Cholesterol and its oxidation products in dairy products: Analysis, effect of technological treatments, nutritional consequences.

Cholesterol oxides in foods can be analysed by high performance liquid chromatography (HPLC) with UV detection, thin layer chromatography (TLC) and gas chromatography (GC) with flame ionisation (FID) and mass spectrometry (MS) detection. The latter method in ''selected ion monitoring (SIM)'' mode was finally chosen for its numerous advantages: selectivity, sensitivity, repeatability, mildness, vast range of applications and robustness. This technique requires a multistep clean-up procedure : mild saponification of the sample, extraction of unsaponifiable material and purification on an aminopropyl cartridge. Elimination of lipid compounds and quantitative separation of cholesterol permit the simultaneous analysis of cholesterol and its oxides, This method was applied to measurement of cholesterol oxides in milk powder, infant formula, cheese, butter oil, lard and egg powder which could be modified by i) photo-oxidation, ii) exposure to long storage under oxygen, iii) exposure to heat treatment. The conclusion of this work is that, under proper conditions of processing, storage and heating, none of the products studied presents a hazard for the consumer. An important amount of cholesterol oxides were generated only in dairy products exposed to harsh conditions of storage where the factors ''oxygen and light'' or ''oxygen and low water activity'' coexisted (accelerated tests).