Continuous production of mixed lactic starters containing probiotics using immobilized cell technology

The production of a mixed lactic culture containing Lactococcus lactis subsp. lactis biovar. diacetylactis MD and Bifidobacterium longum ATCC 15707 was studied during a 17-day continuous immobilized-cell culture at different temperatures between 32 and 37 degrees C. The two-stage fermentation system was composed of a first reactor (R1) containing cells of the two strains separately immobilized in kappa-carrageenan/locust bean gum gel beads and a second reactor (R2) operated with free cells released from the first reactor. The system allowed continuous production of a concentrated mixed culture with a strain ratio whose composition depended on temperature and fermentation time. A stable mixed culture (with a 22:1 ratio of L. diacetylactis and B. longum) was produced at 35 degrees C in the effluent of R2, whereas the mixed culture was rapidly unbalanced in favor of B. longum at a higher temperature (37 degrees C) or L. diacetylactis at a lower temperature (32 degrees C). Strain redistribution in beads originally immobilizing pure cultures of L. diacetylactis or B. longum was observed. At the end of culture, the strain ratio (7:1 L. diacetylactis/B. longum) in bulk bead samples was similar to that of individual beads. The determination of the spatial distribution of the two strains in gel beads by immunofluorescence and confocal laser-scanning microscopy showed that bead cross-contamination was limited to a 100 microm peripheral layer. Data from this study validate a previous model for population dynamics and cell release in gel beads during mixed immobilized-cell cultures.