Partitioning of caffeine and quinine in oil-in-water emulsions and effects on bitterness

The bulk vegetable oil-water partition coefficient of caffeine and quinine was determined by a shake-flask method as log K_ow  = -1.32 and 2.97. These values were consistent with the effect of oil concentration on the distribution of the bitterants in an oil-in-water emulsion (0-2 and 0-20 wt% oil stabilized with 0.125 and 1 wt% whey protein isolate, respectively). For example, in a 20% o/w emulsion, approximately 90% of the total caffeine remained in the aqueous phase, whereas in a 2% o/w emulsion, only ∼20% of the quinine remained in the aqueous phase. The intensity of the bitter taste of caffeine and quinine in emulsions was assessed by a large cohort (n = 100) of untrained participants. An increase in fat in the emulsions (from 0.5 wt% to 2 wt% oil emulsions stabilized with 0.125 wt% whey protein isolate) caused a significant decrease in perceived bitterness that was accompanied by a decrease in the aqueous concentration of the hydrophobic bitterant quinine Specifically, the bitterness of quinine was reduced ∼13% in the o/w emulsion with more fat, and this drop paralleled a drop in the aqueous concentration and was generally consistent with aqueous dose-response functions published elsewhere. For the hydrophilic bitterant caffeine, there was no significant change in the perceived bitterness or aqueous concentration with changing oil concentration. We conclude that the perceived bitterness of a hydrophobic bitterant like quinine in an emulsion depends on the aqueous concentration rather than the overall concentration.