A big part of the reason science ignored taste for so long is because it’s incredibly complicated. Not only are there thousands of flavor receptors, known as papillary cells, on the tongue, each one functions as its own little multi-functional unit.
Certainly some progress had been made observing flavor and receptor interactions over the years. The sensations of sour and salty for example. Researchers had long known that those flavors are created by hydrogen and sodium ions delivering minute electrical charges directly to papillary cell membranes. Those charges, which are received by the cell and carried along to nerve endings, can be measured.
The remaining flavor receptors, however: bitter, sweet and umami, are much more troublesome. Until fifteen years or so ago, no one had been able to prove how they worked. For a long time it had been theorized that flavor compounds that came into contact with a papillary cell bonded with molecules called “receptor proteins” on or near the cells’ surface.
But no one had ever observed it. First, because it’s darn hard to watch a single molecule interact with the surface of a single cell. Second, because it was thought that flavor molecules bonded to receptor proteins only very weakly — temporarily — so any bonding that did occur was over before it could be seen.
As frustratingly elusive as this concept was from a researcher’s point of view, it made quite a bit of sense. If flavor molecules bonded tightly to our receptor proteins, they’d “tie up” our tongues’ flavor receivers as we ate. Once, say, a sweet sensation hit a receptor it would stick, preventing that receptor from registering any of the other other flavors. It’s only via this continuous action of bonding and releasing, bonding and releasing, that the same receptors on our tongues are able to make contact with the full array of flavors in a mouthful of food, like leaves in a stream flowing over a rock.
Of course this was all just a theory until some very clever researchers at Columbia University finally isolated some of these proteins in 1996. What they found was that taste proteins on the tongue work in a way that’s not entirely dissimilar to the way rods and cones work on the retina of the eye. The problem now is to figure out how the impressions made on the receptor proteins make their way into the papillary cells themselves and ultimately to the brain. That’s the current gap in understanding that taste science is even now trying to work out.