I think it's neat that it's 2010 and we still have no idea how we perceive taste. We were supposed to have hovercars and metallic jumpsuits by now and we still only have what can be regarded as a pretty basic understanding of our sense of taste. In the last year or two, though, the scope of what constitutes has expanded tremendously.
What we know of taste has thus far been largely observational. We know that taste and smell are inextricably linked since people who lose one also lose the other. We know that the mental constructs of tastes we create and store in our brains are plastic; they can be enhanced and revised by further encounters with a taste. We know our sense of taste can be tricked by chemicals that mock the flavors of foods. We know that there are five specialized taste receptor types; umami (my favorite), sour, sweet, salty and bitter.
That last bit appears to be fluid. A 2009 study out of UC San Diego covered in the New York Times found that our sour receptors have another life after hours providing us with our perception of carbonation. Previous studies have found that carbonated beverages served by researchers to test subjects in hyperbaric chambers still still produce the perception of carbon dioxide, even though bubbles don't form in such an environment. Further investigation by the UCSD crew found a gene that produces an enzyme called carbonic anhydrase, which converts carbon dioxide in the blood into usable protons and ions. The gene that produces carbonic anhydrase is found in the genetic code of human sour taste receptors.
Data like this that's providing a further understanding of taste is revealing a larger scope of the sense. There may be five types of receptors, for example, but there's a lot more than five taste sensations, carbonation being yet another avenue that needs exploring. What's more, it's becoming apparent that our experience of taste isn't simply the receptors on the tongue sending a message to our brains where a taste memory (something like, "Man, Juicy Fruit is awesome!") is constructed and maintained. It looks like it's a lot more complex than that.
Further research (here's a good article from Science News surveying it) has found that taste sense cells are found in the stomach and other organs and all play a role in our sense of hunger and taste. We fool them, for example, with artificial sweeteners, which would appear to be better than real sugar. Not so much, we're learning.
Since they've been fooled, the sweet taste receptors are activated, and they send a message to their counterparts in the gut, which in turn release insulin. Insulin is intended to release glucose into the blood in order to store new energy-packed sugar. Although the sugar never comes, the result is the same: loads of glucose enters the bloodstream. And since we tend to overuse artificial sweeteners because of their perceived healthy nature, we set ourselves up for a nasty case of type 2 diabetes.
That artificial sweeteners can fool the body into developing diabetes is in and of itself interesting. But learning that our sense of taste includes receptors in the gut and the ability to sense carbon dioxide makes me want to eat Pop Rocks and Coke for research purposes.