Novel glucoamylase-resistant gluco-oligosaccharides with adjacent α-1, 6 branches at the non-reducing end discovered in Japanese rice wine, sake

Sake, a traditional Japanese rice wine, contains various oligosaccharides (Sake oligosaccharides; SAOs) derived from rice starch. We previously found that SAOs reach a high degree of polymerization (DP). In this study, we developed a hydrophilic interaction liquid chromatography-time-of-flight/mass spectrometry (HILIC-TOF/MS) based analytical method to separate isomeric SAOs. Isomers of SAOs with DP = 6, 7, and 8, which were named DP6-1, DP7-1, DP8-1 and DP8-2, respectively, were purified from sake and their structures were determined by two-dimensional NMR spectroscopy. These were novel oligosaccharides containing two alpha-1, 6 bonded branches on an alpha-1, 4-linked glucose main chain. Interestingly, adjacent double alpha-1, 6 branches that have not been identified in starch, were found in DP6-1, DP7-1, and DP8-1, suggesting the presence of the branching pattern in starch. DP6-1 was poorly digested by fungal glucoamylase, and this may be attributed to its adjacent double branches at the non-reducing end.

Automated summary

This study identified novel isomeric structures in oligosaccharides from sake, containing double branches that had not been identified in starch. These isomers showed poor digestion by fungal glucoamylase, possibly due to the presence of adjacent double branches at the non-reducing end.