QUANTITATIVE HIGH-RESOLUTION C-13 NMR ANALYSIS OF LIPIDS EXTRACTED FROM THE WHITE MUSCLE OF ATLANTIC TUNA (THUNNUS-ALALUNGA)

Lipids extracted from albacore tuna (Thunnus alalunga) were studied using high-resolution C-13 nuclear magnetic resonance (NMR) spectroscopy. Simultaneous quantitative determination of free fatty acids, lipid classes, and fatty acids alpha-beta positional distribution in triacylglycerols was obtained. Glyceride and phospholipid composition was studied using glyceryl resonances. Lipids extracted from fresh white muscles of Atlantic tuna had a different triacylglycerol and phospholipid content, related to fish weight. Free fatty acid carbonyl resonances were detected at the lower, field of the C-13 NMR carbonyl region and used for its quantitative determination. NMR data agreed with those obtained using UV spectrophotometry, showing that NMR is a suitable method to follow lipolytic alterations. The careful study of glyceryl and carbonyl resonances also leads to the direct determination of fatty acid alpha-beta distribution in triacylglycerols. The quantitative analysis of fatty acid composition and distribution was then achieved, and results obtained using high-resolution NMR were in good agreement with those obtained by gas chromatography. Polyunsaturated fatty acids showed a preferential location in the beta-acyl position, whereas monounsaturated fatty acids were found preferentially esterified in the alpha-position. The method may be used to monitor the composition and changes in fish lipids and oils during processing or storage.