An empiric steady state model of lactate production in continuous fermentation with total cell retention

Continuous lactic acid fermentation with total cell recycling was carried out with the strain Lactobacillus paracasei ATB 160111 in a membrane reactor system. The aim of this study was to examine the dependence of lactic acid production and cell mass formation on the hydraulic dilution rate, D, and the availability of both yeast extract and peptone in a modified glucose medium. An experimental design was used and experimental data were obtained, which could be fitted by sigmoidal equations with a high accuracy within the parameter ranges investigated of 0.1 h^-1 ≤ D ≤ 0.5 h^-1 and 2.5 g/L ≤ peptone/yeast extract ≤ 50 g/L. A steady-state lactate concentration was established in all tests and a maximum lactate productivity of 26 g /(L × h) was reached at a dilution rate of 0.5 h^-1 and a nutrient concentration of 15 g/L. Increasing the concentrations of peptone and yeast extract beyond a value of 15.9 g/L did not lead to an increase in productivity, although total cell mass was accumulated up to 200 g/L. Further examinations showed that only a certain part of the biomass was always active and balanced cell growth was clearly not limited by the lack of any nutrient component in the medium, however, it was affected strongly by product inhibition. A significant decrease in lactic acid yield occurred when the biomass concentration exceeded a value of about 80 g/L. The empiric models used in this study allowed the performance behavior of different strains, for which no biologically well-founded data are yet known, to be compared. Whether empirical sigmoidal models can be also used for the description of process states in continuous fermentation processes should also be looked into.