MICROBIAL DYNAMICS OF CO-ENTRAPPED AND SEPARATELY ENTRAPPED MIXED CULTURES OF MESOPHILIC LACTIC-ACID BACTERIA DURING THE CONTINUOUS PREFERMENTATION OF MILK

Four strains of mesophilic lactic acid bacteria were separately or coe ntrapped in K-carrageenan/locust bean gum gel beads and used for conti nuous prefermentation of UHT skim milk in a stirred-tank bioreactor. L actic acid and cell productivities of the immobilized cell bioreactor were particularly high and remarkably stable. during eight weeks of co ntinuous milk prefermentation (about 18 g h(-1)l(-1) of lactic acid an d 4.9 x 10(12) CFU h(-1) l(-1), respectively, but important variations of the bacterial populations in prefermented milk and gel beads occur red in any case (co- or separate entrapment). The strain Lactococcus l actis subsp. lactis biovar diacetylactis CDI1 became dominant, account ing for approx. 90% (released cells) and 78% (immobilized cells) of th e total population. Microscopic observations of sections of gels beads showed a progressive destructing of the bend surface with rupture and release of entrapped viable cells from peripheral cavities of the gel . It is believed that these cavities close again after releasing all o r part of their cell content, entrapping the different strains of the mixed culture and initiating a new colonization step and a cross-conta mination of the beads. On the other hand, experimentations over seven- week periods with pasteurized milk showed the high resistance of the i mmobilized cell bioreactor to psychrotrophic contamination. (C) 1997 b y Elsevier Science Inc.