Utilisation of microwave-NaOH pretreatment technology to improve performance and l-lactic acid yield from vinasse

Pretreatment of vinasse is necessary to break down its complex wooden structure, improve the efficiency of enzymolysis, and provide an adequate substrate for fermentation during l-lactic acid production. Independent microwave and NaOH pretreatment processes were carried out in an earlier study. Here a two-way microwave-NaOH coupled method, was investigated to determine the effects of simultaneous saccharification and fermentation (SSF) of vinasse on l-lactic acid production. Compared with the former individual microwave and NaOH pretreatment methods, hemicellulose and lignin in vinasse were more significantly reduced using microwave-NaOH coupled pretreatments, reducing crystallinity to 14.77%. l-lactic acid yield was also higher in the coupled method than the sum of the yields from the independent pretreatment processes. The optimal conditions for the coupled pretreatment method were: 0.06 g g^-1 (vinasse dry weight) NaOH; 523 W microwave power; 1:2 solid-liquid ratio; and 8 min microwave pretreatment time. These conditions resulted in an 86.0 mg g^-1 l-lactic acid yield. The microwave power and the amount of alkali used were approximately 25% lower than those used prior to the optimisation process. The reduced values indicate that optimisation can significantly decrease energy and reagent consumptions. Thus, microwave-NaOH coupled pretreatment exhibits a synergistic effect for improving l-lactic acid production.