well as low values) with the lowest individual values reported for each other species.

The interest began at the onset of this research. While he estimated that thresholds for around 3300 odourants had been determined for humans, this number quickly fell for other mammals: a couple of hundred for different primates, 72 in the mouse, 45 in the rat, and a grand total of 15 odourants tested with dogs! In the comparison of actual threshold values with rats, bats, mice, monkey and otters, humans come out very well on the majority of odourants on which they and we have both been tested. And dogs? Well, they beat us on 10 of the 15 odourants for which there are data in common. They are mostly better than us at detecting carboxylic acids, for example, but much worse for alcohols and slightly worse for the few acetic esters on which we have been compared.

But the key issue here is not which compounds are higher or lower, but rather that the total number of odourants tested on dogs is so low, that even the slight advantage that dogs have in number of odourants for which they have tested as more sensitive is insufficient to be the basis of a supposed fact regarding dog superiority.

So, from where does this notion arise? Clearly, not from the scientific literature. My guess is from watching dog behavior. When your nose is that close to the ground, there’s an awful lot of information that you can gather that never reaches human noses, held aloft as they are. And let’s face it, dogs seem so very interested in all sorts of smells. As well, there are scientific data that, if not exactly proof, very strongly suggest this interpretation. As reported in an earlier blog (http://prescotttastematters.blogspot.it/2014/01/nothing-to-be-sniffed-at.html), a clever experiment by Porter and colleagues [2] showed in 2007 that humans, like dogs, could successfully track a scent in an open-air environment once their noses were down amongst the grass.

All that appears to be left to our canine companions is their much better ability to recognize odours, including those of other dogs and multiple humans, and also their skills at being great detectors of drugs, explosives and so on. But even shortly after birth, babies can recognize their mother’s underarm odour, distinguish it from that of other mothers of newborns [3], and the rest of us are generally able to distinguish scents from older and younger people [4].  

In contrast, though, humans are notoriously poor at odour identification. We can’t reliably even label foods that we eat regularly purely by their smell.  Unless, of course, we are trained. There is no possibility that you or I, if we aren’t trained, can identify by smell the origin of a wine, coffee, or tea, but there are those who have trained for extensive periods that can do just that. Moreover, I can smell two wines and know that they differ but not know in what way. In contrast, my wine enthusiast friends or trained wine judges can immediately spot the “sweaty saddle” note in one and the “stewed prunes” in another, and this ability also serves to help them recognize each wine next time they encounter it. Indeed, there are few odour discrimination, recognition or identification tasks that humans do not seem capable of performing given sufficient training. It’s possible that if we spent the same amount of time as dogs sniffing each other’s ….. you get the picture.

At this same recent AChemS, there was another intriguing presentation that gave us further reasons to be proud of our own sense of smell. The reasons for having two eyes or two ears are obvious, namely stereoscopic vision and binaural hearing, both of which help to locate the origins of shapes and sounds in our environment. But two nostrils? Data presented by Wang and Chen from Baylor College of Medicine in the USA [5] show that sniffing a pair of odourants with both nostrils enhances detectability of an odourant, relative to sniffing the same odourant with both nostrils. Hence, pulling in different smell information with each nostril stimulates a comparison process that “sharpens” the information available. Not such a minor sense after all.


1. Laska, M., Busting a myth: Humans are not generally less sensitive to odors than nonhuman mammals, in Association of Chemoreception Sciences. 2015: Bonita Springs, Fl, USA.
2. Porter, J., et al., Mechanisms of scent-tracking in humans. Nature Neurosci., 2007. 10(1): p. 27-29.
3. Cernoch, J.M. and R.H. Porter, Recognition of maternal axillary odors by infants. Child Dev, 1985. 56: p. 1593-1598.
4.    Mitro, S., et al., The smell of age: perception and discrimination of body odors of different ages. PLoS One, 2012. 7(5): p. e38110.
5.    Wang, J. and D. Chen, Stereo olfaction sharpens sense of smell in Association for Chemoreception Sciences. 2015: Bonita Springs, Fl, USA.

