Agri-food industry wastes as substrates for lactic acid production – an overview of different strategies

Agro-industry wastes include chemically complex and different substrates, from starch-rich wastes from the conditory industry and bread production to lignocellulose-rich substrates like corn stalks or wheat straw. These substrates could be suitable for fermentation and production of bio-based chemicals by microbial transformations if suitable pretreatment is performed and microorganisms are robust enough to perform biotransformations with high efficacy. Lactic acid bacteria as fastidious microorganisms require optimal C/N ratio in media to grow and produce lactic acid with high yields which presents a challenge when combining different waste substrates which are of varying composition. The presence of undesired components in waste substrates and low pH induced by produced lactic acid cause microbial stress in working microorganisms. Additionally, waste substrates have a complex microbiome and often high moisture content. It is difficult and energy-consuming to stabilise and preserve them for later utilization and open fermentations are a convenient approach. We studied different strategies, both in process engineering and adaptive evolution of microorganisms to achieve higher yields in lactic acid production on these substrates. Possibilities for improvement of lactic acid production included immobilization of microorganisms on organic and inorganic carriers, adaptive evolution for the better valorization of stllages and other substrates and some specific aspects related to the application of cold plasma in the pretreatment of waste substrates for fermentation.