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Milk samples were collected in early lactation (8–14 days in milk; DIM) and late lactation (199–326 DIM) from 11 cows to assess the differences in bovine milk fatty acid (FA) and triacylglycerol (TAG) composition, FA positional distribution in the TAG structure, and milk solid fat content (SFC). Cows in early lactation that were in negative energy balance, mobilised C16:0, C18:0, and C18:1cis9 from their body fat stores, increasing the concentrations of these FA in milk fat. These high concentrations of C18:0 and C18:1cis9 enhanced the synthesis of high molecular weight TAG and decreased milk fat SFC in early lactation. For both lactation stages, alterations in the total FA concentrations in the TAG structures resulted in changes in the abundances and the proportions of the FA at the sn-2 and sn-1 (3) positions in the TAG structure, yet without affecting the main esterification preferences of the FA.

Triacylglycerols (TAG) composition and structure in milk fat determine the physical, nutritional and functional characteristics in milk fat. TAG are composed of three fatty acids (FA) bound to a glycerol molecule. Therefore, changes in milk FA composition are expected to influence milk fat TAG composition and structure. Feed and animal related factors are known to affect milk FA composition. Feed related factors refer to alteration in the dairy cows’ feeding regimes, whereas animal related factors refer to factors such as lactation stage, breed, and genetics. This thesis aimed to assess the effect of feed and animal related factors on the variation in FA and TAG composition and structure and to determine its influence on the milk fat solid fat content (SFC). Milk fat SFC is one of the physical properties that define the suitability of a fat for a specific food product application. Therefore, it was selected as the physical property of study in this thesis to highlight the effect of milk FA and TAG variations.In chapters 2 and 3, the effect of the seasonal feeding regimes on the FA and TAG composition and structure were assessed in a non-seasonal milking system. Both chapters were based on weekly-pooled milk samples collected from fourteen dairy factories in the Netherlands, during one year, from May 2017 to April 2018. Seasonal FA and TAG composition variation were presented in chapter 2, where the largest FA variations were found between summer and winter, while spring and autumn were considered transition periods. The FA variations included a smaller proportion of de novo synthesized FA and an increased proportion of biohydrogenation products and conjugated linoleic acid (CLA) isomer FA in spring and summer. Moreover, the proportion of linolenic acid (ALA) in milk fat was greatest during late spring and summer, reflecting the increased intake of fresh pasture in these periods. The high availability of short-chain FA (SCFA) and C16:0 in winter increased the formati