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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.

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