Abstract
The degradation and oxidation of quercetin, as affected by cholesterol during heating at 150 °C, was kinetically studied using non-linear regression models. Both TLC and HPLC were used to monitor the changes of quercetin, cholesterol and cholesterol oxidation products (COPs) during heating. The formation of COPs, including triol, 7-keto, 7α-OH and 7β-OH, was completely inhibited during the initial 30 minute heating period in the presence of 0.02% quercetin, accompanied by reduction in cholesterol peroxidation and degradation. However, the quercetin degradation or oxidation proceeded fast, with the rate constants (h−1) in the presence of nitrogen, oxygen and the combination of oxygen and cholesterol being 0.253, 0.868 and 7.17, respectively. When cholesterol and quercetin were heated together, the rate constants (h−1) of cholesterol peroxidation, epoxidation and degradation were 1.8 × 10−4, 0.016 and 0.19, respectively. The correlation coefficients (r2) for all the oxidative and degradation reactions ranged from 0.82–0.99. The kinetic models developed in this study may be used to predict the degradation and oxidation of quercetin as affected by cholesterol during heating.
Highlights
Flavonoids are ubiquitously distributed in food plants with approximately 90% occurring as glycosides in nature [1,2]
The kinetic study of the cholesterol oxidation pathway during heating has been well documented, showing the cholesterol oxidation is initiated by the second-order free radical chain reaction to form cholesterol hydroperoxides, followed by degradation to generate various cholesterol oxidation products (COPs), including 5,6 -EP, 5,6 -EP, 7 -OH, 7 -OH, 7-keto and triol [10,11,12,13]
A 0.02% level of quercetin was efficient in inhibiting most COPs formation, with a significant residual amount of quercetin over a 30-min heating period (Figure 1)
Summary
Flavonoids are ubiquitously distributed in food plants with approximately 90% occurring as glycosides in nature [1,2]. Quercetin and its glycoside derivatives are the most abundant in plant vegetables [2,3], which have been reported to be effective in scavenging free radicals during lipid oxidation [4,5,6]. The kinetic study of the cholesterol oxidation pathway during heating has been well documented, showing the cholesterol oxidation is initiated by the second-order free radical chain reaction to form cholesterol hydroperoxides, followed by degradation to generate various cholesterol oxidation products (COPs), including 5,6 -EP, 5,6 -EP, 7 -OH, 7 -OH, 7-keto and triol [10,11,12,13]. As the consumption of COPs in excess may be detrimental to human health [15,16], the inhibition of COPs formed during heating of cholesterol-rich foods is of great importance
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