Elucidation of the mechanism of lactic acid growth inhibition and production in batch cultures of Lactobacillus rhamnosus
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Batch cultures of Lactobacillus rhamnosus were carried out at different pH values in order to study the limitation of growth and lactic acid production by the hydrogen ion, non-dissociated lactic acid and internal lactate concentrations. The effect of pH between 5 and 6.8 was studied at non-limiting concentrations of glucose; this is more significant for the lactic acid fermentation rate than for the maximum specific growth rate, as shown by the incomplete substrate consumption at lower values of medium pH and by the constant maximum cell mass obtained within the range of pH values studied. To check whether these results were a direct consequence of the different concentrations of the non-dissociated form of lactic acid at different external pH values, specific growth rates and lactic acid productions rates were calculated for each external pH value. The same specific growth rates were observed at the same non-dissociated lactic acid concentrations only at pH values of 5 and 5.5. For higher values of pH (pH > 6) the specific growth rate falls to zero as the non-dissociated lactic acid concentration decreases. This shows that generalisations made from studies performed within very narrow ranges of pH are not valid and that the non-dissociated form of lactic acid is not the only inhibiting species. The internal pH was measured experimentally for each external pH value in order to calculate the internal lactate ion concentration. This form is described to be the inhibitory one. The results obtained confirmed that the specific growth rate reached zero at approximately the same lactate concentration for all the pH values studied.
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