The basis of the integration of taste and smell appears to
arise from their co-exposure. In the context of eating, we
never experience the odours in flavours without accompanying
tastes. This has two consequences. The first of
these is that the hedonic properties of tastes become
attached to the odour [10,11], an example of a general
associative learning process known as evaluative conditioning
[12]. In other words, odours paired with sweetness
become liked; odours paired with bitterness typically
become disliked (but see below). The second process,
also based in associative learning, reflects the metabolic
value of those food ingredients that give rise to tastes
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qualities(e.g. sugar; glutamate) or otherwise have value as
nutrients (e.g. fat). Odours paired with the ingestion of
calories, as occurs with food consumption, can become
liked even when the taste is unpleasant, which explains
how we can develop strong preferences for bitter drinks
such as coffee or beer, or ‘painful’ foods that contain chilli.
While these two learning processes are seemingly similar,
they can be dissociated by, for example, conditioning
liking for an odour paired with a non-nutritive sweetener
such as aspartame (therefore based only on the pleasantness
of its sweet taste) or alternatively pairing the odour
with the energy of sugar, but without metabolic need
(satiety), in which case the amount of increased liking is
limited [13].