Abstract 41

Triacylglycerol represents the major lipid fraction of human milk and it has been shown to undergo spontaneous hydrolysis following prolonged storage or freezing and thawing of milk. The present study was undertaken to determine the characteristics and major lipid products of spontaneous hydrolysis of human milk. METHODS: Aliquots of fresh milk (n=5) were pasteurized or treated with either 3mM eserine (known to inhibit bile salt-stimulated lipase activity) or 10mM sodium taurocholate, prior to storage at -20°C or -70°C. Hydrolysis of triacylglycerol was then induced by repeated freezing and thawing of the milk samples. Lipids were extracted using chloroform/methanol and fractionated by thin layer chromatography. The fatty acid composition of lipid fractions was determined by gas chromatography. RESULTS: Hydrolysis was completely inhibited by pasteurization, reduced by the addition of eserine and was enhanced by the presence of bile salt. In the presence of bile salt, there was a preferential hydrolysis of oleic acid (26.9 ± 1.1 vs 31.8 ± 1.0, p < 0.005, expressed as % by weight in the remaining triacylglycerol fraction). Oleic acid (48%) and linoleic acid (25%) were the predominant free fatty acid products and palmitic acid was the predominant fatty acid in the monoglyceride fraction (55%). CONCLUSIONS: These results indicate that spontaneous hydrolysis of human milk triacylglycerol is most likely due to bile salt-stimulated lipase and that the activity of this lipase may be altered by storage conditions and the presence of bile salt. This may have implications for lipid digestion in preterm infants who may be deficient in bile salts.