Cholesterol oxidation products in milk: Processing formation and determination

Cholesterol oxidation products (oxysterols) are commonly present in foods derived from animals. The main limitations for analysis of these products are associated with the long sample preparation protocols that require saponification. In this research, a protocol was optimized for determination of oxysterols in milks under the process to obtain given commercial milk products. UHT milks, powder milks, condensed milk, and evaporated milk were selected for this study. The levels of the different oxysterols are discussed attending to the type of milk. UHT and skimmed powder milks provide the highest concentration of oxysterols. Particularly, high concentrations were found for 7β‐hydroxy‐cholesterol (up to 205.6 ng/mL), 5‐cholesten‐3β‐ol‐7‐one (up to 21.8 ng/mL), cholesterol 5α,6α‐epoxide (up to 14.9 ng/mL), and 25‐hydroxycholesterol (up to 5.1 ng/mL). An increase in cholesten‐3β‐ol‐7‐one and cholesterol 5α,6α‐epoxide, resulting from the skimming process, was also observed. Additionally, two different heating modes, microwave‐assisted and conventional heating, have been mimicked to study their influence on the formation of oxysterols in milk. Both heating protocols contributed to the formation of oxysterols but short periods of microwave irradiation (60 s) can lead to levels of oxysterols similar to conventional heating for 5 min. Practical applications: The results of the research described in this article, supported by previous studies in the field, allow to advice about the strong influence of microwave heating of milk on the formation of toxic oxides from the lipid fraction. Although conventional heating can be more tedious and time consuming, it seems to be healthier.