Proton nuclear magnetic resonance and Raman spectroscopic studies of Japanese sake, an alcoholic beverage

The hydrogen-bonding property of water-ethanol in Japanese sake, a kind of brewage, was examined on the basis of both 1H NMR chemical shifts of the OH of water-ethanol and the Raman OH stretching spectra. In 20% (v/v) EtOH-H2O solution, amino acids as well as organic acids caused low-field chemical shifts, i.e., the development of a hydrogen-bonding structure. Additional functional groups, apart from the essential amino- and carboxyl groups, in amino acids caused differences in their effects. The low-field chemical shifts caused by solutes were demonstrated under constant pH conditions maintained by sodium hydrogen citrate. Using both the measurement of 1H NMR chemical shifts and Raman OH stretching spectra, the strength of the hydrogen bond of water-ethanol in Japanese sake products was found to be correlated with the total concentration of organic acids and amino acids. Glucose or saccharides should not have a strengthening effect on the hydrogen bond of water-ethanol. The effects of the main inorganic ions and amines were also discussed. It was concluded that chemical components originating from the starting material, rice, or products produced by microorganisms during the ethanol fermentation affect the hydrogen-bonding structure in Japanese sake.