Acetic acid as catalyst for microwave-assisted pretreatment of sugarcane straw aids highly specific butyric acid bioproduction

Abstract Sugarcane straw was pretreated with acetic acid in a microwave-assisted process for further use in fermentative butyric acid production by Clostridium beijerinckii Br21. An experimental design with temperatures between 170 and 220 °C and acetic acid concentrations between 0.3 and 0.9 % (w/v) provided the optimal conditions (220 °C and 0.3 % acetic acid (w/v)) for maximum total sugar recovery (74 %). Three hydrolysates originated from sugarcane straw pretreatment under optimal conditions were used as substrates for fermentation to produce butyric acid. Hydrolysate 1 was the liquid fraction resulting from pretreatment; Hydrolysate 2 was the enzymatic hydrolysate of the pretreated solid fraction, whereas Hydrolysate 3 was the enzymatic hydrolysate obtained from the whole slurry. In all the fermentations in which the hydrolysates were employed, butyric acid was the only product that was generated by C. beijerinckii Br21: 19.5 g/L was the maximum concentration obtained, and the yield was 0.46 g of butyric acid per g of sugar. C. beijerinckii Br21 was also able to consume the acetic acid that was used in the pretreatment step. Therefore, sugarcane straw pretreatment by microwave in the presence of acetic acid is a promising method to obtain highly specific butyric acid bioproduction.