Inhibition of sweetness

M. L. Shulman, S. D. Shiyan, and A. Y. Khorlin, Specific irreversible inhibition of sweet-almond p-glucosidase by some p-glycopyranosylepoxyalkanes and p-D-glucopyranosyl isothiocyanate, Biochim. Biophys. Acta, 445 (1976) 169—181. 

Lactisole [13794-15-5] the sodium salt of racemic 2(4-methoxyphenoxy)propionic acid, is a sweet-taste inhibitor marketed by Domino Sugar. It was affirmed as a GRAS flavor (FEMA no. 3773). At a concentration of 100 to 150 ppm, lactisole strongly reduces or eliminates the sweet taste of a 10% sugar solution. This inhibition appears to be receptor-related because lactisole also inhibits the sweet taste of aspartame. The 5 -( —)-enantiomer [4276-74-8] (38), isolated from roasted coffee beans, is the active isomer the i -(+)-enantiomer is inert (127). 

Organic acids may exhibit other sensory properties. For example, citric acid possesses sweet-and-sour sensory notes, and succinic acid has a salty-bitter taste. On the other hand, the typical taste and flavor of Emmental cheese can be ascribed to the propionic acid and a few other compounds, such as proline. In fact, taste and flavor result from the combination of different food constituents in definite proportions. Raw meat smells much like lactic acid, which arises from postmortem anaerobic glucolysis and determines the pH of meat, its final properties, and microbial stability. This same organic acid has been related to the inhibition of certain pathogenic bacteria in yogurt (3). Table 1 lists the reported threshold concentrations for various organic acids in different media (4-6). 

Covalent cross-links both between and within the tropocollagen molecules confer strength and rigidity on the collagen fiber. These cross-links are formed between Lys and its aldehyde derivative allysine. Allysine is derived from Lys by the action of the copper-containing lysyl oxidase which requires pyridoxal phosphate for activity. The disease lathyrism is caused by the inhibition of lysyl oxidase by the chemical (3-aminopropionitrile in sweet pea seeds, and results in defective collagen due to the lack of cross-links. 

Human taste response is modified by several plant-derived substances. The detergent sodium dodecyl sulfate, as well as triterpene saponins from the leaves of several plant species (most notably Gymnema sylvestre and Ziziphus jujuba) will temporarily inhibit the sweet taste sensation in man the duration of the effect being about one hour for G. sylvestre and about fifteen minutes for Z. jujuba. The mechanism of action seems to be related, in part, to the surfactant properties of the materials. Structures of the modifiers and possible mechanisms of action are discussed. 

Once a purified gymnemic acid became available, much psychophysical work was done to understand the nature of the sweetness inhibition effect. The work of Bartoshuk and co-workers illustrates the course taken (15). The results of a typical experiment are shown in Figure 3. The sweetness of a sucrose solution was almost completely suppressed after holding a gymnemic acid solution in the mouth for a few seconds. Further experiments were carried out to determine the effect of gymnemic acid on the other taste qualities (sour, bitter and salty). No effect of gymnemic acid on these tastes was observed. Early work with GA extracts had produced an apparent inhibition of bitter taste, but this effect was later attributed to cross-adaptation to the taste of the crude leaf extract, which was itself quite bitter. Experiments with refined (and tasteless) extracts showed no bitterness suppression. 

Sodium dodecyl sulfate, the gymnemic acids and the ziziphins have all been termed "surface active" taste modifiers because they all possess detergent-like properties. These molecules all have a polar and a non-polar end and they are capable of penetrating the phospholipid membranes that are believed to be components of sweetness receptors. Any speculation about the mechanism of action of these substances must take into account the experimental observations concerning miraculin, monellin, and thaumatin, which were presented at the beginning of this article. Those observations suggested that transport of the modifier to the cell s interior was not occurring and the inhibition effect is manifested at the surface of the cell. 

Gynmemic acid(s) Inhibit(s) sweet taste for several hours (powder sugar tastes like sand and sugar solution tastes like plain tapwa-ter). Salty, bitter, and sour tastes are not affected. Various triterpene glycosides isolated from the leaves of Gymnema sylvestre. 

Do these diverse compounds give rise to a common perception of sweetness or to qualitatively different sensations Sweetness does indeed appear to be a unitary percept (Breslin et al. 1994,1996). However, some sweeteners may be discriminable on the basis of their activation of other sensory transduction mechanisms or differences in the temporal properties of their sensory action. For example, the sweetener sodium saccharin activates bitter receptors in some people (Kuhn et al. 2004 Pronin et al. 2007), and also inhibits sweet taste at high concentrations (Galindo-Cuspinera et al. 2006). Sweet proteins such as thaumatin and monellin can have a slow onset or evoke a prolonged sweetness compared with sugars (Faus 2000), likely owing to a relatively high affinity for the sweet taste receptor. 

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