Thermal oxidation of cholesterol: preliminary evaluation of 2-methyl-6-heptanone and 3-methylbutanal as volatile oxidation markers

Cholesterol oxidation in food and model systems is usually monitored by evaluating cholesterol oxidation products, but the analysis is time-consuming and expensive. Therefore, the determination of volatile compounds deriving from cholesterol thermoxidation could be valuable to identify other possible oxidation markers. Cholesterol alone and in the presence of a triacylglycerol mixture (tripalmitin, tristearin, and triolein) were thermoxidized at 170 °C for 15 min. In both model systems, the total volatile compounds increased three times when oxidation time rose from 5 to 15 min. The main classes of volatile compounds were aldehydes, ketones, alcohols and hydrocarbons, displaying a similar behavior in both systems. After 5 min of oxidation, 2-methyl-6-heptanone was the main volatile compound, followed by 3-methylpentane, 2,3-dimethyl-1-pentene and 3-methylbutanal. To verify if 2-methyl-6-heptanone could be used as volatile marker of cholesterol oxidation, data were compared with the total cholesterol oxidation products content of each system. A significant correlation between total cholesterol oxidation products content and 2-methyl-6-heptanone amount was found when cholesterol was oxidized alone (r2 = 0.994) and in presence of triacylglycerols (r2 = 0.998). When egg yolk was thermoxidized at 80 °C for 6 h, 3-methylbutanal was the volatile compound that better explained the oxidative trend in this food system, showing a significant correlation with cholesterol oxidation rate (r = 0.91). In conclusion, 2- methyl-6-heptanone and 3-methylbutanal could represent an easy and cheaper strategy for monitoring cholesterol oxidation in model systems and food samples, respectively; however, a deeper investigation on the amount and type of volatile compounds generated from cholesterol oxidation according to the food matrix, should be carried out.