ABSTRACT
Aims: To develop a mathematical model for prediction of variation in oxygen concentration inside the bed of tray type bioreactor for solid state fermentation and comparison of oxygen profile of unsteady state and pseudo steady state approximation. Place and Duration of Study: All the simulations were performed at the Applied and Industrial Microbiology laboratory, Indian Institute of Technology, Madras, from October 2013 to September 2014. Methodology: Models for various reaction kinetics (zero order, first order and Monod’s kinetics) were derived from the general model. Ordinary differential equations (Pseudo steady state approximation) and partial differential equations (unsteady state assumption) were solved by numerical techniques – Finite difference method (FDM) and Runge-kutta method. Simulation runs were carried out for various parameters such as bed height gas phase oxygen concentration, saturation constant, and porosity of the bed. Results: Oxygen profiles of unsteady state and pseudo steady state assumption were compared and results show lower oxygen concentration in case of unsteady state assumption. Concentration of oxygen was low for the organism following first order when compared to zero order and Monod kinetics. Results of simulation runs revealed that the oxygen concentration decreases as the bed height increases irrespective of the kinetics of the reaction. And it increases with increasing gas phase oxygen concentration, saturation constant and porosity. Conclusion: Mathematical model with unsteady state assumption was reported and it can be employed in calculating the design and operational parameters for solid state fermentors to yield optimal productivity.
ABSTRACT
Aims: To evaluate the effect of volumetric oxygen transfer coefficient on the production of xylitol by Debaryomyces nepalensis and to enhance the yield and productivity of xylitol by fed-batch fermentation using xylose as substrate. Place and Duration of Study: All experiments were performed at the Applied and Industrial Microbiology Laboratory, Indian Institute of Technology Madras, from March 2013 to May 2014. Methodology: Batch cultivations were carried out in a 7.5 L fermentor under various oxygen transfer coefficients in the range 12 to 39.6 h-1 in order to understand the effect of oxygen on xylitol production. Fed-batch studies were performed in 2.5 L bioreactor with a working volume of 1 L. The cultures were initially grown as batch cultures. Feed containing xylose and nitrogen source was added to the medium intermittently. Samples were periodically collected at regular intervals of time and the concentrations of xylose, xylitol and glycerol were determined by HPLC. Results: Maximal xylitol yield (0.64 g/g) and productivity (0.43 g/L·h) were obtained at kLa 13.68 h-1. The effect of pH was also studied at this kLa. A pH value of 6.0 was found to be favorable for xylitol accumulation. Fed-batch fermentation involving feeding of xylose and nitrogen source was used for xylitol production by D. nepalensis. Within the fed-batch phase, the yield of xylitol was 0.83 g/g and the productivity was increased to 0.83 g/L.h with a final product concentration of 90 g/L. Conclusion: Higher kLa favors biomass production whereas product formation was favored at lower kLa. Fed-batch process resulted in enhancement of final product concentration by 73%.