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Your question is a good one and difficult to answer without knowing more about your specific culture conditions. The buildup of lactic acid during fed batch culture could come from several reasons. There is a correlation between lactate amounts and the reduction of culture productivity, so this is a big problem without an easy solution. Some cells are more sensitive than others. Hybridoma culture can be a particular problem since they stop producing antibodies once the lactic acid cut off point is reached. Lactic acid accumulation in media occurs because at low oxygen concentrations glucose is metabolized to pyruvate via glycolysis. If oxygen is available then pyruvate is decarboxylated to form Acetyl CoA in the mitochondria. During anaerobic conditions and especially in the stationary phase when the cell density is high and oxygen low Lactic acid is formed. If you measure the oxygen concentration this may be evident. You might add galactose or another more difficult to metabolize carbon source to reduce lactate or you could limit glucose.