Understanding small and large milk fat globule phenotype variation in dairy cows through milk lipidomic characterisation

© 2019 Leonie WalterMilk fat globules (MFGs) are spherical structures comprising a neutral lipid core that is surrounded by a three-layer membrane. This MFG membrane is of nutritional benefit for infants and adult consumers. Individual variation in the size of MFGs is observed within a herd of the same breed, and this milk production trait, if selected for through breeding programs, could be exploited for a more targeted milk production for specific technological streams. For example, large MFGs are desirable for butter making, whereas small MFGs are preferred for cheese making and direct consumption due to improved sensory properties and increased relative abundance of the beneficial membrane material. The initial experiment of this work aimed to determine how much the average MFG size is affected by on-farm and animal related factors within a herd subjected to the same diet and environmental conditions. Milk fat globule size of the whole herd was repeatedly measured over a one-year period and the effects of these parameters were estimated using a linear mixed effect model. This analysis showed that stage of lactation, parity and milk yield can affect MFG size, while the impact of fat yield, concentrate intake and number of milkings per day was limited. However, the individual variation within the herd outweighed the effect of individual factors, supporting the possibility of a genetically determined regulation of MFG size. Based on the data collected for the first experimental chapter, cows were selected for the second and third experiment, which aimed to characterise the small and large MFG phenotypes through an in-depth lipidomics analysis. This analysis included the characterisation of the fatty acid (FA) profile of the MFG core by gas chromatography and the identification of the whole milk lipidome through targeted liquid chromatography tandem mass spectrometry. The analysis of the MFG core FA profile revealed that the cows with the small MFG (SMFG) phenotype produced milk with higher proportions of unsaturated FAs compared to large MFG (LMFG) cows. This was related to an increased uptake of preformed FAs with a chain length of 18 or more carbons, which are sourced directly from the diet or from lipid mobilisation. This characteristic of the SMFG group could potentially lead to the production of milk with an improved nutritional profile. The results of the third experiment present the most extensive milk lipidomic analysis in the literature to date, with 301 detected lipid species. The results also revealed, for the first time, a potential role for ether phosphatidylethanolamine (ePE) in the regulation of MFG size, showing a higher relative abundance of ePE in the milk from LMFG cows. Ether PEs can reduce the fluidity of biological membranes and are predicted to promote lipid droplet fusion. The milk from SMFG cows, on the other hand, contained higher total phosphatidylcholine (PC) to PE ratios and a higher relative abundance of unsaturated PC species, both attributes that are predicted to prevent lipid droplet fusion. In the final experiment of this thesis an in vitro model using cells purified from raw milk and grown on permeable membrane supports was established, which offers the potential to test some of the novel findings of this thesis in future experiments