Lactic Acid Production from Lignocellulosic Residues: A Case Study Toward Process Integration

In this study, the production of lactic acid from municipal forestry and greening wastes processed by steam explosion (SE) and enzymatic hydrolysis is addressed. The liquid fraction hydrolysate (LFH) obtained after filtration of the steam-exploded slurry (previously subjected to detoxification) and the enzymatic hydrolysate resulting from the water insoluble solid fraction (WIS-derived hydrolysate, WDH) are studied as substrate for lactic acid fermentation using an integrated strategy. The fermentation process of WDH using Bacillus coagulans and Lactobacillus rhamnosus shows similar sugar conversion yields of about 0.9 g g⁻¹ (based on consumed glucose). In contrast, the use of detoxified LFH (d-LFH) completely inhibits B. coagulans, making necessary a previous dilution with the WDH to trigger fermentation when using this microorganism. The best integrated strategy using both d-LFH and WDH streams and B. coagulans as fermentative strain has a 1:3 d-LFH:WDH ratio, yielding 0.93 g lactic acid g⁻¹ of consumed sugars and a lactic acid concentration of about 50 g L⁻¹.