Reduction of Yeast Extract Supplementation in Lactic Acid Fermentation of Whey Permeate by Immobllized Cell Technology

Abstract

A stable 91-d continuous fermentation of whey permeate, supplemented with various concentrations of yeast extract (0 to 10 g·L–1), was carried out in a two-stage bioreactor for lactic acid production. The first fermentation stage contained Lactobacillus helveticus cells entrapped in κ-carrageenan and locust bean gum gel beads. The second free cell fermentation stage in series was continuously inoculated by cells released from the first stage. At a high concentration of yeast extract (10 g·L–1). maximum productivity was 41.3 g·L–1 per h for the first stage immobilized cell bioreactor containing 54% (vol/vol) gel beads, but the residual sugar concentration was unacceptably high (19.1 g·L–1). For the two-stage system, a total productivity of 13.5 g·L–1 per h could be obtained with 1 g·L–1 of residual sugars. An uncoupling effect between lactic acid and biomass production was observed at low yeast extract supplementation (0 to 1 g·L–1). Simulation analysis of the performance of a two-stage immobilized cell system operating at 1 g·L–1 residual sugar showed maximum productivities of 9.3 and 23.0 g·L–1 per h with 33 and 54% gel beads, respectively, and 10 g·L–1 yeast extract.