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In this study, galactose was treated under subcritical water conditions at 140 °C and 5 MPa in a mixture of 10 mmol/L sodium phosphate buffer (pH 7.0) and ethanol using a tubular reactor. The effects of ethanol concentration (0−60 wt %) on the isomerization and epimerization of galactose, respectively, to tagatose and talose were investigated. Both ethanol and buffer synergistically improved the isomerization behavior. Increasing the ethanol concentration up to 60 wt % improved the tagatose yield from 13 to 16% at 300 s. In contrast, the yield of talose slightly decreased from 1.5 to 1.2% at 300 s when ethanol was added to the phosphate buffer. The pH of treated solution at 25 °C decreased with an increase in the duration of treatment under each treatment condition. However, the decrease in pH was moderately suppressed by the addition of ethanol, which could be one of the reasons for the improvement in tagatose yield. The addition of other organic solvents, such as alkanols, polyols, acetonitrile, and pyridine, also improved the tagatose yield.

Glucose and fructose were treated in subcritical water in the presence of alkali or alkaline earth metal chlorides. All salts accelerated the conversion of saccharides, and alkaline earth metal chloride greatly promoted the isomerization of glucose to fructose. In contrast, alkali metal salts only slightly promoted this isomerization and facilitated the decomposition of glucose to byproducts such as organic acids. The selectivity of the glucose-to-fructose isomerization was higher at lower conversions of glucose and in the presence of alkaline earth metal chlorides. The pH of the reaction mixture also greatly affected the selectivity, which decreased rapidly at lower pH due to the generated organic acids. At low pH, decomposition of glucose became dominant over isomerization, but further conversion of glucose was suppressed. This result was elucidated by the suppression of the alkali-induced isomerization of glucose at low pH. Fructose underwent decomposition during the treatment of the fructose solution, but its isomerization to glucose was not observed. The added salts autocatalytically promoted the decomposition of fructose, and the reaction mechanism of fructose decomposition differed from that of glucose.

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