Dynamic formation of melanoidins and parallel competitive pathways during sludge thermal hydrolysis affected by heating temperature

The melanoidins formed through the Maillard reaction during thermal hydrolysis pretreatment (THP) of sludge contribute to darkening, refractoriness, and inhibition of effective wastewater and sludge treatment. However, the dynamic production and structural evolution of THP-derived melanoidins are not fully understood. This study aimed to investigate the factors influencing melanoidins production and the associated mechanisms during THP. Response surface methodology analysis revealed that temperature had the greatest impact on melanoidins production compared to heating time and initial pH. Comparing treatments at 135 °C, 165 °C, and 195 °C, melanoidins production initially increased with rising temperature and then declined. Structural changes and substance transformations during melanoidins production at different temperatures were elucidated via two-dimensional correlation Fourier-transform infrared spectroscopy and gas chromatography-mass spectrometry, respectively. Few competitive reactions occurred at 135 °C; however, carbonization competed with the Maillard reaction for substrates at 165 °C, while carbonization and caramelization led to substrate competition and degradation of the already formed melanoidins at 195 °C. These findings shed light on the dynamic process and mechanisms underlying melanoidins production during THP, and provide fundamental insight into melanoidins regulation for future research.