Friday, 31 October 2014

(De)constructing flavours

… and in breaking news, chocolate flavour and vanilla flavour are different from one another. But what actually makes flavours different? Clearly, both of these flavours are sweet, so we recognize that the non-sweet bit – the inherent chocolaty-ness or vanilla-ness – is the essential difference. And these qualities are, of course, due to volatiles – in other words, they are smells, perceived via the retronasal (back of the mouth) route to our smell receptors in the nose. So, clearly flavours are combinations of smells and tastes.

But what about other sensory information? If we wanted to accurately describe the flavour of a curry we’d also no doubt also say the flavour is hot or spicy; similarly, the flavour of Coca-Cola is fizzy and that of vodka has ‘bite’. The inclusion of this type of sensory information in our definition of flavours is not especially controversial – after all, a curry without the spiciness seems not a curry at all. More to the point, our mouths contain endings of the trigeminal nerve that are responsible for picking up those chemical signals that lead to perceptions of chemical heat, temperature, and sensations of dryness, prickliness and bite. These same nerve endings may also contribute to the experience of taste.

Stepping outside the mouth though, we are confronted with vast quantities of sensory information that contributes to our experience of a food’s sensory properties. Several overviews in recent years have discussed the importance of vision, hearing, and touch (outside the mouth) in influencing our perceptions of what goes on in the mouth during eating [1, 2](see also previous posting). In a new book by Spence and Piqueras-Fiszman [3], the recent research on the impact of properties like colour, shape and weight in the cutlery, crockery and physical environment of food perceptions and preferences are detailed. Does that mean each of these influences must be included within our definition of flavour?

Some researchers do, in fact, suggest this. They propose, for example, that food features such as crunchiness, the result of both sounds and feedback from pressure sensors in the teeth and gums, are intrinsic to the flavour of some foods. Previously, I discussed the role of expectations – essentially the product of our memory for what belongs with what – as another important influence on flavour experiences. So, potentially, if we wished to be broad enough, flavours could be perceptions involving multiple near and far senses, as well as input from memories. Suddenly, the mouth and what goes on inside it becomes only part of our concept of flavours.

Does it really matter how broadly we define flavours? We can distinguish between the sound of an orchestra and the impact that the acoustics and seat comfort of the hall in which it is heard have on our experience of the music. Similarly, there are good reasons to divide our food experiences into mouth-based sensations, on the one hand, and other sensory information that impinge on our perception or enjoyment of these sensations, on the other. Thus, we are able to experience flavours without any input from any senses except smell and taste. To be chocolate flavour, a little square of fat, sugar and miscellaneous odour compounds does not require the colour brown, or even the sensation of melting, but it does require chocolate aroma and sweet taste in all cases.

Recent studies of the brain’s processing of smell and taste have identified a network of neural structures that appears to encode for flavour, as distinct from odours and tastes separately [4]. Indeed, the brain appears to process the same odour differently, depending on whether it is experienced in the mouth as part of a flavour or external to the mouth, when sniffed. The brain is fundamentally a processor of multisensory signals, largely because integration of different sources of sensory information is biologically useful. An important question, though, relates to the adaptive significance of the ‘construction’ of flavours – why do discrete neural circuits, for example, represent flavours rather than simply odours and tastes separately? It is generally proposed that, in the case of foods, it is the combination of tastes and odours together that reliably tell us whether an object is a food that is fit to eat. However, it is clearly not only about identifying foods. While it can be argued that it is taste and odour together that allow us recognize pear as a pear, in practice, once it is familiar the pear odour is sufficient. In a world without taste, trial and error would allow one to distinguish pears from apples and could even tell you whether or not pears were safe to eat.

The most important consequence of integrating odours and tastes may be primarily about pleasure. From the perspective of food preferences, flavours seem to be fundamental units. This is because, at birth (or in the case of salt, shortly thereafter), we are hedonically inflexible when it comes to basic tastes – sweet, sour, salty, bitter, umami. Our likes and dislikes appear to be pre-set as an adaptive mechanism to ensure intake of nutrients (sweetness, saltiness, umami) and avoid toxins or otherwise harmful substances (bitterness, sourness). On the other hand, there is little evidence that odour preferences are other than the result of experience, a process that may begin in the womb.

Repeatedly experiencing odours with tastes attaches additional meaning to the odour that is primarily hedonic, that is, pleasure-related. The pear flavour that is not bitter, not too sour, and quite sweet provides pleasure in the eating. In other words, we are motivated to consume it because of its sweet taste and the prior associations with the calories that the sweetness, and subsequently (through repeated experience), that the pear flavour itself signals. And, of course, this occurs even prior to eating: pear odour repeatedly paired with a sweet taste itself becomes pleasant.

The perceptual consequences of integration of odours and tastes can be interpreted in the same way. The well-known phenomena of food odours being described in terms of tastes – the sweet smell of vanilla or the sour smell of vinegar – also arise from the repeated pairing of the odour with a taste, sweetness or sourness, respectively. But, these perceptual qualities also have hedonic consequences: sweet-smelling odours are pleasant and this quality may in itself motivate consumption even if we cannot identify the actual odour or its source. Conversely, something with a bitter or sour smelling odour is unlikely to be eaten, especially if we cannot recognize the odour. As such, these perceptual changes to odours may help compensate for the fact that odour identification is particularly difficult even for common foods.

Hence, the key purpose of odour/taste integration is not that it aids identification per se (although it might), but rather that it gives an hedonic value to the flavour, which crucially it the defining characteristic of the food. Thus, flavours can be most accurately seen as “objects” constructed for their hedonic qualities. Initial, “gut” responses to foods are almost always hedonic, and this naturally precedes accepting or rejecting the food. Thus, what we perceive when we sit down to dinner are, thankfully, integrated pleasure-inducing perceptions – spaghetti al pomodoro and a nice Chianti – rather than a collection of independent, hedonically-diverse tastes, odours and textures.


1.         Delwiche, J., The impact of perceptual interactions on perceived flavor. Food Qual Pref, 2004. 15(2): p. 137-146.
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 2014. p. In press.
3.         Spence, C. and B. Piqueras-Fiszman, The Perfect Meal. The multisensory science of food and dining.2014, Chichester: John Wiley & Sons.
4.         Small, D.M., et al., The role of the human orbitofrontal cortex in taste and flavor processing. Ann. N.Y. Acad. Sci., 2007. 1121: p. 136-151.