Starting from previous structure-activity relationship studies of taste modifiers based on homoeriodictyol, dihydrochalcones, deoxybenzoins, and trans-3-hydroxyflavones as obvious analogues were investigated for their masking effect against caffeine. The most active compounds of the newly investigated taste modifiers were phloretin, the related dihydrochalcones 3-methoxy-2',4,4'-trihydroxydihydrochalcone and 2',4-dihydroxy-3-methoxydihydrochalcone, and the deoxybenzoin 2-(4-hydroxy-3-methoxyphenyl)-1-(4-hydroxyphenyl)ethanone. Starting with the whole set of compounds showing activity >22%, a (Q)SAR pharmacophore model for maskers of caffeine bitterness was calculated to explain the structural requirements. After docking of the pharmacophore into a structural model of the broadly tuned bitter receptor hTAS2R10 and docking of enterolactone and enterodiol as only very weakly related structures, it was possible to predict qualitatively their modulating activity. Enterodiol (25 mg L(-1)) reduced the bitterness of the 500 mg L(-1) caffeine solution by about 30%, whereas enterolactone showed no masking but a slight bitter-enhancing effect.

In order to find new flavor modifiers, various short chain gingerdione derivatives were synthesized as structural analogues of the known bitter masker homoeriodictyol and evaluated by a sensory panel for masking and sweetness enhancing activities. 1-(4-Hydroxy-3-methoxyphenyl)hexa-3,5-dione ([2]-gingerdione) and the homologue 1-(4-hydroxy-3-methoxyphenyl)hepta-3,5-dione ([3]-gingerdione) at concentration ranges 50-500 mg kg (-1) showed the most promising masking activity of 20-30% against bitterness of a 500 mg kg (-1) aqueous caffeine solution. Additionally, both compounds were able to reduce the bitterness of a 5 mg kg (-1) quinine solution by about 20%; however, the bitter tastes of salicine, the model peptide H-Leu-Trp-OH, and KCl solutions were not reduced. Whereas for bitter masking activity a vanillyl moiety seems to be important, some of the tested isovanillyl isomers showed an interesting sweet enhancing effect without exhibiting a significant intrinsic sweetness. The isomer 1-(3-hydroxy-4-methoxyphenyl)hexa-3,5-dione ([2]-isogingerdione) at 100 mg kg (-1) caused a significant and synergistic increase of 27% of sweet taste of a 5% sucrose solution.

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