Empirical Kinetic Modelling and Mechanisms of Quercetin Thermal Degradation in Aqueous Model Systems: Effect of pH and Addition of Antioxidants

Quercetin (Qt) is a natural flavonoid of high biological significance, and it occurs in a wide variety of plant foods. Although its oxidation by various means has been extensively studied, its behavior with regard to thermal treatments remains a challenge. The study described herein aimed at investigating Qt thermal decomposition, by proposing an empirical sigmoidal model for tracing degradation kinetics. This model was employed to examine the effect of addition of antioxidants on Qt thermal degradation, including ascorbic acid, L-cysteine, and sulfite. Furthermore, degradation pathways were proposed by performing liquid chromatography-tandem mass spectrometry analyses. Upon addition of any antioxidant used, the sigmoidal course of Qt thermal degradation was pronounced, evidencing the validity of the empirical model used in the study of similar cases. The antioxidants retarded Qt degradation in a manner that appeared to depend on Qt/antioxidant molar ratio. No major differentiation in the degradation mechanism was observed in response to the addition of various antioxidants, and in all cases protocatechuic acid and phloroglucinol carboxylic acid were typical degradation products identified. Furthermore, in all cases tested the solutions resulted after thermal treatment possessed inferior antioxidant properties compared to the initial Qt solutions, and this demonstrated the detrimental effects of heating on Qt. The empirical model proposed could be of assistance in interpreting the degradation behavior of other polyphenols, but its validity merits further investigation.