Submerged culture process for biomass and exopolysaccharide production by Antarctic yeast: some engineering considerations

Abstract

Production of biomass and extracellular polysaccharide (EPS) from psychrophilic Sporobolomyces salmonicolor AL₁ in a stirred bioreactor was studied. The aspects of production technical-scale parameters, namely, bioreactor flow field, biomass and EPS production rates, oxygen mass transfer per input power, as well as important product properties, such as rheology and stability of EPS mixtures, were considered. The bioprocess was found to proceed in non-Newtonian flow with consistency coefficient rising typically to 0.03 Pa.sⁿ and flow index declining to 0.7. Flow modeling was carried out and showed good homogenization for substrate delivery at agitation rates exceeding 400 rpm. Agitation rates lower than 400 rpm were considered counterproductive due to flow field non-uniformity. The cell density reached 5 g/l and EPS production yield reached 5.5 g/l at production rate 0.057 g EPS/l per hour (0.01 g EPS/g biomass per hour). Oxygen uptake rate and oxygen transfer rate were in the range of 0.5–1.7 mmolO₂/l per hour and 2–4.7 mmolO₂/l per hour, respectively. The mass transfer coefficient at reaction conditions was found to be in the range ${K_L}a\tilde{\mkern6mu} 0.004-0.01{{\mathrm{s}}^{-1 }}$. The bioprocess biological performance was higher at moderate agitation speed and revealed biomass diminution and cell inactivation by increasing impeller revolutions and shear rate. The product EPS was found to introduce shear-thinning behavior in water solutions with apparent viscosity of up to 30 mPa.s and to stabilize 1–2 % oil-in-water emulsions improving their lipophilic properties. The emulsion dispersion index was found to be comparable with the one of Arlacel 165, the emulsifier used in cosmetic. The long-term performance of the complex cream mixtures of the glucomannan prepared in commercial format was found promising for further application.