We used mixtures of aqueous solutions of a bitter substance, urea, which is strongly suppressed by sodium-containing compounds6; a sweetener, sucrose; and a salt, sodium acetate, which has a fairly mild taste6 and so is suitable for studying the flavour-modifying effects of sodium ions. Subjects (21 volunteers) were required to judge the extent of bitterness, sweetness and ‘otherness’ of all possible combinations of three concentrations of urea (0.0, 0.5, 1.0 M), four of sucrose (0.0, 0.1, 0.3, 0.5 M) and three of salt (0.0, 0.1, 0.3 M) using the method of magnitude estimation6. We evaluated the solutions, 12 per day (twice) over three consecutive days, in a counterbalanced order. Data were standardized and normalized6.

As predicted, there was a selective suppression of the taste components by sodium acetate (Fig. 1). The bitterness of urea was suppressed much more by the salt than was the sweetness of sucrose. Consequently, the sucrose-urea mixtures with added salt were relatively less bitter and more sweet than when sodium acetate was not added. Moreover, at the higher concentrations of sucrose (0.3, 0.5 M) and both concentrations of urea (0.5, 1.0 M), the absolute sweetness intensity was increased by adding either 0.1 or 0.3 M sodium acetate compared with when no sodium acetate was added (one example is shown in Fig. 1). This presumably occurred by releasing sweetness from suppression by the bitterness of urea7. As expected8, the addition of sodium acetate to sucrose in the absence of urea never had an enhancing effect on sweetness (data not shown).

Figure 1: The normalized reported magnitude of the taste of various solution mixtures is shown.
figure 1

The intensity of urea and sucrose at the highest concentrations were roughly the same (left). Statistical analysis revealed that in mixtures, the highest concentrations of sucrose and urea (without sodium acetate), mutually and roughly equally suppressed their intensities (centre). When sodium acetate was added, also at the highest concentration, intensity of the bitterness greatly decreased, being suppressed by sodium ions6, whereas the sweetness intensity increased to levels that approximated the sweetness in pure deionized water (right). Relative to binary mixture levels, asterisk denotes increase (P<0.0001) and star denotes descrease (P<0.0001). These trends were evident for other concentrations tested. Detailed analyses availabel from the authors.

Although this simple three-component aqueous system does not fully mimic the complex food systems in which salts are used, it illustrates at least one mechanism by which a salt increases both the relative and absolute intensity of palatable components of foods. This mechanism has not commonly been considered in taste mixture studies, which have tended to concentrate either on two-component mixtures, or on complex foods where interpretations are difficult.

Our data show that, in addition to adding desired saltiness to food, salts potentiate flavour9 through the selective suppression of bitterness (and perhaps other undesirable flavours), and the release from suppression of palatable flavours such as sweetness. The desire for NaCl and other salts in foods as diverse as (often bitter) vegetables, oily foods and meats may be due in part to their ability to suppress unpleasant flavours10. This may explain why it is difficult to make low-sodium foods acceptable.

Biophysical evidence11 implies that it will be extremely difficult to develop a salty-tasting sodium-free substitute for NaCl. However, the multiple sensory functions of salts in foods should be considered, as the differential flavour-suppressing effect shown here might be duplicated by non-sodium substances, such as bitterness blockers.