Waste-reducing and energy-saving lactic acid production using activated carbon

Lactic acid (LA), a renewable material widely used in various industries, is produced through fermentation of biomass. However, LA fermentation requires the neutralization of LA to prevent the inactivation of LA-producing microorganisms, resulting in a significant amount of waste (gypsum, approximately 75 wt % per weight of LA produced) and high energy consumption during the purification of LA. Herein, we developed an LA fermentation method without neutralization using activated carbon (AC) and engineered yeast. This method exhibited high LA productivity comparable to that of conventional fermentation methods and generated zero waste from desalination. The high LA productivity was due to LA adsorption on AC, leading to an LA production concentration of 101.2 g L⁻¹ and glucose-to-LA conversion yield of 88.0 %. The LA adsorbed on AC was easily extracted by immersion in bioethanol near room temperature (30 °C). Therefore, the LA extraction has low energy consumption, and the LA-containing bioethanol can be directly used to produce ethyl lactate. In addition, because AC can be reused repeatedly for LA adsorption and extraction, an LA recovery rate of up to 99.9 % can be achieved. Consequently, the total electricity consumption of LA production can be reduced by up to 34.8 % compared with that of conventional methods. Our proposed fermentation method is not only waste reducing and energy saving but also highly productive, making it promising for the eco-friendly production of organic acids and biomass materials.