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.
