Relationships between hydrodynamics and rheology of flocculating yeast suspensions in a high‐cell‐density airlift bioreactor

Abstract

In this article a hydrodynamic and rheological analysis of a continuous airlift bioreactor with high-cell-density system is presented. A highly flocculating recombinant strain of Sacharomyces cerevisiae containing genes for lactose transport (lactose permease) and hydrolysis (beta-galactosidase) was exploited to ferment lactose from cheese whey to ethanol. The magnetic particle-tracer method was used to assess the effect of operational conditions (air-flow rate, biomass concentration) on hydrodynamic behavior of an airlift bioreactor during the fermentation process. Measurements of liquid circulation velocity showed the existence of a critical value of biomass concentration at which a dramatic deceleration of net liquid flow appeared with increasing biomass quantity. Rheological analysis revealed exponential increase of viscosity of the yeast floc suspension at the same biomass concentration of about 73 g/dm3 corresponding to 42.8% v/v of solid fraction. These facts have a particular importance for the successful processing of a high-cell-density airlift bioreactor as only a circulated flow regime will be favorable to keep the solid particles in suspension state and evenly distributed throughout the bioreactor.

Continuous Beer Fermentation Using Immobilized Yeast Cell Bioreactor Systems

“…Increasing solids loading provoked a decrease in liquid circulation velocity and an increase in critical air flow rate and mixing time (102,110,111).…”

Effect of Mixing on Microorganism Growth in Loop Bioreactors

“…The microorganisms suspended in the liquid phase usually have a density that is close to that of water and their volumetric flow rate can, at present, be incorporated as a part of the liquid phase. At relatively low volumetric concentrations (up to 30 g cells/L dry weight) the presence of bacteria in water will exert little influence on its characteristics but, the rheology of the microbial suspensions can become more complex at high-volumetric solid loadings of interacting and flocculated microorganisms [43]. The operational characteristics of the multiphase bioreactor are usually based on the volumetric flow rate of the liquid phase to be processed (which includes the suspended microorganisms as an integral part of it) and the gas to liquid volumetric flow ratio, G/L.…”

Experimental analysis of an effect of the nutrient type and its concentration on the rheological properties of the baker’s yeast suspensions

“…(2). The dependence of the apparent viscosity  a on the shear rate  is presented in Figure 5 for the different values of the yeast concentration X in the suspension and for the period of the time t = 1 h. As it is seen, apparent viscosity  a decreases with the increase of the yeast concentration up to X = 150 kg/m 3 and it increases when concentration X is higher than 200 kg/m 3 and shear rate is higher than 300 1/s. …”

“…Klein et al 3 undertook the study related with the rheological properties of the yeast cells. Authors stated that two regions can be distinguished in rheological characteristics for such suspension systems.…”