Determination of lipids in infant formula powder by direct extraction methylation of lipids and fatty acid methyl esters (FAME) analysis by gas chromatography

Fatty acid methyl esters (FAMEs) of pure triglyceride standards, oils, and fat from dry matrixes were formed by transesterification using sodium metho xide in methanol-hexane. FAMEs were produced by direct addition of sodium m ethoxide-hexane to samples and heating to simultaneously extract and transe sterify acyl lipids. FAMEs were quantitated by capillary gas chromatography (GC) over a fatty acid concentration range of 0 to 1.7 mg/mL (r greater th an or equal to 0.9997). Total fat was calculated as the sum of individual f atty acids expressed as triglyceride equivalents, in accordance with nutrit ion labeling guidelines. Saturated, polyunsaturated, and monounsaturated fa ts were calculated as sums of individual free fatty acids. Absolute recover ies determined from individual fatty acids in test samples ranged from 69.7 to 106%, Recoveries (relative to the C13:0 internal standard) for individu al fatty acids in test samples ranged from 95 to 106%, Reproducibility was constant at each fatty acid level in the reaction mixture (n = 5, coefficie nt of variation [CV] <2%). Absolute recovery determined from the sum of tot al fatty acids in standard reference material (SRM) 1846 (powdered infant f ormula) was 96.4%. Analysis of SRM 1846 gave results that agreed closely wi th the certified fat and fatty acid values. Analysis of commercial infant f ormula gave results that were comparable to those obtained with AOAC Method 996.01. The direct extraction methylation procedure is rapid, and the tran sesterification of acyl lipids to form FAMEs is complete within 15 min. Cla ssical saponification and refluxing are not required. This method provides FAMEs free of interferences and easily quantitated by GC or confirmed by GC /mass spectrometry (MS). Unambiguous MS identification of individual FAMEs derived from pure standards, SRM 1846, and powdered infant formula product was obtained.