Control of a mixed culture of Lactobacillus delbrueckii and ralstonia eutropha for the production of PHB from glucose via lactate

The mixed culture system was considered in the present research where sugars such as glucose were converted to lactate by Lactobacillus delbrueckii and the lactate was converted to poly beta-hydroxybutyrate (PHB) by Ralstonia eutropha in one fermenter. Noting that the high lactate concentration inhibits the cell growth for both cells, we attempted to control it at low level. The lactate concentration was measured on-line using an on-line enzymatic lactate sensor with FIA system. The control system was 2 x 2 multivariable system where glucose and lactate concentrations were controlled at their set points. Since the lactate was produced by L. delbrueckii and consumed by R. eutropha, it was quite difficult to control the lactate concentration with the conventional control policy due to interaction of both control loops. In order to compensate for the interaction between the two control loops, we considered to manipulate the glucose feeding rate depending on the difference between the on-line measured or estimated lactate concentration and its set point. The manipulation was made by adjusting the on period of on-off control so that the proportinal control property was retained. The usefulness of the proposed control policy was verified by both computer simulation and by the fed-batch experiments. Since the lactate concentration could be estimated on-line by the amount of NaOH solution supplied to keep the culture pH at constant level to some extent, we also studied the effect of its accuracy on the control quality and thus PHB production. (C) 1999 Elsevier Science S.A. All rights reserved.