pH control and the “shoulder” effect in parallel fermentation pathways

In an effort, to develop comprehensive mathematical models for use in optimizing fermentation processes, product rate data taken at constant pH's for the lactic acid fermentation ( Lactobacillus delbrueckii ) were analyzed. It was observed that the rate of acid synthesis‐time trajectories exhibited a “shoulder” effect at pH's less than 5. That is, a nearly constant rate of synthesis for up to 10 hr in the late growth phase, out of a maximum total fermentation time of 70 hr. This effect was used as a clue from which to structure the proposed model with parallel pathways. Simulating shunting pathways does, in fact, demonstrate that, the notion of parallelism is consistent with the expression of a shoulder. It is postulated, therefore, that a differential pH effect between two parallel pathways can account for the presence of a shoulder when both routes prevail, and no shoulder when either pathway predominates.