Abstract
Lipid oxidation is one of the major causes of food spoilage for lipid-rich foods. In particular, oil-in-water emulsions, like mayonnaises and spreads, are prone to oxidation due to the increased interfacial area that facilitates contact between the lipids and hydrophilic pro-oxidants present in the water phase. Polar, amphiphilic lipid species present at the oil/water interface, like the mono- (MAGs) and di-acylglycerols (DAGs), act as oxidation starters that initiate subsequent oxidation reactions of the non-polar lipids in the oil droplets. A comprehensive two-dimensional liquid chromatography (LC×LC) method with evaporative light-scattering detection (ELSD) was set up to study the composition of the complex mixture of oxidized polar and non-polar lipids. The LC×LC-ELSD method employs size exclusion chromatography (SEC) in the 1D (1st dimension) to separate the various lipid species according to size. In the 2D (2nd dimension), normal-phase liquid chromatography (NPLC) is used to separate the fractions according to their degree of oxidation. The coupling of SEC with NPLC yields a good separation of the oxidized triacylglycerols (TAGs) from the large excess of non-oxidized TAGs. In addition, it allows the isolation of non-oxidized DAGs and MAGs that usually interfere with the detection of a variety of oxidized products that have similar polarities. This method facilitates elucidating how lipid composition affects oxidation kinetics in emulsified foods and will aid in the development of more oxidation-stable products.
Highlights
Lipid oxidation in food products is a crucial problem that causes undesirable changes in a food’s flavor, texture, nutritional value and reduces its shelf life
The separation system envisaged here would separate the sample according to the different size classes of lipids present in the first dimension (1D) and subsequently separate the various oxidation products within each size group in the second dimension (2D)
size exclusion chromatography (SEC) and normal-phase liquid chromatography (NPLC) present a satisfactory degree of orthogonality, since SEC separates the sample molecules according to size with little or no contribution of polarity, whereas NPLC separates according to polarity with just a limited size influence [19]
Summary
Lipid oxidation in food products is a crucial problem that causes undesirable changes in a food’s flavor, texture, nutritional value and reduces its shelf life. Oil-in-water emulsions, such as mayonnaises, salad dressings and infant formulas are among the most widely consumed lipid-rich foods [1,2]. In these oil-in-water emulsions, lipid droplets are dispersed in a continuous water phase, and stabilized by emul-. Sifiers such as free fatty acids and mono- and di-acylglycerols (MAGs and DAGs), proteins and phospholipids. In such food products, lipid oxidation generally proceeds from the exterior of the oil droplet (interface) to the interior, making it important to understand how the compounds present at the interface impact oxidation kinetics [3]. Analysis of the various lipid classes and their oxidation products is key
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