Lactic acid is used traditionally in the food, pharmaceutical, and chemical industries. And recently new applications, such as degradable plastics made from poly(lactic) acid, have a potential to greatly expand the market for lactic acid, if more economical processes could be developed.
For the cost savings of fermentation process, the composition of the fermentation substrate had to be optimized. In batch cultures of Lactobacillus delbrueckii, the effects of various nitrogen sources including yeast extract on the fermentation characteristics such as cell growth rate, the final lactic acid concentration and productivity were experimentally studied. It was demonstrated that cell growth was affected by two main factors, inhibition by lactic acid and limitation by nutritional components.
In order to increase the productivity significantly, a continuous stirred tank reactor with cell recycle was employed. A cell density of 145.2 g dry weight/L and a volumetric productivity of 73 g/L h were obtained with an effluent concentration of 85 g/L lactic acid. The productivity achieved by this system was 25-fold higher than those obtained by the corresponding batch cultivations. Once the lactic acid concentration reached the steady state, lowering the yeast extract concentration caused the reduction of the lactic acid concentration without affecting the biomass concentration.
Finally, the formation of D-lactate was investgated. During the various cultures, a small amount of D-lactate always formed, even though a majority of lactate was L-isomer. It was supposed that the relative amount of the D-lactate was affected by glucose limitation, and there seems to exist a certain relationship between the concentration of D-lactate and acetate.
