CELL REUSE IN SUBMERGED LACTIC-ACID BATCH FERMENTATION

The reusability of biomass in lactic acid batch fermentation with free cells of Lactobacillus paracasei was studied in a 2-1 fermenter and i n a 50-1 fermenter. In lab-scale fermentation experiments, 33 to 100% of the cell mass formed was reused in the subsequent batch in each cas e. In a series of seven consecutive batches, maximum values of lactate formation productivity of 6.32 to 11.54 g/l x h were observed at init ial cell concentrations of 2.1 to 24.6 g/l. In all of the experiments, the initial cell viability was 78% or greater than 78%, and the final cell viability did not fall below 70%. At cell concentrations above 2 0 g/l, the productivity of lactic acid formation did not increase furt her, but remained constant. Because its level could be influenced by v arying the proportions between the content of yeast extract, peptone a nd initial cell mass (1:1:2, 1:1:1 and 3.3.1) in the medium and no inh ibitory effects were observed, this finding can be attributed to nutri ent limitation. A low degree of cell reuse was reached in an analogous series of experiments carried out in a 50-1 fermenter. In this case, the initial cell concentration varied between 0.5 and 1.1 g/l, and the refore cell growth was not limited by nutrients in the first period of fermentation. Lactate production was still stable after six cell-reus e operations. The lactic acid yield did not fall below 90%. Temporary storage of the biomass in a refrigerator for a time interval of one to two weeks caused no significant impairment of overall lactate product ion, but a proportional prolongation of the lag phase occurred with in creasing duration of storage.