High Production of d ‐Tagatose by the Addition of Boric Acid

An l ‐arabinose isomerase mutant enzyme from Geobacillus thermodenitrificans was used to catalyze the isomerization of d ‐galactose to d ‐tagatose with boric acid. Maximum production of d ‐tagatose occurred at pH 8.5–9.0, 60 °C, and 0.4 molar ratio of boric acid to d ‐galactose, and the production increased with increasing enzyme concentration. Under the optimum conditions, the enzyme (10.8 units/mL) converted 300 g/L d ‐galactose to 230 g/L d ‐tagatose for 20 h with a yield of 77% (w/w); the production and conversion yield with boric acid were 1.5‐fold and 24% higher than without boric acid, respectively. In 24 h, the enzyme produced 370 g/L d ‐tagatose from 500 g/L d ‐galactose with boric acid, corresponding to a conversion yield of 74% (w/w) and a production rate of 15.4 g/L·h. The production and yield of d ‐tagatose obtained in this study are unprecedented.