Abstract
Previously published results from a novel, homogeneously distributed shear, tubular fermenter used to study continuous chemostat (high shear) and biofilm (low shear) fermentations (Brink and Nicol, 2014a), were compared to results obtained in the same reactor at intermediate shear conditions as well as batch (biofilm) fermentations of Actinobacillus succinogenes.
It was found that the steady-state volumetric production rates increased by nearly an order of magnitude (1.8 g.L-1.h-1 vs 15–17 g.L-1.h-1) as the shear was reduced from the chemostat (1.83 m.s-1) to the lower shear biofilm conditions. The biofilm results indicated similar volumetric production rates for the different shear conditions, while the measured extracellular polymeric substances (EPS) in the biofilm exhibited a significant shear dependence; EPS fractions of 0.50 ± 0.05 g.g-1 vs. 0.16 ± 0.02 g.g-1 for the low and intermediate shear conditions respectively. The cell-based biofilm production rates were shown to be dependent on shear conditions at succinic acid titres less than the growth-maintenance boundary (10 g.L-1), with a reduction in cellular production rate associated with an increase in shear. Under maintenance conditions the cell-based production rates appeared independent of the shear conditions in the fermenter, with the cell-based production rates decreasing with increasing succinic acid titres. The industrial implications are that for succinic acid titres in excess of 10 g.L-1, the same mass of biomass under intermediate shear conditions should exhibit a greater volumetric production rate due to the higher fraction of cells as opposed to EPS.
The initial values for the batch cell-based succinic acid production rates corresponded to the steady-state values for biofilms grown under the same continuous conditions. However, during transient operation the production rates exceeded the steady-state values; a lag in the product inhibition response was observed. The transient production rates eventually ceased at succinic acid titres in excess of approximately 60–72 g.L-1; a residual glucose concentration of 20 g.L-1 was measured at the highest succinic acid titre (72 g.L-1), indicating product related inhibition as opposed to substrate depletion. The transient results indicate that a larger average cell- based production rate can be obtained during transient operation when compared to the steady fermentation conditions due to a lag in the succinic acid inhibition during transient operation.
