Effect of controlling lactate concentration and periodic change in DO concentration on fermentation characteristics of a mixed culture of Lactobacillus delbrueckii and Ralstonia eutropha for PHB production

A 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 e utropha in one fermenter. Based on the experimental studies on the effect o f lactate concentration on the cell growth of both microorganisms, the lact ate concentration was controlled at less than 5 g/l using an on-line enzyma tic lactate and glucose sensors with the FIA (flow injection analysis) syst em, and by manipulating the glucose feeding rate. Since L. delbrueckii pref ers anaerobic conditions while R.eutropha prefers aerobic conditions, we st udied the effect of DO concentration on fermentation characteristics of eac h microorganism. For the mixed culture, we considered the control scheme of a two inputs and three outputs multivariable system. It was experimentally shown that the periodic fermentation resulted in superior PHB yield with r elatively high productivity as compared with the cases where DO concentrati on was controlled to be constant either at less than 1 ppm or 3 ppm.