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 cellbased
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.
