Short communicationDetermination of triacylglycerols in milk fat from different species using UPLC–Q-TOF–MS
Introduction
In recent years, researchers have been interested in the composition of triacylglycerols (TAGs) in dietary fats because of their influence on physiology and nutrition (Ali et al., 2019; Yener & van Valenberg, 2019; Zhou et al., 2014). Milk fat is a complex source of TAGs, and a number of factors such as species, climate, diet, and stage of lactation may greatly influence its chemical composition (Ali et al., 2019; Beccaria et al., 2014; Kanwal, Ahmed, & Mirza, 2004). The vast number of fatty acids (FAs) that can be esterified to the three hydroxyl groups of the glycerol backbone, with varying chain lengths and a varying number of double bonds (DBs), resulting in a wide variety of TAGs (Beccaria et al., 2014). Due to many unique TAGs and their related positional isomers, determining TAGs composition in milk lipid is difficult. However, chromatographic techniques may separate these TAGs based on their molecular weight and degree of saturation of FAs (Ali et al., 2019).
TAGs in milk lipids have been identified via various of approaches (Ali et al., 2019; Beccaria et al., 2014; Fontecha, Goudjil, Ríos, Fraga, & Juárez, 2005; Lísa, Holčapek, & Boháč, 2009; Ross, Hansen, & Tu, 2011; Zhang, Wei, Tao, Jin, & Wang, 2021; Zhou et al., 2014). For analysis of TAGs, sample preparation and analysis should be done with care to avoid lipid oxidation and hydrolysis (Milinsk, Matsushita, Visentainer, Oliveira, & Souza, 2008). Both reversed-phase liquid chromatography (RPLC) and silver-ion-chromatography (SIC) have been used to characterise TAGs in the fat fraction of mozzarella cheese (Romano, Giordano, Chianese, Addeo, & Musso, 2011). Separation of TAGs by RPLC is based on the PN (partition number), which is equal to the carbon number (CN) of the three FAs minus two times the number of DBs [PN = CN – (2 × DB)] (Beccaria et al., 2014). Separation of TAGs by SIC is based on saturation degree and the DB position or configuration in each FA (Dugo, Favoino, Tranchida, Dugo, & Mondello, 2004).
Due to its rapid and sensitive separation and precision in mass measurements, ultra-performance liquid chromatography coupled with quadrupole time-of-flight mass spectrometry (UPLC–Q-TOF–MS) has been used to investigate complicated lipid matrices. This approach was recently developed and has been applied to the study of TAGs in single cell oils (Zhang et al., 2016). Application of UPLC–Q-TOF–MS for the characterisation of TAG composition of fresh milk fat from different species (camel, cow, goat, yak, and donkey) is scarce. In this work, UPLC–Q-TOF–MS was used to investigate the TAGs composition of milk fat from different species, such as camel, cow, goat, yak, and donkey. The identification of TAG composition in this study was based not only on the carbon number but also on the number of double bonds. The results obtained in this work would provide an in-depth insight into milk fat, which may also contribute to its further nutritional evaluation.
Section snippets
Materials
A standard TAG mixture (glyceryl tridecanoate:glyceryl tridodecanoate:glyceryl trimyristate:glyceryl trioctanoate:tripalmitin at 1:1:1:1:1, weight ratios, 17811-1AMP) was purchased from Sigma–Aldrich Chemicals (Shanghai, China). HPLC-grade methanol (>99.9%) and n-hexane (>99.9%) were bought from J&K Scientific, Ltd. (Beijing, China). All other chemicals were of analytical and chromatographic grade and were obtained from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). Donkey and yak milk
Results and discussion
Observations on the fatty acids occurring in the TAGs of milk fat from the different species studies here were reported in our previous study (Karrar et al., 2022), which highlighted large interspecies differences. Milk fatty acid composition of the different species was found to have three major fatty acids, oleic acid (C18:1), palmitic acid (C16:0), and myristic acid (C14:0). Camel milk had the highest level of C18:1 n-9, while donkey milk had higher levels of C18:3 and C18:2. Donkey and
Conclusions
In this study, TAG composition profiles of cow, yak, donkey, goat, and camel milk fat were characterised. Differences in levels of TAGs were seen among milk sources with camel milk outscoring other types due to its highest levels of TAGs with longer carbon chains and more unsaturation. The reported TAG identities could provide valuable information about the nutritional value of milk from different species. Further studies to investigate the effects of age, season, lactation stage, feed, as well
Declaration of competing interest
None.
Acknowledgements
This work was supported by the National Natural Science Foundation of China, China (32150410365).
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