Thermal oxidation of cholesterol: preliminary evaluation of 2-methyl-6-heptanone as volatile marker

Cholesterol oxidation in food and model systems is usually monitored by evaluating cholesterol oxidation products (COPs), but the analysis is time-consuming and expensive. Therefore, the determination of volatile compounds deriving from cholesterol thermoxidation could be valuable to identify a possible marker of its oxidative status. Cholesterol alone and in the presence of triacylglycerols (TAG) (tripalmitin (29%); tristearin (12%) triolein (53%); cholesterol (6%)), 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 volatile compounds were aldehydes, ketones, alcohols and hydrocarbons and they displayed 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-methylbutanale. All these volatile compounds increased with similar ratio in both model systems, but only a significant effect (p≤ 0.05) of cholesterol oxidation on 2-methyl-6-heptanone formation was found. To confirm that 2-methyl-6-heptanone could be suggested as possible volatile marker of cholesterol oxidative status, the 2-methyl-6-heptanone data were compared with the COPs content of each system. COPs formation was faster when cholesterol was heated alone than in presence of TAGs. The most abundat COPs was 7alfa-hydroxycholesterol, followed by 7-ketocholesterol, 7beta-hydroxycholesterol, beta-epoxycholesterol, alfa-epoxycholesterol, and 25-hydroxycholesterol. A significant correlation between the total COPs content and 2-methyl-6-heptanone amount was found when cholesterol was oxidized alone (R2=0.994) and in presence of TAG (R2=0.998). In conclusion, 2-methyl-6-heptanone could represent an easy and cheaper strategy for monitoring the oxidation of cholesterol; however, a deeper investigation on the relationship between volatile compounds and cholesterol oxidation in food matrices should be carried out.