Effects of pretreatment on the microbial community and l-lactic acid production in vinasse fermentation

Microwave–alkali and steam–alkali coupled pretreatments were carried out to improve the yield and optical purity of l-lactic acid produced using vinasse fermentation. Lactobacillus casei was inoculated into the system to initiate fermentation. Polymerase chain reaction denaturing gradient gel electrophoresis was used to analyze the microbial community during fermentation with and without the pretreatments. The original bacterial genus in vinasse was essentially inactivated, whereas L. casei became the dominant genus after 24 h of fermentation. The system subjected to microwave–alkali coupled pretreatment released more reducing sugars and produced more lactic acid (up to 30.32 g/L), which is twice that without pretreatment. In addition, the proportion of lactic acid in the organic acids also increased. The optical purity of the l-lactic acid produced under the microwave–alkali coupled pretreatment reached 91%, which is 2% higher than that under the steam–alkali coupled pretreatment and 7% higher than that under the control conditions. Therefore, the microwave–alkali coupled pretreatment is an effective method for the highly efficient bioconversion of vinasse into bioenergy.

► Microwave–alkali and steam–alkali coupled pretreatment could effectively inactivate indigenous flora. ► Lactobacillus casei inoculated to the vinasse quickly became the dominant flora, the optical purity of l-lactic acid improved. ► Yield and optical purity of l-lactic acid with microwave–alkali coupling pretreatment increased by 49.8% and 57.2%, respectively. ► Vinasse with microwave–alkali coupling pretreatment had a stronger ability to release reducing sugars continually, which provided a richer nutrition source.