Monday, 23 March 2015

The hot topic

A few times every year, and without too much effort, you’ll find articles about the “hows and whys” of liking for hot (spicy) foods. Most recently, there was “Learning to Like Spicier Food” and “What’s Driving the Global Chili Pepper Craze” in the USA magazines The Atlantic and Forbes, respectively [both featured conveniently in Taste Matters on Flipboard]. Now, I’d like to be able to say that these particular articles actually revealed the drivers of the chilli ‘craze’ or the exact mechanisms of chilli liking. However, scientists such as Paul Rozin attempted to answer both questions some years ago [1], but without reaching an obvious conclusion, and the same questions continue to be raised.

Chilli is not like other foods. We don’t, for example, tend to see long articles exploring the conundrum of ice cream or chocolate preferences. It’s fat and it’s sugar and they’re delicious …. what’s the issue? Chilli, on the other hand, contains compounds that, when they aren’t being added to foods, are being sprayed into the faces of criminals, students and other less desirable members of society. Indeed, the effects in both cases are not entirely dissimilar: tearing, pain, facial flushing and excessive salivation. In neither case, does our body seem to be welcoming a dose of capsaicin.

And yet …. chilli, from its origins as a relatively localized crop in central and south America, has become the most used spice worldwide, and the second most added food ingredient after salt. We can’t all be masochists, surely?  There are two questions to answer. The first of these is why any individual would voluntarily eat something that was painful. Secondly, why have so many societies – Korea, Mexico, India, Vietnam, Thailand, amongst many others – made hot spices a central part of their cuisine’s flavour principle? These are not the same question, but on a different scale. In very many societies, sections of the population eat odd things – oysters, natto, Vegemite – without such delicacies taking over the world.

A variety of explanations have been proposed to attempt to explain the ubiquity of chilli and other pungent spices (pepper, ginger, cinnamon, mustard). These include that fact that the chilli plant is a fast growing source of vitamins, especially C and A; that it increases salivation and hence better digestion of foods; and that it promotes perspiration and thus heat loss in warm climates. These all sound plausible reasons, but they lack any evidence to support them [2].

There really is no obvious connection between the cuisines of those countries that have adopted chilli, nor any obvious difference between those that are and are not especially spicy. Why Korea and not Japan, for example? Had Japan not been resistant to foreign influences until relatively recently, would its soba be spicy? Geographical isolation aside, one clue to the widespread uptake of chilli may lie in the nature of dietary staples in chilli consuming countries. In Mexico, the cuisine is based on corn, in South-East and other parts of Asia, rice – both bland staples. It may be that chilli provides a way of providing interest and flavor impact to otherwise somewhat monotonous diets.

To be convincing though, this argument needs to apply elsewhere, rather than in just warm-climate cultures and their chilli-dominated cuisines. The expeditions of Christopher Columbus to central America are widely regarded as the source of the introduction of chilli to the rest of the world. It is useful to recall why he was sailing in the first place, and that is to find a convenient route to the sources of spices in Asia. In other words, spice-wise he really was very lucky. Europe as a whole proved a tough sell for chilli, but it is unclear why. It may have been the growing climate as much as the heat of the fruit itself. Certainly, chilli is still used in countries such as Spain, Italy and parts of central Europe. Hungary, of course, took the hot chilli and converted it in hot and mild versions of its national spice, paprika. Even Britain has a history of applying other pungent spices to foods with its traditional use of hot mustard and horseradish, and now curries from the subcontinent being the most popular restaurant foods. In the USA, hot sauces are more popular than ketchup. Hence, even in countries where dietary variety is not an issue, spiciness continues to increase.

It is well known that both flavour impact and complexity in foods are valued, making foods more pleasurable. Pungency in its many forms provides both. Adding bubbles to soft drinks, heat or cooling to foods, and bite to alcohol are all ways of engaging a completely distinct sensory system, mediated by the trigeminal nerves in the tongue, palate, nose and eyes. While this system contains pain fibres, it also responds to other tactile and temperature sensations that can enhance and amplify tastes and odours. 

Even when the burn of a hot dish becomes a little too impactful, and the body produces defensive responses such as salivation and flushing – we are aware that it is harmless. In high chilli-consuming countries, the spice is introduced to children gradually in the secure context of family meals, and there is therefore no anxiety associated with the pain. 

