Production of ascorbic acid glucoside by alginate-entrapped mycelia of Aspergillus niger

The mycelia of Aspergillus niger, cultivated in a medium containing 45 g l^-1 maltose, 66 g l^-1 yeast extract, and 5 g l ^-1 K₂HPO₄ at 30°C and 200 rpm, were used as a biocatalyst in the glucosylation of ascorbic acid. Free mycelia from 3-day-old culture, when used in a 6-h reaction with maltose as the acyl donor, gave 16.07 g l^-1 ascorbic acid glucoside corresponding to a volumetric productivity of 2.68 g l^-1 h^-1 and a conversion of 67%. Mycelia from 3-day-old cultures were entrapped in calcium alginate beads and used as a catalyst in the glucosylation of ascorbic acid. An ascorbic acid-to-maltose molar ratio of 1:9 was found to be optimum, and the conversion reached 75% after 12 h. The concentration of ascorbic acid glucoside produced at this molar ratio was 17.95 g l^-1, and the productivity was 1.5 g l^-1 h^-1. The biocatalyst was repeatedly used in a fixed bed bioreactor for the synthesis of ascorbic acid glucoside and approximately 17 g l^-1 of ascorbic acid glucoside corresponding to a volumetric productivity of 1.42 g l^-1 h^-1 was produced in each use. The conversion was retained at 70% in each use. The entrapped mycelia also exhibited exceptionally high reusability and storage stability. The product was purified to 85% by anion exchange and gel permeation chromatography with a final yield of 75%.