Effect of Maillard reaction on the formation of humic acid during thermophilic phase of aerobic fermentation

• lard reaction was the main contributor to humic acid production during thermophilic phase. • Bacterial changes associated with humic acid formation were identified. • Humic acid formation was independent of microorganisms during thermophilic phase. • The increase of humic acid was consistent with the pattern of protein and reducing sugar consumption. This study aimed to explore the main pathway of humic acid (HA) formation during the thermophilic phase (TP) of aerobic fermentation, clarify the contribution of Maillard reaction. These experiments were carried out on cow dung, chicken manure and rice straw. Results indicated that the maximum temperature reached 60.2℃ during TP led to a sharp decrease in microbial abundance, while the production of HA increased. The network analysis indicated that microorganisms did not participate in the formation of HA and may be dominated by abiotic pathways. In addition, proteins and sugars were consumed at the highest rate during TP, and the trends were similar to HA formation. These findings suggested that the formation of HA has relationship to Maillard reaction, because TP provided suitable reaction conditions for Maillard reaction. Therefore, these results elucidated the contribution of Maillard reaction in HA formation during TP, and provided theoretical support for directional humification.