Agitation effects and kinetic constants of exoglucomannan production by Antarctic yeast strain in a stirred tank bioreactor

Abstract

Exoglucomannan production by Antarctic yeast Sporobolomyces salmonicolor AL1 is studied at semi-tech scale in a 5 L stirred tank bioreactor and the bioreaction kinetics is quantified. The organism's unconventional response to agitation is analyzed in terms of the agitation-induced mechanical stress. The yeast maximum production activity was observed at agitation rate 400 rpm and conserved or decreased at further increase in mixing intensity. Referring to the relationship of cell growth and aeration intensity, the various production activity, oxygen availability and cell growth are considered as a starting point to elucidate the possible reasons for the anomaly. At suspicion of shear detrimental effect on the yeast cells, the hydrodynamic stress acting on cell particles is determined and the microorganism morphology at low and high mixing intensity is examined. Biological stability is registered and the agitation effect is attributed to depressed metabolic activity at the evolving dissolved oxygen tension rather than to direct effect of hydrodynamics. A kinetic model is proposed. The specific growth rate (μ, h-1) and growth-associated (g EPS g-1 cells), and non-growth associated (g EPS g-1 cells h-1) production constants are determined and compared with reported estimates for similar reference EPS fermentations. The model and its parameters are determined in well-mixed cultures and could be upgraded further to account for mixing non-ideality and mass transfer in larger vessels.