Binding of Caffeine and Quinine by Whey Protein and the Effect on Bitterness

Many drugs and phytochemicals are bitter, leading to noncompliance with prescriptions and avoidance of healthy foods and a need to suppress their taste. The goal of this study was to investigate the binding of bitterants (quinine and caffeine) by whey protein isolate (WPI) and the effect on perceived bitterness. Caffeine interacted minimally with WPI, while the proportion of unbound quinine decreased exponentially with protein concentration. Molecular modeling was used to show the energy of the quinine-Β-lactoglubulin interaction was an order of magnitude greater than the caffeine-Β-lactoglobulin interaction. Untrained assessors were used to assess the bitterness of caffeine (1.8, 5.7, and 18 mM) and quinine (0.056, 0.10, and 0.18 mM) solutions with 0% or 1% WPI. There was no significant effect of protein on the bitterness of caffeine solutions, but WPI decreased the bitterness of quinine relative to the same concentration in water. This is generally consistent with our hypothesis that higher binding results in lower bitterness; however the magnitude of reduction was not large and the bitterness of the protein-quinine solutions was greater than would be expected for the unbound quinine present.