3-49: Continuous succinic acid fermentation by Actinobacillus succinogenes

Fermentations were performed in an external recycle bioreactor using CO₂ and D-glucose at feed concentrations of 20 and 40 g L^-1. Aggressive biofilm formation prevented kinetic analysis of suspended cell  fermentation, while perlite packing enhanced the volumetric productivity by increasing the amount of immobilised cells. The highest productivity of 6.35 g L^-1 h^-1 was achieved at a dilution rate of 0.56 h^‑1. A constant succinic acid yield of 0.69±0.02 g/(g of glucose consumed) was obtained and found to be independent of the dilution rate, transient state and extent of biofilm build-up – approximately 56% of the carbon reaching phosphoenolpyruvate ended up as succinate. Byproduct analysis indicated that pyruvate oxidation proceeded solely via the formate-lyase pathway. Cell growth and corresponding biofilm formation were rapid at dilution rates higher than 0.35 h^-1 when the product concentrations were low (succinic acid<10 g L^-1), while minimal growth was observed at succinic acid concentrations above this threshold.