Fat is one of the most important nutrients in milk; however, unsaturated fatty acids in milk, especially polyunsaturated fatty acids, are prone to oxidation. As transparent packaging has been increasingly applied by milk producers, the light-induced oxidation of milk fat has become one of the most important causes of the quality deterioration and rancidity of milk, which is detrimental to the flavor of milk. Furthermore, the oxidation products pose a huge threat to human health. This article reviews the mechanism, the main influence factors and the evaluation methods of milk fat photooxidation, which will provide a theoretical basis for evaluating and inhibiting photooxidation in milk, ensuring the quality of milk, and guiding the production, packaging and marketing of milk.

A comparative analysis of fatty acid contents between ultra-high temperature (UHT) milk and reconstituted milk was carried out using 1H nuclear magnetic resonance (1H NMR) spectroscopy. The results showed that the relative contents of linolenic acid, linoleic acid, oleic acid and saturated fatty acid in UHT milk and reconstituted milk were (0.98 ± 0.11)% and (0.90 ± 0.04)%, (1.15 ± 0.36)% and (1.11 ± 0.36)%, (27.30 ± 0.56)% and (26.78 ± 0.78)%, and (43.26 ± 1.56)% and (43.81 ± 2.34)%, respectively. Orthogonal partial least squares discriminant (OPLS-DA) analysis and independent sample t test were performed on the 1H NMR spectrum peaks and fatty acid contents of the two milks. It was found that the relative content of linolenic acid was significantly different (P < 0.05) between the two milks. Nutritional assessment showed that UHT milk was more suitable than reconstituted milk for people with hyperlipidemia and/or high cholesterol, while the nutritional value of other fatty acids was similar between both milks.

In this study, ultra-high temperature (UHT) milk and reconstituted milk were discriminated by high resolution quadrupole-time of flight mass spectrometry (LC-Q-TOF-MS) combined with chemometrics based on non-targeted metabolomic approaches. Sample pretreatment was performed prior to chromatographic separation on a C18 column, and the mass spectrometer was operated in the full scan mode. The obtained data were preprocessed before being imported into SIMCA-P 14.1 for principal component analysis (PCA) and partial least squares-discrimination analysis (PLS-DA). A total of 14 characterization factors were selected in both positive and negative ion modes to establish a discriminant model. The results showed that this model could be used as a robust approach to discriminate between UHT and reconstituted milk and provide a theoretical basis for the identification of reconstituted milk.