Process monitoring and control using innovative optical sensors for fermentative lactic acid production in membrane bioreactor system

Lactic acid (LA) is an important organic acid, traditionally used in food, pharmaceutical, leather and textile industries as an acidulant, a flavouring or a preservative. Since the beginning of the 21st century, the expanding demand of LA is mainly caused by the application to synthesize polylactic acid (PLA), which is a biodegradable, compostable and biocompatible plastic. However, the current price of LA has strongly limited the competitiveness of PLA compared with petroleum-derived plastics. Therefore, reducing the production cost by increasing the volumetric productivity has become a critical goal, and various methods focused on increasing the titre, production rate and final yield of LA have been employed in both manufacturing and downstream separation processes. In industry, the microbial conversion of sugars (in fermenters) is preferable to the chemical synthesis of LA, because fermentation can achieve stereospecific LA production by using different strains of microorganisms, whereas chemical synthesis provides only racemic mixture of D,L-LA. However, the productivity of LA in conventional batch or fed-batch fermentations is often limited by the product inhibition. Commonly, the inhibitory effect occurs when the LA concentration in the fermentation broth reaches a critical level. In this study, the investigations of batch fermentation indicate that the growth of Bacillus coagulans PS5 is obviously inhibited at the concentration of LA > 40 g·L-1. A membrane unit integrated reactor system, i.e. membrane bioreactor (MBR) system is a promising candidate to surmount this limit by continuous removal of LA and supplement of substrates. The alleviation of product inhibition allows the cells to keep growing during the entire fermentation process, achieving a high level.