The physiology of lactic acid production by Lactobacillus delbreuckii in a cell recycle fermenter

The physiology of lactic acid production by Lactobacillus deZbreuckii NRRL B-445 in a continuous fermenter with partial cell recycle has been studied and compared with that observed in a conventional chemostat .Increased formation of ethanol and acetate occurred in the recycle fermenter although lactic acid remained the major product. The yield of lactic acid from biomass and the molar product ratio, lactate: ethanol + acetate, decreased with increasing recycle ratio. The volumetric productivity of lactic acid was enhanced in the recycle fermenter due to the complete utilization of glucose, the greatest productivity (12.1 Gl 1 h-1 at D=0.3 h-1) being achieved using the lowest recycle ratio (0.5). The change in product profile was phenotypic in nature and due to glucose limitation. The specific activity of lactate dehydrogenase and the specific rate of ATP formation were maintained during cell recycle fermentation. The cellular content of polysaccharide, protein, carbon and nitrogen also remained constant and culture viability was maintained. The biomass growth yield was reduced with increasing recycle ratio. A simple mathematical model was derived which described the biomass concentration produced in the recycle fermenter at steady state over a five-fold range of concentrating effect. The derived model included a constant specific rate of glucose consumption for non-anabolic (e. g. maintenance) functions. The results suggest that catabolism and anabolism were less closely coupled in the cell recycle fermenter than in the conventional chemostrat.