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The isomerization of galactose was performed with a mixture of 25 μl of 100 mM galactose and 100 μl of the crude enzyme solution as a substrate at 60° C. for 1 hour. To measure the activity of galactose isomerization, 100 μl of the crude enzyme solution containing 40 mM of galactose as a substrate was mixed with 1 Ml of reaction buffer (50 mM Tris-HCl, pH 7.0), followed by reaction at 65° C. for 20 minutes. At that time, 5 mM of MgCl2 and 1 mM of MnCl2 were added to the reaction mixture. The activity of the isomerase was measured by the cystein-carbazol-sulfuric acid method (Dische, Z., and E. Borenfreund., A New Spectrophotometric Method for the Detection and Determination of Keto Sugars and Trioses, J. Biol. Chem., 192:583-587, 1951). The protein contained in the crude enzyme solution was quantified with a Bradford assay kit (Biorad, U.S.A.). As a result, the isomerase activity was 0.2045±0.0078 (mg-tagatose/mg-protein·h), indicating that the product of galactose isomerization, tagatose, was successfully generated (FIG. 3).

1. customfollowed by reaction at 65° C. for 20 minutes