Eating is probably one of the most obvious things that utilises all our senses. If something doesn't look quite right, crunch or sizzle in the expected way, smells peculiar, has the right texture or taste as expected it can affect our enjoyment of the food (no matter how prestigious the chef). This sensory overload is what enables the likes of Heston Blumenthal to delight and confuse us ((apparently) - The Fat Duck is definately on my to-do list when we're richer).
When the interplay between our senses and how they are integrated to giving us a full experience of food goes wrong it can be frustrating. Perhaps this is most commonly experienced when we have a head cold and can't smell, but other reasons can be less obvious and more serious. As we age people often lose their appetite, resulting in weight loss and malnutrition. This has been linked to a loss in the sense of taste. I suffer from ulcerative colitis which I sometimes have to control using steroids. Whenever I take the steroids some foods taste weird (particularly milk in tea) and I get a strange metallic taste in my mouth. Wondering why the steroids do this got me interested in how we taste.
Taste and smell are chemistry based senses. When we eat something the chewing that we do combined with our saliva breaks down the food and the different chemicals in the food are released. These then react with sensors in our mouth and nose. I was taught at school that there were 4 tastes: Sweet, Sour, Bitter, Salt. That was a while ago and since then a fifth - Umami has been added. Umami is that deeply savoury flavour grilled meats have. Currently there is also a debate as to whether there is a 6th - fat.
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| Where we detect Sweet, Sour, Salt, Bitter (National Library of Medicine) |
Our mouths have specific cells that detect each of these individual tastes that are spread all over our tongues. Specific taste receptors have been found for Sweet, Sour, Salt and Bitter. These are clustered together into taste buds and each one has a different sensitivity to each of the five tastes. There are thousands of taste buds and this variety combined with the different sensitivities of enables the vast range of different taste combinations to be detected.
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| A Taste Bud (National Library of Medicine) |
Once the chemicals have been detected we need to then integrate them and interpret them to recognise it as chocolate or marmite. This happens in our brains. The taste buds therefore detect the different chemicals in the food and send a signal to our brains via the nervous system. Once in the brain each taste seems to be interpreted by neurones that are clustered together. For example, no matter where on the tongue a bitter taste is detected once the signals reach the brain all the 'bitter' neurones are clustered together.
It is also in the brain that the basic taste information is integrated with the information about the food from our other senses, and particularly the sense of smell. Like taste our sense of smell is triggered by specialist sensors detecting chemicals. Unlike 5 tastes the smell sensors can detect hundreds of different compounds. It is this that gives the flavour to foods beyond the 5 tastes.
So the odd taste in my mouth from the steroids could be because there are some residual tablets (which do taste awful) still in my mouth and binding to the bitter taste sensors. Alternatively the steroid compound itself could be affecting the complex signals that allow taste and flavour to be detected and recognised by the brain. But taste is more than chemistry. It is also a social experience, shared with those around us and influenced by our environment, temperature, texture and smell. I think this is just the start of my interest in taste......
References
http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0072592/
http://www.flavourjournal.com/series/the_science_of_taste
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