Revisiting the production of L( +)-lactic acid from vine shoots: bioconversion improvements by employing thermotolerant bacteria

Vine shoots (Vitis vinifera L.) constitute an abundant lignocellulosic source which is frequently underutilised. Alkaline and acidic pretreatments (with and without washing steps) were compared and optimised to release fermentable sugars from vine shoots. An acidic pretreatment using 1.72% H2SO4 at 134 °C for 17 min (with 10% w/w solid biomass), followed by an enzymatic hydrolysis, offered the most cost-effective results, releasing 40.21 g/L sugars. Three thermotolerant strains, namely, Bacillus coagulans DSM 2314, Geobacillus stearothermophilus DSM 2313, and G. stearothermophilus DSM 494, were assessed to produce lactic acid from vine-shoot hydrolysates under aerobic and non-sterile conditions, without the need of detoxification steps. In addition, wine lees were satisfactorily employed as nitrogen sources for the fermentation, providing similar results to yeast extract and being the only nutrient added to vine-shoot hydrolysates. Under optimal conditions, B. coagulans DSM 2314 produced 29.21 ± 0.23 g/L lactic acid in 24 h, with a sugar consumption of 98.74 ± 0.07% and a yield of 96.38 ± 0.76%, when supplemented with red wine lees. The purity of the isomer L( +) reached 97.59 ± 1.35% of the total lactic acid produced. Although G. stearothermophilus was able to transform the hexoses from vine-shoot hydrolysates into lactic acid, it proved to be inefficient for metabolising pentoses, thus obtaining lower lactic acid values (16–18 g/L).