Importance of molar volumes and related parameters in sweet taste chemoreception

A major new chemical approach to the study of sweet taste chemorecpetion now centres on the role of water. Interaction of sweet stimulus with receptor requires a molecular ''fit'' which in turn demands specific volume requirements of the stimulus in water and probably also in the biophase. Apparent specific volume, rather than partial specific volume, defines taste quality at normal tasting concentrations and sweet taste quality is largely confined to the range 0.51-0.71 cm(3)g(-1) with the ''ideal'' quality of sugar and sugar alcohol sweetness being about 0.60-0.64 cm(3)g(-1). Specific volumes define hydrostatic packing of sweet molecules among water molecules whereas the related intrinsic viscosities define their hydrodynamic behaviour. Both are related to ''characteristic volumes'' which are partial specific volumes at absolute zero. A further solution characteristic is the partial molar isentropic compressibility (K-2s) which defines the compactness of the hydration layer around sweet molecules. This parameter is much more sensitive to structural differences between sweeteners than is partial molar volume and it best represents compatibility with water structure. The K-2s of (D) under bar-galactose, for example, is -2.08x10(-3)cm(3)mol(-1)bar(-1) while that of the conformationally analogous (D) under bar-glucose is -1.76x10(-3)cm(3)mol(-1)bar(-1). (D) double under bar-galactose is therefore less compatible with water than (D) double under bar-glucose and half as sweet. These studies will help to elucidate the mechanistic differences between sweeteners and their mode of interaction with water and flavours in foods.