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Optimization of substrate feed for continuous production of lactic acid by the homofermentative bacterium, Lactococcus lactis IO-l, 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" at a dilution rate of 0.5 h", a dry cell weight of 5 g I -i, a level of lactate in the broth of 20 g 1-1, and a concentration of glucose in the spent medium of about 5 g1-1

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