Insight into formation of various rare sugars in compressed hot phosphate buffer

Abstract

Galactose, glucose, xylose, arabinose, maltose, and lactose were treated in 10 mmol/L sodium phosphate buffer at 140 °C using a tubular reactor under pressurized conditions. Each sugar was isomerized or epimerized to its corresponding ketose and C-2 positional epimer. Isomerization/epimerization resulted in production of various yields of rare sugars (0.8–27 mol%). Yield of rare keto-disaccharide (21–27 mol%) was higher than that of rare monosaccharide (5.8–16 mol%). Rare keto-monosaccharide yield was also different between hexose (13–16 mol%) and pentose (5.8–8.9 mol%). Although disaccharides were partially hydrolyzed to form corresponding monosaccharides, degrees of hydrolysis for these disaccharides were small (~1 mol%). During the treatment, organic acids were formed as byproducts, resulting in a drop in pH. Isomerization and epimerization hardly proceeded for all sugars at a pH lower than 6.3. This means that isomerization by Lobry de Bruyn-Alberda van Ekenstein transformation progressed well only when pH was > 6.3 regardless of kind of sugar.