Efficient and green production of manno-oligosaccharides from Gleditsia microphylla galactomannans using CO2 and solid acid in subcritical water

Abstract

This study aimed to produce manno-oligosaccharides (MOS) from Gleditsia microphylla galactomannans (GMG) using CO2 and solid acid (Amberlyst-35) in subcritical water. The optimal condition for MOS preparation was 3 MPa CO2, 0.1 g/g solid acid (relative to GMG) at 150 °C for 40 min. The maximum MOS yield with a degree of polymerization from 2 to 4 (M2-M4) was 52.19%, which doubled the yield of MOS compared to either using solely CO2 or solid acid. Solid acid showed excellent performance in producing MOS under subcritical H2O–CO2 condition, due to the enhanced mass transfer efficiency and increased H+ concentration in the reaction system. The solid acid can be easily separated and reused. Comparing with traditional methods used to produce MOS, this approach has many merits such as higher galactomannan hydrolysis efficiency (largely reduced time and higher MOS yield), purer M2-M4 product, lower costs, and more environmental-friendly.