Composition of the sterol fraction of caprine milk fat by gas chromatography and mass spectrometry

Abstract

The sterol fraction of milk is of nutritional interest because high levels of cholesterol in plasma (modulated by the cholesterol ingested) are associated with an increasing risk of cardiovascular disease. In addition, some sterols (ergosterol and 7-dehydrocholesterol) are provitamins (D2 and D3 respectively). At the same time, through the study of the sterol fraction, vegetable fats can be detected in milk and dairy products. Sterols are a minor fraction of total milk fat, the main sterol being cholesterol (3 mg/g fat, equivalent to 100 mg/l cows' milk). Small quantities of other sterols (7-dehydrocholesterol, 22-dehydrocholesterol, ergosterol, fucosterol, lanosterol, lathosterol, 24-methylenecholesterol) and several phytosterols have been reported in cows' milk (Walstra & Jennes, 1984). International Dairy Federation (1992) states that in the sterol profile of genuine milk fat there may appear, in addition to the peak of 7-dehydrocholesterol which ranges from 0·7 to 4% of total sterols, < 1% of minor sterols with retention times corresponding to phytosterols.Values for the cholesterol content of goats' milk vary considerably, from 211 mg/l (Pantulu et al. 1975) to 125 mg/l (Lu, 1993), partly owing to the use of different analysis techniques. Some of these values were obtained using non-specific colorimetric methods, which are inaccurate in the presence of cholesterol precursors or phytosterols (Clark et al. 1983; Haugh & Harzer, 1984). Some minor peaks have been assumed to be sterols but have not been identified (García-Olmedo & Barrera, 1985).Conventional methods of sample preparation for sterol analysis prior to gas chromatography (GC), which involve saponification of fat with or without isolation of the sterol fraction by thin layer chromatography, are tedious and time-consuming. Transesterification with KOH–methanol has been successfully used as a rapid alternative for obtaining the unsaponifiable fraction.This paper describes the identification of sterols (cholesterol and other minor sterols) in goats' milk fat using an alkali-catalysed transesterification procedure prior to GC and GC–mass spectrometry (GC–MS) analysis.