Microbial Conversion of Lactose to Lactobionic Acid by Resting Cells of Burkholderia cepacia No. 24

Abstract

Microbial conversion of lactose to lactobionic acid (β-O-D-galactosyl D-gluconic acid) was carried out with the resting cells of a mutant strain of Burkholderia cepacia that showed tolerance to high lactose concentrations, high oxidizing activity, and no β-galactosidase activity. To obtain the cells carrying high lactose-oxidizing activities, the strain was cultivated at 28°C for 216 h in a medium (pH 7.0) consisting of 10% (w/v) lactose, 1.5% (w/v) CaCO₃ (1/2 molar equivalent of lactose), 3.0% (w/v) corn steep liquor, 0.02% (w/v) yeast extracts, and other mineral salts. The oxidizing activity of the resulting cells was most active at around pH 6 and at 55°C and the activity remained almost stable between pH 5 and 9 and below 40°C. Under the optimized conditions, the washed cells of 2.0 U/mL of the oxidase activity converted 15% (w/v) lactose and 2.2% (w/v) CaCO₃ almost completely to calcium lactobionate within 15 h at 40°C. The cells were recovered from the reaction mixtures and used for the repeated bath operations five times. The initial conversion velocity depended on the reaction temperature, and the degree of conversions at 30, 35, and 40°C in the first cycle reached nearly 100% after 27, 18 and 15 h, respectively. In the fifth cycle of the reactions, however, incubation times for the complete conversion should be extended to 54, 36 and 30 h, respectively. Low content of impurities and high conversion efficiency enabled simple refining of calcium lactobionate in a high yield (about 99.3%), which was achieved by the treatment with activated carbon.