Taste, smell and the brain
The terms taste and flavour are often used interchangeably, but they do mean different things: taste is a basic sensation (salty, sweet, bitter, sour, savoury), whereas flavour is a complex perception. Taste seems to have evolved for specific biological purposes and serves vital functions. Taste is like the gatekeeper governing what to go ahead and swallow (e.g. sweet=good; bitter=beware).
Flavour mainly arises when the sense of smell is added to the sense of taste and combined in the brain with other signals to produce a perception — flavour. The sense of smell that is most important is not the aroma of food as we breathe in, but rather the detection of volatile flavour molecules that are released as we eat and that are carried backwards through the nasal cavity as we exhale through the nose.
We can tell that olfaction (smell) is central to flavour by noticing how flavourless food can be when consumed at the height of a head-cold, even though taste receptors in the mouth have not been affected. Normally, the detection of odour molecules begins with them sticking to, and then being absorbed into, mucus membranes that surround our olfactory nerve endings in the upper reaches of the nasal cavity. With a head cold, a build-up of mucus interferes with and dilutes this process, and our ability to smell (and therefore appreciate flavour) is reduced.
The advantage of perceiving flavour with chewing is that chewing releases more complex and volatile molecules than does plated food. Plus, the reactions that occur in the mouth (for example with acids and saliva, or between food components as they intermingle) again release more interesting and complex volatiles.
The taste signals from the mouth go primarily to the brainstem. The brainstem, located just above the spinal cord, is ancient (in evolutionary terms) and involved in vital functions (e.g. heart rate, breathing). Sending taste signals directly to the brainstem enables quick reflex-like avoidance responses to tastes we may have learned are dangerous. From the brainstem, the signals relay to another important sensory centre (the thalamus, located between the brainstem the cortex) before being sent on to the cerebral cortex itself to be integrated with other sensory inputs such as smell (and vision for example) that make up the totality of flavour. So, it is a three-stage process to get taste signals from the mouth to the cortex and to conscious perception.
In contrast, the olfactory nerve endings send signals for smell straight to the highest processing centres of the brain (the frontal lobes of the cerebral cortex) as well as multiple other cortical areas important for perception, memory and experience. This highlights the importance of smell when appreciating flavour. It is smell, mainly retro-nasal (backwards through the nose) that gives nuance to flavour.
However, there is no inherent flavour in food, only the perception of it in our brain. It’s the same with vision — you don’t see with your eyes, you see with your brain. The eyes provide the information and the brain interprets it and ‘sees’. It is why visual illusions can occur.
There are a host of other mouth sensations that are integrated to form a perceived flavour — e.g. mouthfeel, pungency, temperature — and these follow a similar route as do the neural signals for taste. As well, other senses contribute — vision, sound and extra-oral touch (if, for example, eating with fingers)
Finally, there are purely cortical factors, mainly hedonic — e.g. expectation, experience, memory — that modulate the basic sensations in complex ways.
With all of those contributions, it is no wonder that the appreciation of food, well prepared and eaten with mindfulness, can be such a rewarding experience.