Determination of 10-hydroxystearic, 10-ketostearic, 8-hydroxypalmitic, and 8-ketopalmitic acids in milk fat by solid-phase extraction plus gas chromatography-mass spectrometry

Abstract

Fatty acids (FA) bearing oxygenated functions and present in esterified form in triacylglycerols are widespread in nature but very little is known about their occurrence in dairy products. A method based on gas chromatography (with flame ionization detector and mass spectrometry detectors), including the previous isolation of polar FA methyl esters by solid-phase extraction, was applied to quantify oxygenated FA in milk fat. Samples obtained from ewes and goats fed with a variety of oil sources were studied. Fatty acids identified were 8-ketopalmitic, 8-hydroxypalmitic, 10-ketostearic, and mainly 10-hydroxystearic acids. The highest levels of 10-ketostearic acid were obtained in milk from animals fed olive oil (up to 1.5%) and from those fed long-chain n-3 FA-enriched diets (0.5–1.0%). In all samples, 10-hydroxystearic acid, not reported so far in milk, was the second most abundant oxygenated FA (up to 0.8%). The high correlation obtained between contents of 10-ketostearic and 10-hydroxystearic acids would confirm the existence of a common pathway of formation in the rumen, whereas the presence of 8-ketopalmitate and 8-hydroxypalmitate could be putatively attributed to mechanisms of β-oxidation in the tissues. The influence of cis-9 C18:1 and trans-10 C18:1 as precursors of these compounds in milk and the metabolic pathways involved in their formation are discussed.