Surface active taste modifiers: a comparison of the physical and psychophysical properties of gymnemic acid and sodium lauryl sulfate

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Abstract
We have compared the surface active properties of gymnemic acid with those of the wellknown surfactant sodium lauryl sulfate. Aqueous solutions of gymnemic acid produce a surface tension-concentration relation similar to that of water soluble surfactants. Its form suggests that an equilibrium distribution of gymnema between an air/aqueous interface and soluble micellar aggregates is established. Analysis of the surface tension-concentration curve indicates that the critical micelle concentration for gymnemic acid is about 0.25 mM. Physiochemical studies further show that gymnemic acid easily penetrates insoluble monolayers of phospholipid at the air/aqueous interface at concentrations in the range of parts per million. In both its surface tension lowering capacity and its penetrating ability, gymnemic acid closely parallels the action of sodium lauryl sulfate, a detergent commonly used in oral hygiene products. This striking similarity in surface chemical behavior suggests that: (1) surface activity may be a common property of a class of taste modifying substances including gymnemic acid and sodium laurl sulfate, and (2) taste modification is associated with perturbations in receptor membrane function. We have tested the first point by studying sodium lauryl sulfate for taste modifying ability. Psychophysical experiments on human subjects reveal that responses to NaCl, sucrose, and quinine are suppressed to various extents. The sourness of citric acid is unaffected, but it is augmented by a distinct bitter taste. These studies show, that like gymnemic acid, sodium lauryl sulfate reduces perceived stimulus intensity by a percentage which is independent of stimulus concentration. While these studies do not rule out the possibility of a highly specific and stoichiometric effect on receptor proteins, it is more likely that much of their effectiveness results from nonstoichiometric perturbations of membrane structure. Some of the variation in the psychophysical effects observed among surface active agents may reflect hydrophile-lipophile balance and other surfactant properties.

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