Optimization of substrate feed for continuous production of lactic acid by Lactococcus lactis IO-1

Summary Optimization of substrate feed for continuous production of lactic acid by the homofermentative bacterium, Lactococcus lactis IO-1, in glucose medium was investigated. A pH-dependent feed with two pH set-points, a lower set-point for neutralization with alkali and an upper set-point for substrate feed, proved better than continuous substrate feed with one pH set-point for neutralization with alkali only. Built-in electrodialysis with a cell-recycling system was tested and high cell density was achieved as a result of the use of enriched medium. However, specific lactate productivity in this system was not satisfactorily high. pH-dependent feed was combined with turbidity control and a cell recycling. With this system, we achieved high specific lactate productivity of 2 g (g-cell)^-1 h^-1 at a dilution rate of 0.5 h^-1, a dry cell weight of 5 g l ^-1, a level of lactate in the broth of 20 g l ^-1, and a concentration of glucose in the spent medium of about 5 gl ^-1.