Lactic acid production and by-product formation in high cell density culture of lactobacillus casei

Abstract

With the aim to produce lactic acid at a reduced medium cost, various complex nitrogen sources were examined in batch cultures of Lactobacillus casei subsp. rhamnosus. Among the sources tested, yeast extract (YE) enhanced lactic acid production most significantly. The addition of B vitamins of less than 25 mg/L enhanced lactic acid production to some extent in all cases except the media supplemented with nitrogen sources of casein origin. Batch fermentations were performed in order to find an optimum pH for both lactic acid production and by-products minimization. While a pH range of 6.0 - 6.5 was optimal for cell growth and lactic acid production, superior results were achieved at pH 6.0 when both maximum lactic acid productivity and minimum by-product formation were considered. Among the techniques that can increase cell concentration in the bioreactor, cell recycle using a hollow fiber filter and cell immobilization were studied. A continuous fermentor with a partial cell recycle was employed in order to investigate the physiological effects of cultivating L. casei at a high cell density in terms of by-products formation such as acetic acid, formic acid, ethanol, and \% D-lactic acid. The increase of by-products formation in the recycle fermentor was more significantly connected with substrate limitation resulted from a lower dilution rate rather than the cell concentration. The immobilization method of cells was developed using alginate capsules that possess an interphasic membrane and a liquid-core. The capsules were found to offer more space for cellular growth than gel-core beads, and allowed 1.5-fold higher cell concentration than the latter. However, the Ca-alginate structure was unstable during repeated batch fermentations for lactic acid production. Stable lactic acid production was possible in batch culture by cells immobilized in chitosan-coated Ba-alginate capsules. The cell leakage from the capsules was maintained relatively low during repeated b...