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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.

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