Microwave processes: A viable technology for obtaining xylose from walnut shell to produce lactic acid by Bacillus coagulans

Abstract Biorefining uses alternative resources (such as biomass) instead of petrochemical sources to produce bioenergy and platform chemicals. Walnut shell is a promising lignocellulosic biomass for the production of fermentable carbohydrates. In this study, the platform chemical L-(+) lactic acid was produced after walnut shell had been treated with physicochemical and biological processes. So biomass waste can be regenerated to the benefit of the circular bio-economy. Autohydrolysis assisted by microwave processes was used to depolymerize hemicellulose and amorphous cellulose in walnut shell to yield 63.5% w/w of xylose in a liquid fraction (hydrolysate) with a minimum yield of glucose. The optimum reaction conditions to obtain a high xylose yield with a low byproduct titer were 190 °C for 25 min. L-(+) lactic acid was produced by thermophilic Bacillus coagulans DSM 2314 with a carbon source of xylose and glucose which were solubilized in hydrolysate from autohydrolyzed walnut shell. A non-sterile and straightforward fermentation medium coupled with a pH adjustment of the crude hydrolysate in an anaerobic environment led to a L- (+) lactic acid yield of 81% w/w with a productivity of 0.2 g/L/h.