Another piece of evidence supporting idea that chilli is used for flavour impact is the change in response to the burn over time. Regular chilli eaters do show a reduction in burn intensity, but this is minor, and they are not immune from the burning sensations – rather, they actually learn to enjoy them, even if it seems too hot at first [1]. Interviews with both Americans and Mexicans who were asked why they liked chili showed that the majority referred to the piquancy, or the enhancement of flavor of food. Many interviewees claimed to be as sensitive as always to chili, but that they had come to like the hot sensation that they originally disliked. This is also consistent with a view that chilli consumption represents “benign masochism” – effectively getting enjoyment from pain because we know it is harmless. 

A variety of explanations have been put forward to explain the high levels of enthusiasm with which many of us take to very spicy food. It is no surprise to find that real chilli enthusiasts are also “sensation seekers” for all sorts of sensory stimulation. One enduring popular notion is that the burn of chilli releases the body’s own opiates, or endorphins, perhaps via the release of a neurotransmitter called Substance P (which is released in response to pain). The idea is that chilli eating – followed by a little squirt of the body’s own heroin – produces a feeling of well-being could reinforce a sort of spice addiction. Attempts to demonstrate this have yet to produce any evidence for such a mechanism, but it is at least consistent with findings that chilli burn is reduced by sweet tastes, and vice versa [3]. The pleasantness of sweet tastes are also thought to be mediated by endorphins, and it is well known that sweetness can increase tolerance for pain [4].

1. Rozin, P. and D. Schiller, The nature and acquisition of a preference for chili pepper by humans. Motiv. Emot., 1980. 4(1): p. 77-101.
2. Prescott, J. and R.J. Stevenson, Pungency in food perception and preference. Food Rev. Int., 1995. 11(4): p. 665-698.
3. Prescott, J., S. Allen, and L. Stephens, Interactions between oral chemical irritation, taste and temperature. Chem. Senses, 1993. 18(4): p. 389-404.
4.     Barr, R.G., et al., The response of crying newborns to sucrose: Is it a “Sweetness” effect? Physiol. Behav., 1999. 66(3): p. 409-417.

Tuesday, 17 February 2015

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.


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.

Thursday, 15 January 2015

Chemical coffee

During the past few weeks, I have been at the food coalface, engaging in much hands-on flavour research. These flavours, many of which are soluble in alcohol, provide some of the key memory elements for this time of year. While providing an ad-hoc chemical analysis of one such flavour (Result: 2 parts vodka, 1 part vermouth, a drop of orange peel oil) I read a brief web article on why coffee smells and tastes good, the first part of a series on the chemistry of coffee. There was nothing unique about this article – it explained, correctly, that coffee is a mixture of many hundreds of different compounds, the vast majority of which smell not at all like coffee itself.

But the article was very similar in many ways to other articles, especially in the media or on the internet, that seek to explain preferences for food flavours or other odours in terms of their chemistry. Another recent article addressed the question of preference for the odour of bacon in the same way. Now, of course, in some sense this must be true – the chemical compounds in coffee produce an odour that is highly liked by many people. Without this mixture of compounds there is nothing to like.

This article went on to talk about odour-activity values, which is the idea that some odours are detected at much lower quantities (often parts per billion, or even trillion, in air or solvent) than others and therefore can have a disproportionate impact on what is being smelled. Hence, if we look at coffee, acetone is present at a higher proportion than methyl furanol, but even so most coffees (those served in hospitals and government departments excepted) do not smell like nail-polish remover, and it is this latter component that comes closest to coffee odour. Incidentally, this compound tends to be pleasant smelling.

So far, so (mostly) good. But … there are three underlying assumptions to linking chemistry and preference that have to be examined. The first of these is that some compounds, by virtue of their low thresholds, have a disproportionate impact above threshold. This could be true, but is not necessarily true, because thresholds are poor predictors of psychophysical function – that is, the growth of intensity with increasing concentration. A compound with a low threshold may have a low psychophysical function, meaning that above threshold its intensity (and hence, presumably, impact) grows slowly, or it may have a high psychophysical function. Conversely, there is no theoretical reason why compounds cannot be influential below threshold – this has been shown, for example, by the impact of sub-threshold tastes on odour quality [1]. Indeed, inferring much of anything from odour thresholds is tricky – they can, and do, respond to repeated experience with the odour [2].

