Lipolysis and Oxidation in Ultra‐High Temperature Milk Depend on Sampling Month, Storage Duration, and Temperature

Abstract

During storage, some factors (for example, storage duration and temperature) can affect milk stability and consumer acceptability. Thiobarbituric acid reactive substances (TBARSs), lipid classes, and fatty acid profiles in stored ultra-high temperature (UHT) milk were analyzed to assess the effects of storage time and temperature on lipid oxidation and lipolysis. With storage duration up to 12 months, the milk fat phase was separated and showed high levels of oxidation and lipolysis, manifested as increased levels of TBARS and free fatty acids. High oxidation levels decreased the percentage of unsaturated fatty acids (UFAs) in triacylglycerol and phospholipids. Higher storage temperatures (20, 30, and 37°C) resulted in a higher degree of fat aggregation, oxidation, and lipolysis compared with refrigerated storage (4°C). Additionally, sampling month of raw milk (May, July, and November) affected the lipid profiles of UHT milk during storage, with more UFA oxidized in July than in the other 2 months.This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. Food ChemistryLipid oxidation and lipolysis in stored UHT milk . . . separated off and subsamples were used for analysis (for details of the experimental design, see Figure A.1 in the Supplementary Material). Confocal laser scanning microscopy analysisThe samples were prepared as previously described (Lu et al., 2018), by mixing 0.5 mL of UHT milk with 5 µL of Nile Red stock solution (0.1% Nile Red in acetone, Sigma-Aldrich, St Louis, MO, USA) for 15 min at room temperature. The final mixture (20 µL) was placed on a concave microscope slide for confocal laser scanning microscopy. A Zeiss LSM 780 confocal laser scanning microscope (Jena, Germany) was used for microstructure analysis. A 488 nm Argon laser was used as excitation source and the fluorescence emissions of wavelengths between 500 and 530 nm were detected. Images were acquired using a C-Apochromat 63× oil immersion objective (NA 1.32) with a resolution of 1,024 × 1,024 pixels.

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