Effect of Ethanol and Lactic Acid Pre-fermentation on Putrefactive Bacteria Suppression, Hydrolysis, and Methanogenesis of Food Waste

Lactic acid pre-fermentation is widely used as an effective method for the bacteriostatic preservation of food waste, and ethanol pre-fermentation can alleviate acidification and maintain system stability of methane fermentation. This study investigated the effects of methods of ethanol and lactic acid pre-fermentation on bacteriostatic preservation and anaerobic digestion. Results showed that high levels of lactic acid production reduced the pH and decreased the populations of Staphylococcus aureus and Escherichia coli by 99.9 and 99.8%, respectively. Meanwhile, serious acidification during methane production caused the gas production to halt. Ethanol pre-fermentation did not lower the pH of the system but still decreased the S. aureus and E. coli populations by 98.5 and 96.5%, respectively, compared to the control group. This trend was attributed to the presence of some metabolites in the pre-fermentation broth as proven by the Oxford cup method and the selective-medium method. In addition, the total volatile fatty acid content as well as the propionic and acetic acid concentrations were the lowest with ethanol pre-fermentation. However, methane production was 43.9 and 49.6% higher in the ethanol pre-fermentation group than lactic acid pre-fermentation and control groups, respectively. Therefore, ethanol pre-fermentation is an effective treatment that can benefit the bacteriostasis of food waste while promoting hydrolysis, alleviating acidification, and stabilizing the fermentation system to improve the methane production rate.