Seasonal variation in fatty acid and triacylglycerol composition of bovine milk fat.

The aim of this study was to assess the effects of seasonal variation on the changes of the fatty acid (FA) and triacylglycerol (TAG) composition of bovine milk fat (MF) in a nonseasonal milking system. Weekly milk samples were collected from 14 dairy factories and pooled per week as representative samples of the average Dutch bovine milk. The sample collection started in May 2017 and finished in April 2018, resulting in a total of 52 samples, corresponding to each week of the year. The samples were analyzed for MF content (%) and FA and TAG composition using gas chromatography with flame-ionization detection. The increased intake of C18:3 cis-9,12,15 through grass feeding in spring and summer was associated with major changes in MF FA composition, including reduced proportions of de novo synthesized FA and presence of several rumen biohydrogenation products and conjugated linoleic acid isomers in MF. These changes in seasonal FA composition had an effect on TAG seasonal variation. The TAG seasonal variation showed that all TAG groups were significantly different between months. The low molecular weight and the medium molecular weight TAG groups increased in winter and decreased in summer, whereas the high molecular weight TAG groups increased in summer and decreased in winter. Based on pooled monthly samples, MALDI-TOF-mass spectrometry allowed the analysis of even- and odd-chain TAG species in MF based on their total carbon number and number of double bonds. These analyses indicated saturated TAG species to be greatest in winter, whereas monounsaturated, polyunsaturated, and odd-chain TAG species were greatest in summer. Our study showed that TAG seasonal variation in a nonseasonal milking system is influenced by the variation in FA composition throughout the seasons.