Production of Tagatose from Galactose in a Batch-type Reactor Using a Phosphate Buffer under Subcritical Water Conditions

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Isomerization of galactose to produce rare sugars, tagatose and talose, was performed in a Teflon screw-lid container, used as a batch reactor, at approximately 120 °C. Galactose was dissolved at 5% (w/w) in NaH₂PO₄-Na₂HPO₄ buffer at pH 6.0, 7.0 or 8.0. A high yield (approximately 13%) for the conversion of galactose to tagatose was achieved at pH 7.0, with slight changes in the solution color. The tagatose yield increased with increasing buffer concentration of 10 mmol/L; however, it was nearly constant at concentrations of >10 mmol/L. Conversely, little talose was produced at buffer concentrations of <10 mmol/L; however, its yield significantly increased at buffer concentrations of 10 mmol/L. When the galactose concentration was ≥5%, the tagatose yield decreased; however, the tagatose concentration increased with increasing galactose concentration; for example, the tagatose concentration was 1.71% (w/w) when the initial galactose concentration was 25% (w/w).