Culturing lactic acid producing microorganisms has been a human endeavor for thousands of years for food preservation, however there are differences in the characteristics of strains optimal for food preservation versus utilitarian lactic acid production. A food preservation LA microorganism must be a nonpathogenic and for maximum benefit should be probiotic, their growth should be slow to maintain the food for an extended period, relatedly growth should be possible at lower temperatures to extend preservation further, full utilization of sugars may not be desirable to maintain a higher nutrient density for human consumption, and a mixture of products may be desired to produce complex flavors. In contrast utilitarian LA microorganisms should grow fast to minimize the process time and decrease the chances of contamination, they should have a high yield and convert as much of the feedstock to the product as possible.
Lactic acid microorganisms can be grown in the OSE fermentor, airtight containers, or as silage. Lactobacilli are optimal for silage and inoculation schemes could be useful. Operation of a fermentor for lactic acid production would be aided by control of pH, removal of product, and features for continuous operation.
High yield lactic acid fermentation is through optimization of the microorganisms environment. The pH of the media will be lowered by the production of lactic acid and maintaining pH in the range of 5-6 is essential to culture health and maximum yield. It is through the production of acid that the organisms gain energy and maintaining favorable redox conditions allow the cells to continue to harvest energy through fermentation. To maintain pH developed techniques were to add calcium stearate or other salts, which also precipitated the salt form of lactic acid for collection, however an equal amount of waste is produced alongside the purified lactic acid. There are now techniques to split the lactic acid from its basic salt component and recycle it back to the fermentation chamber, most notably through electrodialysis which creates a LA acidic product stream and a basic waste recycle stream. Constant removal of waste products and maintenance of optimal growing conditions creates a more efficient system with higher productivity.
Continuous run fermentation is more efficient with higher productivity than batch runs by maintaining stationary cell densities, cells do not divide and more sugar consumed is turned into the desired product. Continuous run fermentation is through the removal of product rich media and the addition of fresh nutrients while maintaining cell densities. A standard configuration is to remove the product rich broth, filter the broth through a fine mesh that captures the cells, and return an amount of fresh nutrients equal to the product broth removed to the chamber along with the captured cells. Optimization is needed for different microorganisms, end-product concentration inhibition, and optimal nutrient levels in order to maximize efficiency. A bioreactor with necessary sensors makes this process much more manageable.