The effects of chitosan and alginate coatings of alginate beads with entrapped Lactococcus lactis ssp. lactis were studied in batch and continuous fermentations. Chitosan coating reduced the final concentrations of free cells, the initial release of free cells and the rate of lactate production in milk fermented batch-wise to a final pH of 4.7 in five consecutive batch fermentations. An alternative experimental system based on continuous fermentation with controlled pH and a high dilution rate was developed to better study the phenomenon of cell release. To estimate the effects of different bead coatings on cell release, alginate beads were coated with chitosan or alginate, or sequentially with chitosan/alginate or chitosan/alginate/chitosan. Chitosan coating alone seemed to reduce the rate of cell release only in the early stages of the fermentation, while sequential coatings with chitosan and alginate showed significant reduction throughout the whole test period. To examine whether the observed effects of bead coating could be explained only by a decrease in cell activity, the ratios between the rate of cell release and the rate of lactate production were examined during the fermentations for the different beads. This ratio showed qualitatively the same behavior as direct results of volumetric cell release.

Aims: To investigate the growth and release of Lactococcus lactis subsp. lactis in gel beads and to affect rates of cell release by changing the growth conditions. Methods and Results: The rate of release and the distribution of immobilized L. lactis subsp. lactis in alginate beads were studied in continuous fermentations for 48 h. A change in operating pH from 6á5 to 9á25 initially reduced the ratio of the rates of cell release to lactate production by almost a factor of 10 5 . Compared with fermentations at pH 6á5, growth at pH 9á25 also increased the ®nal internal bead biomass concentration by a factor of 5 and increased the ®nal rate of lactate production by 25%. After 48 h, the ratio of the rates of cell release to lactate production was still 10 times lower than in fermentations at pH 6á5. Conclusions: A change in the operating pH from 6á5 to 9á25 reduced rates of cell release throughout 48 h of fermentation and increased the ®nal rates of lactate production and internal bead biomass concentration. Signi®cance and Impact of the Study: These data illustrate that diffusional limitations and corresponding pH gradients can be exploited in affecting the distribution of immobilized growing cells and their concomitant release.

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