… 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.
