Long-term mechanical and biological stability of an immobilized cell reactor for continuous mixed-strain mesophilic lactic starter production in whey permeate

The aim of this study was to characterize the biological and rheological stability of continuous immobilized cell fermentation for the production of mixed-strain mesophilic starters. Three strains of Lactococcus were immobilized separately in kappa-carrageenanlocust bean gum gel beads. Continuous fermentation was carried out in a 1 L pH-controlled stirred tank reactor, operated at 30 degrees C, pH 6.2, and D = 2 h-l in whey permeate medium supplemented with yeast extract (1.5%) and 0.1 M KCl and inoculated with 30% (v/v) bead inoculum (strain ratio 1:1:1). The continuous mixed-strain immobilized cell fermentation demonstrated a high biological stability, and no strain became dominant or was eliminated during the 52 day fermentation. The total and specific free cell populations showed high time stability. All immobilized populations, except MD, were unchanged, but a cross contamination of gel beads initially immobilizing a pure culture occurred, leading to a redistribution of immobilized population in individual beads. After initial modifications of bead rheological properties during colonization batches, the beads demonstrated a high mechanical stability, even with reduced KCl supplementation of the broth medium, in the range 0.1-0 M. This work emphasizes the potential of immobilized cell technology for producing mixed lactic starters in continuous fermentation.