New glycerol glycosides in sake formed by Aspergillus oryzae α-glucosidase A

Sake contains unique glycosides produced by the transglycosylation action of α-glucosidase from Aspergillus oryzae. Besides influencing the taste of sake, some transglycosylation products in sake exhibit beneficial biological activities. In this study, we searched for new transglycosylation products in sake. Liquid chromatography-mass spectrometry (LC-MS) revealed that peaks with m/z values corresponding to the glycosides, diglucopyranosylglycerol, triglucopyranosylglycerol, and tetraglucopyranosylglycerol, are present in sake. The presence of glycosides containing up to four polymerized glucose units is the first observation in sake. The peaks of the compounds were not observed in the sake that was brewed with a rice-koji made by α-glucosidase A (AgdA) gene disruption A. oryzae strain. The in vitro transglycosylation experiment using maltose, glycerol and a recombinant AgdA suggested that the compounds in sake were transglycosylation products composed of glycerol and one to four units of glucose. Nuclear magnetic resonance (NMR) analysis revealed that one of the major products of in vitro synthesis was α-isomaltosylglycerol (α-iMG). α-iMG was detected in commercial sake as a common component at an average of 1095 ppm (mg/L). This is the first study to report the presence of α-iMG in foods and beverages.