The non-enzymatic browning of pine bark during thermal treatment: Color and chemical changes, color kinetics and insights into mechanisms

Abstract

Thermal treatment is an essential modification method for agroforestry biomass, and it causes the most intuitive change in agroforestry biomass, a color change. But there is still a lack of color change mechanism due to the complexity of the thermal behavior. In this work, Masson pine bark was thermally treated at 80–200 °C for 3–9 h. The monosaccharide compositions of untreated pine bark and the content of cellulose, hemicellulose, lignin, total phenolic, and total sugar during thermal treatment were determined to investigate the substrates and pathways triggering non-enzymatic browning reactions. UPLC-HESI-MS/MS, TG, SEM, FTIR, XRD, and color analysis were performed to track the association between chemical changes, color changes and pyrolysis properties. The results showed that the colorimetric parameters L*, a*, and b* values decreased with temperature and time, while ΔE* values increased. The color changes in L*, a*, and b* values were well described by the combined color kinetic model, with corresponding zero-order activation energies of 8.06 kJ/mol, 2.79 kJ/mol, and 3.5 kJ/mol and first-order activation energies of 11.45 kJ/mol, 18.55 kJ/mol, and 18.66 kJ/mol, respectively. And the predictive ability of the combined color kinetic model and the reliability of kinetic parameters was verified at an extended temperature of 220 °C. Furthermore, chemical changes confirmed that the formation of colored compounds (such as 5-hydroxymethylfurfural, furfural, and (+)-Procyanidin B2) by the non-enzymatic browning reactions and the degradation of the colored polymers existed in thermal treatment. Together, insights into the non-enzymatic browning reaction mechanisms and color kinetics provided a scientific basis for controlling color quality and valorized application of agroforestry wastes.