Formation of melanoidins and development of characterization techniques during thermal pretreatment of organic solid waste: A critical review.

• Heat treatment can easily lead to Maillard reaction. • The structure and molecular weight of melanoidins are related to substrate, temperature, pH and other factors. • The characteristics and common characterization methods of melanoidins are studied. • High doses of melanoidins show obvious inhibitory effect on anaerobic fermentation. Melanoidins are Maillard products produced in the thermal pretreatment of organic solid waste represented by food waste and sludge. Their effect on anaerobic fermentation is related to energy recovery efficiency. Based on the researches of melanoidins in food field, this review paper aims to improve the understanding of melanoidins in waste treatment and anaerobic fermentation. The formation mechanism of melanoidins is complex, and the molecular weight and structure are not systematic and standardized. During the Maillard reaction process, the basic characteristics of melanoidins are affected by the type of reactants, temperature, pH and the presence of metal ions. In addition, the physicochemical and biological properties of melanoidin itself involve the mechanism explanation of melanoid behavior. Antioxidant, antimicrobial and metal chelating abilities are related to the inhibition or promotion of melanoidins in anaerobic fermentation, but the specific mechanism (such as cytotoxicity or the effect on interspecific electron transfer) still needs to be further explored. Finally, the characterization of melanoidins can provide more accurate semi-quantitative information with the development of analytical technology. The colorimetric method, ultraviolet/visible (UV/Vis) spectrophotometry, excitation-emission matrix joint region integration method and parallel factor analysis method developed from the color and structure characteristics of melanoidins have become the commonly used characterization techniques. In this review, the formation mechanism and influencing factors of melanoidins from organic solid waste by thermal pretreatment were investigated, the characteristics and characterization techniques of melanoidins were reviewed, and the possible ways of melanoidins affecting high quality products were put forward, which provided a reference for the efficient utilization of organic solid waste by thermal pretreatment coupled with anaerobic fermentation. [ABSTRACT FROM AUTHOR]