Sensitivity Propagations in Response to Start-Up Perturbations During Batch Fermentations with Complementary Cultures

Batch fermentation for poly-β-hydroxybutyrate production by a mixed culture of Lactobacillus delbrueckii and Ralstonia eutropha has been analysed for its sensitivity to perturbations in the initial concentrations. The sensitivities of six key concentrations, including those of the two bacterial species, differ significantly in terms of their variations with time and their magnitudes at a given time. The variations may be related to the mechanism of the fermentation, with lactate being a critical variable connecting the growth and product kinetics of L. delbrueckii and R. eutropha. The progress of the sensitivities emphasizes that disturbances at the start of the fermentation may persist long afterwards, cause widely different excursions in the variables of interest and thus influence the dynamics and control of the process. Since the cells are physiologically nascent in the bioreactor environment during the start-up phase, sensitivities to perturbations here are critical in determining the course of the fermentation.