A novel extractive fermentation process for lactic acid

In order to produce lactic acid directly with high productivity, we have developed a novel extractive fermentation process combining lactic acid production by two-step pH manipulation and extraction of lactic acid by poly(4-vinylpyridine) resin. The culture pH of the fermentation was maintained at an optimum value for the growth of Lactobacillus delbrueckii in the initial stage of the fermentation. The pH control was released to allow the lactic acid excreted by the microorganisms to remain free form after certain amounts of lactate salt and cell mass accumulated in the fermentation broth. PVP resin in a column in situ extracted the produced lactic acid from cell-free fermentation broth coming through a microporous filtration unit from a fermentor. The extraction of lactic acid reduced the lactic acid inhibition and thereby allayed the pH drop of the culture as well. With 400 g resin in the process, approximately 62 g lactic acid could be produced with productivity of 3.3 g/L/hr in a 1.2 L fermentor. Without lactic acid extraction, productivity was only 1.1 g/L/hr in producing 17 g lactic acid. An application has been made for a U.S. patent for the process. A generalized mathematical model was developed to account for the effects of the changing pH and lactic acid as well as lactate. The model was extended to the novel extractive fermentation process. The simulation of the extractive fermentation process predicted the experimental results. An objective function was set up in terms of fermentation, recovery, and material costs. The operating parameters such as resin amount, the number of cycles used (long-term stability of cells), the lactate concentration at the pH control off, and the minimum pH at column exchange were investigated for the optimization of the process. (Abstract shortened with permission of author.)