By combining ultraviolet-visible spectroscopy, three-dimensional excitation-emission matrix fluorescence spectroscopy, parallel factor analysis, methane production tests, and microbial community analysis, the team found that higher hydrothermal temperatures continuously promoted melanoidin formation, especially above 140 °C. These compounds showed a clear dose-dependent inhibitory effect: low doses reduced digestion efficiency and methane quality, while high doses triggered near-complete system collapse.
Food waste is rich in organic matter and is widely considered a promising substrate for anaerobic digestion, a process that converts biodegradable materials into methane-rich biogas. Hydrothermal pretreatment is often used to accelerate the breakdown of complex food-waste components and overcome the slow hydrolysis stage of digestion. However, higher pretreatment temperatures can also promote Maillard reactions between sugars and amino compounds, generating melanoidins. Because melanoidins are structurally complex, difficult to isolate, and lack standard reference materials, their formation and biological effects have remained hard to quantify. These uncertainties have limited the optimization of hydrothermal pretreatment coupled with anaerobic digestion.
A study (DOI: 10.48130/een-0026-0008) published in Energy & Environment Nexus on 21 April 2026 by Guangsuo Yu's & Lu Ding's team, East China University of Science and Technology, reports that melanoidins increase with hydrothermal temperature and inhibit methane production by disrupting methanogenic microbial communities.
The researchers first prepared simulated food waste composed of cooked rice, pork, and cabbage, representing starch-rich, protein-rich, and cellulose-rich components. The waste was hydrothermally treated at 120, 140, 160, 180, and 200 °C for one hour, and the resulting hydrothermal liquid products were analyzed. Dissolved organic carbon increased up to 160 °C, indicating enhanced hydrolysis of macromolecules, but declined at higher temperatures as polymerization and condensation shifted carbon into less soluble products. Meanwhile, pH decreased from 5.62 to 3.59 as temperature increased, showing stronger acidification. The team then used UV-Vis spectroscopy to assess aromaticity and color-related absorption. SUVA254 and absorbance at 350 nm both rose markedly when temperatures exceeded 140 °C, indicating increased formation of aromatic, brown-colored melanoidin-like compounds. To strengthen semi-quantitative characterization, they applied three-dimensional fluorescence spectroscopy and parallel factor analysis. The model identified two fluorescent components, with Component 1 assigned mainly to melanoidins. Its maximum fluorescence intensity increased continuously from untreated samples to the 200 °C group, reaching 374.14 a.u., confirming that melanoidins accumulated progressively with hydrothermal severity. The researchers next prepared food-waste-derived melanoidins and added them to anaerobic digestion systems at four dosage levels: 2.08, 4.16, 6.24, and 8.32 mg·mL−1. Low-dose additions reduced methane content and impaired digestion performance without causing full acidification failure. In contrast, high-dose additions sharply suppressed methane production by 98.15% and 99.24%, with methane content dropping below 10% and final pH values falling well below the optimal range for methanogens. Microbial sequencing showed that melanoidins reshaped bacterial and archaeal communities. Bacterial groups related to acid fermentation remained active, but methanogenic archaea, including key methane-producing taxa, were strongly disrupted. This suggests that melanoidins mainly block the methane-production stage rather than the early acid-production stage.
Overall, the study reveals that hydrothermal pretreatment is a double-edged strategy for food-waste resource recovery. While it can enhance hydrolysis and accelerate substrate breakdown, excessive thermal severity promotes melanoidin accumulation, which weakens methane production and may collapse anaerobic digestion systems. By establishing a semi-quantitative approach for tracking melanoidins and linking dosage effects to microbial responses, the work offers practical guidance for selecting hydrothermal conditions that balance organic-matter solubilization with biological digestibility. Future applications may include pretreatment-temperature control, melanoidin monitoring, and microbial management strategies to improve stable biogas production from food waste.
