The Influence of Sodium Salts on Binary Mixtures of Bitter-tasting Compounds

Russell S.J. Keast 0 Thomas M. Canty 0 Paul A.S. Breslin 0 0 Monell Chemical Senses Center , 3500 Market St, Philadelphia, PA 19104, USA In order to study potential mixture interactions among bitter compounds, selected sodium salts were added to five compounds presented either alone or as binary bitter-compound mixtures. Each compound was tested at a concentration that elicited 'weak' perceived bitterness. The bitter compounds were mixed at these concentrations to form a subset of possible binary mixtures. For comparison, the concentration of each solitary compound was doubled to measure bitterness inhibition at the higher intensity level elicited by the mixtures. The following sodium salts were tested for bitterness inhibition: 100 mM sodium chloride (salty), 100 mM sodium gluconate (salty), 100 and 20 mM monosodium glutamate (umami), and 50 mM adenosine monophosphate disodium salt (umami). Sucrose (sweet) was also employed as a bitterness suppressor. The sodium salts differentially suppressed the bitterness of compounds and their binary combinations. Although most bitter compounds were suppressed, the bitterness of tetralone was not suppressed, nor was the bitterness of the binary mixtures that contained it. In general, the percent suppression of binary mixtures of compounds was predicted by the average percent suppression of its two components. Within the constraints of the present study, the bitterness of mixtures was suppressed by sodium salts and sucrose independently, with few bitter interactions. This is consistent with observations that the bitter taste system integrates the bitterness of multi-compound solutions linearly. Introduction Everyday life exposes us to complex mixtures of bitter tasting compounds. For example, many foods contain multiple compounds that can elicit bitterness (e.g. catechin, theophylline, theobromine, and caffeine in black tea). Similarly, Over-The-Counter pharmaceuticals are often codelivered within a formulation (e.g. dextromethorphan, acetaminophen, and pseudoephedrine in cough syrups). Despite the potential for interactions via the cellular complexity of the bitter taste system [multiple G-proteincoupled receptors and post-receptor transduction mechanisms (Kinnamon and Margolskee, 1996; Wong et al., 1996; Rossler et al., 1998; Huang et al., 1999; Adler et al., 2000; Chandrashekar et al., 2000)], bitterness perception often appears additive when compounds are mixed in binary combination (Keast et al., 2003). For example, adding a weakly bitter alkaloid (e.g. quinineHCl) to a weakly bitter amino acid (e.g. L-tryptophan) results in a final bitterness that is equal to the addition of the weakly bitter alkaloid to itself, or the weakly bitter amino acid to itself (see Figure 1, Equation 4). Therefore, the processes of increasing concentration and mixing together different compounds are related to each other in that they produce similar levels of perceived bitter taste intensity. We investigated the influence of bitterness suppression on binary mixtures of bitter compounds as yet another test of binary bitter mixture interactions. If the suppression of the individual compounds predicts the suppression of bitter mixtures, then there is little evidence of interactions. When bitter compounds are mixed together, the combination solution appears more bitter than either compound would alone. This creates the opportunity for the mixture to appear more difficult to suppress than its components. Because bitterness is more difficult to suppress as perceived intensity increases (Breslin and Beauchamp, 1995), we employed the additional comparison condition of adding salts to individual bitter compounds at double their respective concentrations in the binary mixtures. There are few known bitterness inhibitors, but sodium (Na+) salts have been shown to suppress the bitterness of certain compounds in human psychophysical studies (Bartoshuk and Seibyl, 1982; Breslin and Beauchamp, 1995; Keast and Breslin, 2002a,b). This suppression is mainly an oral peripheral effect of ions (at the cellular/epithelial level) rather than a cognitive effect (central process) of the perceived taste. To demonstrate the peripheral effect, Kroeze and Bartoshuk (1985) applied a bitter stimulus to one side of the tongue and a Na+ salt to the other side of the tongue (split-tongue methodology). The stimuli were applied independently and simultaneously. The intensity of bitterness was reduced more when the stimuli were applied to the tongue in mixture together, compared to independent simultaneous application of the two stimuli on different sides of the tongue. This conclusion is possible because the two lateral halves of the tongue are neurologically independent until the ascending neurons interact in the brain (Tucker and Smith, 1969). This peripheral interaction between sapid compounds could occur with a number of molecules in the taste receptor cells (Keast et al., 2001). Several studies have investigated the effect of these bitterness inhibitors on a variety of individual bitter compounds, but there are few, if any, reports of bitterness inhibition of binary mixtures of bitter compounds. Sodium gluconate was used in this study because of the reduced salty taste (compared to NaCl) associated with its large anion (Ossebaard and Smith, 1995); low perceived saltiness allows us to distinguish between the peripheral inhibition of bitterness by Na + ions and the central cognitive inhibition of bitterness by perceived saltiness (Breslin and Beauchamp, 1995). Kroeze and Bartoshuk (1985) demonstrated cognitive taste suppression using the same splittongue methodology described above. They reported mutual suppression of individual suprathreshold taste qualities, such as sweet and bitter, regardless of whether the compounds were applied independently to either side of the tongue or together as a mixture. This demonstrated that suppression could have a central cognitive, rather than just a peripheral oral, effect. Umami-tasting Na + salts were also included in the present study because a comparison of the bitterness inhibition of several Na+ salts revealed that the umami tasting salts monosodium glutamate (MSG) and adenosine monophosphate (Na2AMP) were the most effective at inhibiting bitterness (Ming et al., 1999; Keast and Breslin, 2002b). It was not known if the added bitterness inhibition efficacy was due to the cognitive influence of the umami taste quality or an oral peripheral effect of the salts. To help further understand the central or peripheral influence on bitterness inhibition, we used the rare phenomenon of within quality taste synergy. Mixtures of certain 5-ribonucleotides (NaIMP or NaGMP) with MSG enhance umami taste beyond additivity (Yamaguchi, 1967; Rifkin and Bartoshuk, 1980). Therefore, the central effect of umami taste can be compared with the peripheral effects of Na+ and glutamate on bitterness by using iso-intense umami solutions containing different m (...truncated)