Milk lipids characterization in relation to different heat treatments using lipidomics

Highlights

• Heat treatment caused a significant reduction of milk TGs, especially TGs with DB ≥ 2.

• UHT treatment led to the most significant glycerophospholipids hydrolysis.

• Heat treatment promoted lipid oxidization and oxidized lipid hydrolyzation.

• Lipids can be used to monitor different heat treatments of milk.

Abstract

Heat treatment is an important processing technique related to milk quality and nutritional value in the dairy industry. In this study, changes in milk lipids in response to different heat treatments were comprehensively characterized using a lipidomic approach. Ultra-performance liquid chromatography coupled with quadrupole time-of-flight mass spectrometry (UPLC-Q-TOF-MS) were used to identify and quantify 29 classes and 788 different lipids. In general, heat treatment promoted milk lipid hydrolysis and oxidation; in particular, ultra-high temperature (UHT) treatment resulted in more phospholipid hydrolysis than did pasteurization and extended shelf-life (ESL) treatment. Heat treatment resulted in further lipid oxidation reactions and a reduction in the amount of mild oxidation products. Moreover, the levels of lysophospholipids and free fatty acids (including oxidized free fatty acids) can be used to distinguish UHT-treated milk. In turn, oxidized phosphatidylcholine, oxidized phosphatidylethanolamine, ether-linked phosphatidylethanolamine, diacylglycerol, triacylglycerol, and oxidized triacylglycerol can be used to differentiate raw, pasteurized, and ESL milk. These biomarkers can potentially be used in the dairy industry to monitor the degree and method of heat treatment of milk.

Introduction

Milk is an important nutrient source in the human diet. One of the main components of milk solids, lipids account for 3%–5% of the total milk composition. Milk lipids profoundly impact the properties and quality of milk and are also important nutrients (Liu, Li, Pryce, & Rochfort, 2020b). Fat in milk exists in the form of fat globules. Triacylglycerols (TGs) are located in the core of the fat globules and account for more than 98% of the total milk fat (Liu, Li, Pryce, & Rochfort, 2020a). The TG core is wrapped in the milk fat globule membrane (MFGM) composed of phospholipids, glycolipids, and proteins (Gallier, Gragson, Jimenez-Flores, & Everett, 2010). Polar lipids in the fat globule membrane have a significant impact on human health (Kosmerl, Rocha-Mendoza, Ortega-Anaya, Jimenez-Flores, & Garcia-Cano, 2021), such as improving neurodevelopment, reducing the risk of cardiovascular diseases, and regulating cholesterol absorption (Silva et al., 2021, Snow et al., 2010). Lipids in milk are attracting increasing attention.

To extend milk storage time and ensure its safety, heat treatment has become a necessary step in commercial milk production (Li et al., 2021, Zhang et al., 2018). However, heat treatment leads to the deterioration of sensory quality (Oupadissakoon, Chambers, & Chambers, 2009) and changes in milk lipid composition (Jadhav, Annapure, & Deshmukh, 2021). Heat treatment causes casein and whey protein to bind to the MFGM surface through the disulfide interchange reaction, which causes structural changes in the MFGM (Sharma, Oey, & Everett, 2015). Moreover, heat treatment affects fat globule stability and lipid digestion (Lund, Nielsen, Nielsen, Ray, & Lund, 2021). Although pasteurization does not cause significant changes in the milk fatty acid (FA) composition, it significantly increases the levels of oxylipins derived from arachidonic acid and 18-carbon polyunsaturated fatty acids (PUFAs; linoleic acid and α-linolenic acid) (Pitino et al., 2019). In addition, studies on the volatile components of milk have shown that heat treatment leads to the formation of methyl ketones in milk, which impacts milk flavor (Reis et al., 2020). The influence of heat treatment on milk FAs has been extensively studied, but the biological function of lipids and their potential nutritional value are usually related to specific lipid species or even the stereo-structures of lipid molecules (Contarini & Povolo, 2013). Therefore, it is necessary to comprehensively characterize lipid changes caused by heat treatment at the molecular level.

The development of high-resolution mass spectrometry has greatly promoted qualitative and quantitative studies of milk lipids (Liu et al., 2020b). Several reports have proven the feasibility of using liquid chromatography-quadrupole time-of-flight mass spectrometry (LC-Q-TOF-MS) for non-targeted milk lipidomics, including identifying 411 species of lipids in human milk (Zhao et al., 2021) and characterizing human milk phospholipid profiles at different lactation stages (Song et al., 2021). In addition, differences in lipid composition between infant formula and human milk were discovered using a similar method (X. Zhang et al., 2021). However, there is still a lack of research using this technique to explore the relationship between heat treatment and lipid changes in milk.

In this study, we used ultra-high performance (UHP) LC-Q-TOF-MS/MS to comprehensively characterize the lipid composition of milk after processing using different heat treatments. Our results revealed changes in milk lipids specific to the heat treatment and provide a reference for identifying the thermal-processing degree of milk.