Effect of mixing and mass transfer conditions on gellan production by Auromonas elodea

Abstract

The effect of fermentor hydrodynamics on gellan fermentation kinetics and rheological properties of the culture broth were studied using various mixing and mass transfer conditions. Different impeller systems, such as a helical ribbon (HR250), Rushton turbines (RT600) and a pitched-blade turbine combined with an in-flow turbine (CT600) were tested along with extra oxygen supply (HR250Ox) or reduced nitrogen amount in the culture medium (HR250N). The highest gellan productions (around 13 g/ l of native gellan) were observed when oxygen transfer capacity was good ( i.e. HR250Ox, HR250N, CT800 and RT600). The volumetric power input was found to be a good tool to evaluate the gellan synthesis progress especially with the helical ribbon impeller where no stagnant zone formation occurred. Macromixing conditions affected the rheological properties of the final broth. For instance, the highly heterogeneous conditions (with RT600) led to a more shear-thinning broth with a lower yield stress value than the most homogeneous conditions (with HR250Ox). Good correlations between yield stress value and gellan concentration were established with respect to the fermentation pattern.