MCT Oil Coating Improves the Oxidative Stability of Surface Lipids in Corn Extrudates

Lipid-containing extrudates are highly susceptible to lipid oxidation because of their porous structure and high specific surface area. The objective of this study is to investigate the effect of a low-oxidizable medium-chain triglycerides (MCT) coating on the oxidative stability. Therefore, cornmeal is extrusion cooked with commercial vegetable oil and various water contents (10–18% on dry matter) and partially coated with 5% w/w MCT oil. Lipid oxidation in the extrudates is examined during storage at +40 °C by measuring hydroperoxide concentrations in different lipid fractions (surface, inner-surface, and matrix-incorporated lipids). Hexanal is analyzed using headspace-gas chromatography. Coating allocation is studied by fatty acid profiles of the lipid fractions, by fluorescence microscopy, and computerized microtomography. Application of an MCT coating leads to lower hydroperoxide concentrations and significantly reduces hexanal formation during storage. In particular, the surface lipids are stabilized. MCT oil diffuse only to small extent inside the extrudates and its effect on lipid oxidation can be attributed to differenct effects: filling surface microcracks, reducing the release of volatiles, and dilution of oxidizable lipids. This effect is strongest in porous extrudates with low feed-water content. In this study, we examine whether it is possible to improve the oxidative stability of extrudates by application of a lipid-based coating. The results of this study suggest that it will be possible in the future to produce extrudates that are protected from oxidation by a thin natural oil film. We expect that especially products such as kibbles could benefit from treatment with a low-oxidizable lipid-based coating. In these products, fats are already used in combination with other functional ingredients. The combination of an MCT coating with a palatant or antioxidant is, therefore, an obvious alternative and of high relevance for manufacturers.