The second assumption behind the chemistry of preferences idea is that, in the sort of mixtures that constitute most foods and drinks, individual odour qualities can stick up their hand and be noticed. Thus, in something that smells good, the pleasant odours in that mixture somehow force their way into our attention against the background of those odours that are less pleasant … or vice versa. Certainly, in the latter case, it is easy enough to spoil a pleasant odour by dropping in something nasty.

Here, too, though the science doesn’t back up this assumption. David Laing showed in the early 1990s that in typical mixtures of very many odours, the influence of individual odours rapidly disappears as the mixture approaches and then passes four components. This is true even if you are highly familiar with each of the components and it is also true even if you are an odour expert such as a flavourist or perfumer [3]. The implications of this work are profound and the results of Laing’s well-replicated experiments have met with some resistance from those who aim is to link chemistry of individual components to the smell of the compound made from those components.

In essence, then, a complex, pleasant mixture such as coffee really needs to contain no compounds that are either coffee-like or pleasant. The odour quality of the mixture is a gestalt – both more than the sum of its parts and mostly unpredictable from the contributions of its parts. So, how does this new quality emerge? This is as yet unknown, and to a degree, mysterious. How, for example, can a wine made from sauvignon blanc grapes smell like white wine (a gestalt), sauvignon blanc wine (also a gestalt), and also have notes that smell like cut grass, asparagus, cat’s pee (seldom present … ok, perhaps in New Zealand wines) and so on?

We can say that it is not about chemical interactions in the mixture. Laing’s experiments were done with mixtures of odours not odourants. Hence, the odour mixture property is perceptual - that is, a product of olfactory processing and interpretation in the brain. Beyond that, we know little. But this fact does help to explain how cognitive processes such as attention can facilitate or hinder (to a real, but limited degree) the blending of different chemical compounds into a distinct odour quality [4].

The final assumption underlying a chemistry of preferences is that some odours are inherently liked or disliked and that this is a property of the odourant compound. Like other negatives, it is impossible to prove that this is not the case, at least with some odours. Cadaverine, the product of rotting flesh, could well be a candidate as it appears to be universally repulsive. However, we can show that likes and dislikes for the majority of odours, and certainly food odours, are learned through repeated exposure, or pairing with pleasant tastes/experiences or metabolically useful nutrients [5].

These mechanisms of associative learning are well known, but we really only need to look at variations between individuals or across cultures to see this convincingly demonstrated. Let’s take one example. Sheepmeat (lamb; mutton) is well-accepted in many Western countries, where children begin to be exposed to its odour and flavour from a very young age. In those countries in which eating sheep is not the norm, there is considerable resistance to adding it to the local cuisine. This isn’t based on sympathy for the cute little lambs, but rather the presence in their meat of the compounds skatole and octanoic acid, the former fecal and the latter like a barnyard on a hot day [6]. To you these compounds are the inviting aromas of a roast dinner; to many others, they are ….. somewhat less so. But importantly, with repeated exposure, even these demonstrably initially unpleasant odours can come to get the gastric juices flowing.

The chemically-independent origin of preferences allows too for cognitive influences, explaining how we can be encouraged to like or dislike foods because of what we know about their origin or composition. You are much more likely to find the presence of trichloranisole found in some wines as unpleasant if you know about its origin as a taint from cork. Alternatively, it may be that many people show a liking for the vile vegetable du jour, kale, merely because they feel that it has health benefits.

1.         Dalton, P., et al., The merging of the senses: integration of subthreshold taste and smell. Nature Neuroscience, 2000. 3(5): p. 431-432.
2.         Dalton, P., N. Doolittle, and P.A. Breslin, Gender-specific induction of enhanced sensitivity to odours. Nature Neuroscience, 2002. 5(3): p. 199-200.
3.         Laing, D.G. and G.W. Francis, The capacity of humans to identify odours in mixtures. Physiology & Behavior, 1989. 46: p. 809-814.
4.         Le Berre, E., et al., Perceptual processing strategy and exposure influence the perception of odour mixtures. Chemical Senses, 2008. 33: p. 193-199.
5.         Prescott, J., Chemosensory learning and flavour: Perception, preference and intake. Physiology & Behavior, 2012. 107, 553-559
6.         Prescott, J., O. Young, and L. O'Neill, The impact of variations in flavour compounds on meat acceptability: a comparison of Japanese and New Zealand consumers. Food Quality and Preference, 2001. 12(4): p. 257-264.