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

Galactose, glucose, xylose, arabinose, maltose, and lactose were treated in 10 mmol/L sodium phosphate buffer at 140 degrees 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 (similar to 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.

Automated summary

This study found that under high temperature and pressure conditions, various sugars can be isomerized or epimerized into rare sugars. The yields vary slightly between different sugars, with higher yields observed for rare disaccharides than rare monosaccharides, and higher yields for hexose compared to pentose. Additionally, the formation of organic acids during the process leads to a decrease in pH, and the isomerization and epimerization reactions hardly occur at pH levels lower than 6.3.