Optimization of lactic acid production from apple and tomato pomaces by thermotolerant bacteria

The production of lactic acid (LA) through biomass fermentation represents a promising alternative to the chemical synthesis. The use of agri-food by-products as fermentable carbohydrate sources can improve process sustainability by reducing waste and valorizing residual biomass. This study assessed the use of apple and tomato pomaces for producing LA through fermentation using thermotolerant bacteria under aerobic and non-sterile conditions. Three bacteria were evaluated and Heyndrickxia coagulans DSM 2314 was selected for its ability to produce LA from hydrolyzates of apple pomace (APH) and tomato pomace (TPH). The fermentation conditions were optimized to maximize LA production from APH, TPH and a mixture of both hydrolyzates. Therefore, LA productions ranged from 36.98 ± 0.41 to 40.72 ± 0.43 g/L, with yields from 0.86 ± 0.02 to 1.01 ± 0.01 g/g. Yeast extract was necessary as a nitrogen source for fermenting APH, while TPH and the mixture of both hydrolyzates did not require any supplementation. Other nitrogen sources, such as wine lees, urea and NH₃Cl, were tested for fermenting APH. However, mixing this hydrolyzate with TPH proved to be the most viable alternative. This study demonstrates the potential for valorizing apple and tomato pomaces into LA under feasible fermentation conditions.