Bitter Truths

Salt, sweet, sour, and bitter--these, we're assured, are the basic tastes. Somehow the cells in our taste buds can communicate the whole symphony of flavors in a Sunday dinner from these basic building blocks.

The sense of taste has been a tricky one for scientists to study. For a long time, biologists were willing to assume that the sensory structures of the tongue and mouth were fairly passive relays, picking up and sending on basic information to the brain where all the real work was done. The closest analogy might be to the sense of touch, as pressure sensors indicate how strongly something presses upon us, and over how large an area.

Now, with vastly improved microtechnology to guide their studies, the experts believe the taste bud should be seen as a kind of biological microprocessor. A host of taste cells--as many as 100 at a time--react to the chemical and electrical information in a bit of food or drink. Each cell responds to some degree to sweet, sour, bitter, and salty compounds. This interplay provides fairly precise information, which is integrated by the whole taste bud. The integrated message then goes to the brain, where the flavor judgments are made.

The best sensory analogy may be to sight rather than touch. Retinal cells also take basic information--the primary colors of the visible spectrum--and somehow team up assure our brains that we're seeing a whole varied landscape of subtle shades and hues.

That's one of the suggestions now finding favor among scientific students of taste, according to a recent issue of the journal BioScience. One of the advocates of the microprocessor model, Stephen D. Roper of Colorado State University, reported that he and his colleagues were able to measure the electrical activity of individual taste cells. They worked with the tongues of mud puppies, a kind of salamander that has unusually large taste cells. Those big cells appear to pass electrical current to neighboring cells as well as to neural connections leading to the salamanders' brains. Such electrical chitchat among cells certainly hints that there's processing underway right in the taste bud, not just in the distant brain. And it is similar to what goes on in groups of cells in the retina of the eye.

Though Roper's hypothesis is still controversial--mud puppies, after all, could be unique as well as convenient to study--another point about taste is pretty well settled. Of the basic taste sensations, the most important is bitterness.

Myles Akabas of Columbia University in New York is a leading advocate of this view. Part of his argument stems from wonderfully logical natural evidence: plant alkaloids are often bitter, and often poisonous. Evolutionary pressures would strongly favor sensitivity to bitter tastes. Akabas is not at all surprised that mammals are more than a thousand times more sensitive to bitter substances than to sweet or salty ones. He also finds it appropriate that newborn mammals only develop that sensitivity to bitterness at about the age they are weaned--that is, when they would begin to eat potentially dangerous substances.

The importance of accurately tasting bitterness has historical evidence in its support as well. Professional tasters worked for nobles and monarchs in medieval times, to sample food and drink before it entered the royal mouth. Those tasters underwent long and careful training to detect faint traces of bitterness. Often enough, in a kind of early arms race, the same dignitaries who needed tasters also employed alchemists to devise lethal compounds for their enemies, something apparently edible in which the bitter taste was disguised. Generally the tasters won out, saved by their acute mammalian sensitivities.

In fact, this particular arms race still continues--only now chemists are out to fool the taste buds of rodents, not kings. Akabas hopes his work will pay off someday in a truly successful rat poison.