Isomerization of maltose to maltulose in a pressurized hot phosphate buffer

Abstract

Rare sugars, which are saccharides that exist in small amounts in nature, have received considerable attention. However, only a limited number of rare sugars are available in markets, owing to their low abundance. The establishment of a low-cost method for the mass production of rare sugars may promote the elucidation of its function and availability in markets. We observed that common sugars can be efficiently isomerized to rare sugars in phosphate buffers maintained at temperatures slightly above 100 °C. Therefore, the purpose of this study was to investigate conditions for the efficient production of maltulose, which is a rare disaccharide, whose functionality has not been fully elucidated. Maltose was isomerized to maltulose in a 10-mmol/L phosphate buffer (pH 7) at 108 °C, resulting in a yield of approximately 20%. Buffer concentrations higher than 10 mmol/L promoted the formation of colorants and reduced the isomerization efficiency. The maltose concentration had little effect on the coloration of the reaction mixture; however, an increase in the maltose concentration resulted in a gradual decrease in the maltulose yield. Furthermore, the higher the reaction temperature, the more maltulose was efficiently produced with lower coloration in a short reaction period. Maltose dissolved in a 10-mmol/L phosphate buffer (pH 7) was isomerized to maltulose, resulting in a yield of ca. 20% at 108 °C within 60 min.