Lactic acid (LA) is a key organic acid widely used in food, pharmaceuticals, leather, and textiles as an acidulant, flavoring, or preservative. In the 21st century, the demand for LA has surged due to its role in synthesizing polylactic acid (PLA), a biodegradable and compostable plastic. However, high LA prices hinder PLA's competitiveness against petroleum-based plastics. Thus, reducing production costs by boosting volumetric productivity has become essential, leading to various methods aimed at enhancing LA titre, production rate, and yield in both manufacturing and downstream processes. The microbial conversion of sugars in fermenters is preferred over chemical synthesis, as fermentation allows for stereospecific LA production using different microorganisms, while chemical methods yield only a racemic mixture. Nevertheless, conventional batch or fed-batch fermentations often face limitations due to product inhibition, which occurs when LA concentration exceeds a critical level. This study reveals that Bacillus coagulans PS5 growth is significantly inhibited at LA concentrations above 40 g·L-1. A membrane bioreactor (MBR) system presents a promising solution to overcome this limitation by continuously removing LA and supplying substrates, thereby alleviating product inhibition and enabling sustained cell growth throughout fermentation, ultimately achieving higher LA levels.
