Kinetics, bioenergetics and optimization of extracellular polysaccharide production by Methylomonas mucosa in batch and fed-batch fermentations
Abstract
Methylomanas mucosa, an obligate methylotroph, produces an extracellular polysaccharide (mucosan) when cultured on a defined medium containing minimal salts and methanol. A detailed experimental and theoretical study of growth and product formation kinetics of M. mucosa was conducted. A mathematical model which combines principles of kinetics and bioenergetics was developed and successfully used for optimization of mucosan production. Fed-batch fermentations at constant residual methanol concentrations were run by using computer control and a methanol sensor. The effects of three important variables, methanol concentration, nitrogen concentration and oxygen transfer rate on growth and product formation kinetics, were studied and the dependence of specific growth rate $\mu$ and specific product formation rate $\pi$ on these variables was mathematically expressed. It was shown that oxygen limitation during the cell growth phase is undesirable for polysaccharide production and M. mucosa switches off non-growth associated mucosan production during oxygen-limited cell growth. Equations for the oxygen uptake rate and the carbon dioxide evolution rate were derived by using macroscopic material balances and principles of microbial energetics as a basis. The mathematical treatment based on bioenergetics gives values of various maximum yields and provides a theoretical basis for calculating effects of change in polysaccharide structure on maximum yields and respiratory quotient. It also provides some insights into the modes of energy utilization and "priorities" of M. mucosa. The kinetics and energetics models were combined and an algorithm was proposed to calculate specific rates under oxygen limitation. Correlations for k$\sb{\rm L}$aC* and methanol stripping losses were determined and were included in the model. The model could predict the effects of methanol concentration, nitrogen concentration and oxygen supply rate on growth and product formation quite well. Based on this model, optimum C/N (Carbon/Nitrogen) ratios for batch fermentations were determined and a novel scheme for optimizing mucosan production in a fed-batch culture was proposed and experimentally implemented with almost a 50% increase in final mucosan concentration. The polysaccharide was separated, purified and rheologically characterized. Rheological data were mathematically correlated.
Degree
Ph.D.
Advisors
Tsao, Purdue University.
Subject Area
Chemical engineering
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