Effects of Heating Rate on Aldose-to-Ketose Isomerization and Browning in Phosphate Buffer

In the chemical isomerization of common sugars to rare sugars, browning of the reaction mixture due to byproduct formation remains a major challenge. In this study, two aldopentoses, two aldohexoses, and three aldo-disaccharides were utilized as substrates to investigate how the heating rate from room temperature to approximately 115°C at 0.754±0.011 and 0.401±0.007 °C/min (fast vs. slow heating) influences both ketose yield and the degree of browning, the latter of which was evaluated by the change in absorbance at 440 nm (ΔA440). For all tested sugars, ΔA440 was markedly lower under fast heating than under slow heating conditions during rare sugar production. This trend suggests that the activation energy for isomerization is higher than that for the concurrent browning pathways. Furthermore, the tendency for browning followed the order of pentoses > hexoses > disaccharides. These findings demonstrate that fast heating provides a practical strategy to suppress thermal degradation and browning while maintaining efficient rare sugar production.