Optimization of nutrient feed concentration and addition time for production of poly(β-hydroxybutyrate)

PHB is an intracellular storage compound produced by many organisms under conditions of nutrient limitation (N, P, etc.) when carbon source is present in excess. Model-based fed-batch cultivation is one of the techniques to induce desired nutrient limitations intelligently for over production of such partially growth-associated polymeric products. In the present study, batch kinetics of PHB synthesis by Wautersia eutropha was established in a bioreactor. Batch kinetics featured a biomass concentration of 19.68 g/l with a PHB content of 10.89 g/l in 60 h. A mathematical model was then developed, which adequately described the batch kinetics. The model equations were extrapolated to fed-batch cultivation by incorporating the dilution terms due to fresh nutrient (carbon and nitrogen) feeding. Model-based fed-batch fermentation was conducted by feeding nitrogen (7 g/l) from 20–30 h at the rate of 70 ml/h and fructose (300 g/l) from 24–34 h at the rate of 80 ml/h constantly which yielded 32 g/l biomass containing 14 g/l PHB in 50 h. The productivity was 1.5-times greater than that obtained in batch increasing from 0.18 to 0.28 g/l-h. The model proved to be highly instrumental in the design of suitable fed-batch cultivation.