Salt transforms and softens bitter and sweet flavors.
TRUE GASTRONOMES HAVE TWO GREAT FEARS: gout and a diet without salt. To guard against gout they abstain, at least occasionally, from gamey meats; but against a salt-free regime they find themselves powerless and dread the doctor who prescribes it. This fear is doubly well founded. Gary Beauchamp and his colleagues at the Monell Chemical Senses Institute in Philadelphia have shown that the absence of the salt taste is not the sole inconvenience of this regime. Without salt, agreeable tastes forfeit their prominence, and they are unable to prevent disagreeable tastes from asserting themselves.
In earlier chapters I examined the action of salt on the texture of foods without discussing its taste. Salt is important because it increases the ionic strength of aqueous solutions, making it easier for odorant molecules to separate themselves from food. This is why unsalted soup has no flavor and why adding salt amplifies its odor, which is an important part of flavor. Sodium chloride is also a taste molecule that stimulates the papillary receptors. Does it have other virtues from the point of view of flavor? Does it really bring out the flavor of a dish, as some maintain?
In examining these questions experimentally, Beauchamp and his colleagues did not limit themselves to sodium chloride but also tested other salts such as lithium chloride, potassium chloride, and sodium aspartate. They sought to make sense of a paradoxical state of affairs: Whereas most psychophysiological studies test pairs of tastes and succeed in showing that salt either suppresses the accompanying taste or has no effect on it, every gourmet knows that unsalted foods lose much of their interest. Cooks who add salt to their pie dough—even a pinch in the case of a sweet pie—do so not in order to make the dough salty but to give it flavor.
Filtering Tests
The Monell Institute team of psychophysiologists wanted to know whether salt selectively filters tastes, weakening unpleasant tastes while enhancing pleasant ones. Convinced that it was not enough to examine pairs of tastes, they compared aqueous solutions containing one or more of three substances: urea (bitter), sucrose (table sugar), and sodium acetate. There were reasons for choosing these three: Sucrose added to urea softens its bitterness, and sodium acetate contributes sodium ions without imparting too salty a taste. Ten subjects were asked to evaluate the intensity of bitter, sweet, and other sapid sensations produced by combinations of urea, sugar, and salt in different concentrations (three for urea and salt, four for sugar).
As predicted, sodium acetate reduced the bitterness of urea. What gastronomic empiricism did not predict, however, was that salt masked the bitterness much more effectively than sugar. Mixtures of sugar, urea, and salt turned out to be sweeter and less bitter than unsalted mixtures of urea and sugar. Moreover, in strong sugar concentrations, the sweet character was increased by the addition of sodium acetate, probably because salt offsets the weakening of the sweet intensity caused by the bitterness of urea. Consistent with the hypothesis, the addition of sodium acetate by itself to sugar, in the absence of urea, did not increase the intensity of the sweet taste.
These studies were conducted for many other compounds and showed that sodium ions selectively suppress bitterness (and probably other disagreeable tastes as well) while intensifying agreeable tastes. It is therefore a question not of bringing out a single basic taste but rather of modifying the proportions of a combination of tastes. Adding salt to a variety of dishes—vegetables (both bitter ones, such as endive, and sweet ones, such as carrots and peas), certain fatty foods, and meats—may have become habitual because there is an unconscious wish to eliminate unpleasant tastes and to reinforce the natural sweetness of many foods. The recent experiments seem also to explain why some coffee lovers put a pinch of salt in the filter: to remove the bitterness of caffeine.
It is not yet known how the stimulation of taste receptors produces these effects, but we do finally know why salt-free diets make us wince.