Long-Chain Fatty Acids Elicit a Bitterness-Masking Effect on Quinine and Other Nitrogenous Bitter Substances by Formation of Insoluble Binary Complexes.

We have previously found that fatty acids can mask the bitterness of certain nitrogenous substances through direct molecular interactions. Using isothermal titration calorimetry, we investigated the interactions between sodium oleate and 22 bitter substances. The hydrochloride salts of quinine, promethazine, and propranolol interacted strongly with fatty acids containing 12 or more carbon atoms. The (1)H NMR spectra of these substances, obtained in the presence of the sodium salts of the fatty acids in dimethyl sulfoxide, revealed the formation of hydrogen bonds between the nitrogen atoms of the bitter substances and the carboxyl groups of the fatty acids. When sodium laurate and the hydrochloride salt of quinine were mixed in water, an equimolar complex formed as insoluble heterogeneous needlelike crystals. These results suggested that fatty acids interact directly with bitter substances through hydrogen bonds and hydrophobic interactions to form insoluble binary complexes that mask bitterness.