d-Lactic acid production from cellulose: dilute acid treatment of cellulose assisted by microwave followed by microbial fermentation

Cellulose chemical robustness still remains a burden for value-added compounds production from biomass. The goal of this study was to take profit of the synergetic effects of dilute acid treatment and microwaves for the hydrolysis of highly crystalline cellulose. Afterwards, the liquid phase obtained after this treatment was treated microbiologically to transform the hydrolyzed product, containing glucose, into lactic acid. The hydrolysis reaction was performed in a microwave reactor. The samples were irradiated for 30 min up to 4 h at 400 watts maintaining the temperature at 393 K. At the optimal conditions, 88 % of cellulose was hydrolyzed. Glucose was the main obtained product. Through bio-fermentation Lactobacillus delbrueckii converted selectively all the present glucose into 98 % optically pure d-lactic acid, without suffering any inhibition from the rest of hydrolyzed products. The combination of cellulose hydrolysis under microwave irradiation and bio-fermentation at the conditions performed in this study opens a new alternative route to obtain valuable chemical platform products from cellulose.