The overall volumetric oxygen transfer coefficient in high viscosity, alginate-rich media

Abstract

Understanding the impact of alginate, or similar high-viscosity polymers, on the oxygen transfer in aerobic bioprocess is critical in bioprocess development. Further, in the case of alginate lyase production, it is common for the media to also include significant concentrations of NaCl, which alters the ionic strength and impacts K L a. The results show that in alginate containing media, as the concentration of alginate increases, the fluid viscosity increases, leading to the reduction in K L a. However, comparison of the water-air and water-alginate-air systems revealed that at increased agitation rates (i.e. > 200 rpm) the K L a is enhanced in the water-alginate-air system as compared to the water-air system. This enhancement of K L a was seen up to 10.5 g/L of alginate. We propose this is due to two competing effects. Under lower alginate concentration, the influence of NaCl on bubble size and formation properties enhanced K L a in comparison to the water-air system. However, an additional increase in alginate concentration resulted in the reduction of K L a as the effect of increased viscosity, which reduces the liquid phase mass transfer, became the dominant effect. These results suggest that for fermentation utilising viscous media, operating under lower alginate concentrations may prevent oxygenation issues. Furthermore, the observations made emphasise the need to consider additives such as NaCl in fermentation media when estimating K L a. • Oxygen transfer as a function of alginate concentration in NaCl supplemented fermentation media were studied. • The K L a was repressed as alginate concentration increased due to increasing effect of fluid viscosity. • Enhanced K L a was observed up to 10.5 g/L alginate as NaCl influences media viscosity and the kinetics of oxygen transfer. • Further increase results in repressed K L a as the liquid mass transfer markedly impacted by increased media viscosity. • The results suggest oxygenation issues may be prevented during fermentation by maintaining lower alginate concentrations.