Continuous fermentation of a supplemented whey permeate medium with immobilized Streptococcus salivarius subsp. thermophilus

Abstract

Abstract Streptococcus salivarius subsp. thermophilus was immobilized in k -carrageenan/locust bean gum mixed gel beads (0·5–2·0 mm diameter), using a dispersion process in a two-phase system. Inoculated beads were used to ferment a supplemented whey permeate medium continuously in a 0·75-liter bioreactor equipped with external pH control and mechanical stirring. The pH-stat fermentations were conducted at optimal pH of 5·8, inoculation level of 20% (v/v) and various dilution rates (D = 0·5, 1·0, 1·5, 2·0 and 3·0 h−1). Cell production, carbohydrate utilization and acid production were measured at a steady state. High cell release rates from the gel beads into the broth medium, and growth of free cells in the bioreactor allowed for a steady and efficient inoculation and fermentation of the feed, with cell counts in the outflow varying from 1 × 109 to 3·0 × 108 CFU/ml for D from 0·5 to 3·0 h−1. Lactic acid concentration in the effluent was 17·3 and 4·3 g/liter for D = 0·5 and 3·0 h−1, respectively. Biomass yield showed a slight but not significant tendency to increase with increasing dilution rates. Gel bead integrity was only slightly affected after 72 h continuous fermentation, particularly for bead diameters in the range 1·3–1·7 mm, even though the selected level of broth supplementation with KCl was deliberately suboptimal for the mechanical properties of the gel beads. This process could be used for efficient continuous inoculation and fermentation of dairy fluids, or for continuous single or mixed strain starter production.