Impact of pH-mediated in situ enzymatic pretreatment on food waste decomposition and biomethane production: Performance, mechanism, and key metabolic pathways

pH value universally presents the great influence on hydrolysis efficiency of food waste (FW) by in situ enzymatic pretreatment, while the influencing mechanism of pH value on enzymatic decomposition of FW is not clear at present. This study innovatively in situ prepared a microbial-based compound enzyme (MCE) from FW, as well as systematically deciphered the impact and underlying mechanism of pH-mediated MCE pretreatment on FW decomposition and biomethane production. The dissolution, biodegradability and monomer formation of FW were found to be promoted by MCE pretreatment, and the highest hydrolysis efficiency was obtained at pH 5, while the biomethane yield was accordingly increased by 30.89 % compared to the control. The dissection of Fourier transform infrared spectroscopy (FTIR) results unraveled that the reaction order of FW components was protein → lipid → starch → cellulose in the acid-enhanced process, while the order shifted to starch → protein → cellulose → lipid in the alkali-enhanced process. Moreover, the looser protein structure was obtained after MCE pretreatment due to α-helix/(β-sheet + random coil) value was declined from 60.35 % to 57.01 %-49.03 %. These changes induced by pH-mediated MCE pretreatment reshaped the bacterial microbial community in favor of biomethane production, and the metagenomic analysis further demonstrated that MCE pretreatment had the highest abundances of genes related to metabolic functions of level 2 and 3 at pH 5, while the abundances of over 90 % key enzyme-encoding genes associated with hydrolysis, acidogenesis and methanogenesis were also increased during the anaerobic digestion process, therefore harvesting the highest biomethane yield.