Epimerization and Decomposition of Kojibiose and Sophorose by Heat Treatment under Neutral pH Conditions

We evaluated the stabilities of kojibiose and sophorose when heated under neutral pH conditions. Kojibiose and sophorose epimerized at the C-2 position of glucose on the reducing end, resulting in the production of 2-O-α-D-glucopyranosyl-D-mannose and 2-O-β-D-glucopyranosyl-D-mannose, respectively. Under weak alkaline conditions, kojibiose was decomposed due to heating into its mono-dehydrated derivatives, including 3-deoxy-2,3-unsaturated compounds and bicyclic 3,6-anhydro compounds. Following these experiments, we propose a kinetic model for the epimerization and decomposition of kojibiose and sophorose by heat treatment under neutral pH and alkaline conditions. The proposed model shows a good fit with the experimental data collected in this study. The rate constants of a reversible epimerization of kojibiose at pH 7.5 and 90 °C were (1.6 ± 0.1) × 10^-5 s^-1 and (3.2 ± 0.2) × 10^-5 s^-1 for the forward and reverse reactions, respectively, and were almost identical to those [(1.5 ± 0.1) × 10^-5 s^-1 and (3.5 ± 0.4) × 10^-5 s^-1] of sophorose. The rate constant of the decomposition reaction for kojibiose was (4.7 ± 1.1) × 10^-7 s^-1 whereas that for sophorose [(3.7 ± 0.2) × 10^-6 s^-1] was about ten times higher. The epimerization reaction was not significantly affected by the variation in the buffer except for a borate buffer, and depended instead upon the pH value (concentration of hydroxide ions), indicating that epimerization occurred as a function of the hydroxide ion. These instabilities are an extension of the neutral pH conditions for keto-enol tautomerization that are often observed under strong alkaline conditions.