Relationship between fat globule size and fatty acid composition in ewe’s milk

It is well know that the fatty acid composition of milk is affected by several factors and probably also milk fat globule size is influenced by quantity and type of fatty acids available to synthesise milk fat. Finding regarding the relationship between fat globule size and fatty acid composition are not totally in agreement because of the criteria currently used to define the fat globules size classes of milk are not the same for all the authors, making it difficult to compare the results. Furthermore, trials were nearly always carried out on cow milk. The aim of present study was to verify if sheep milk fat globule size is linked to the milk fatty acid composition. Bulk milk from Massese ewes were collected and morphometric analysis of the fat globules (number of fat globules ml-1 of milk and diameter) and milk fat extraction were performed. Milk fat was analysed for milk fatty acid composition. The frequency distribution of total measured milk fat globules was evaluated according to their size. Fat globule diameters were divided into nine classes of 1.5 μm width: from class 1 (1.5-3 μm) to class 9 (>13.5 mm). Finally, fat globules were further classified into two groups: group S (1.5-6 μm), and group L (>6 mm). The results show that regard the transition from size of 5 μm to 6 μm of milk fat globules diameter there is an inversion of the sign of correlations; moreover, some fatty acids shows a greater number of significantly correlation with milk fat globules size: milk with a higher percentage of globules bigger than 6mm are positively correlated with C18:3 w3, C23:0 and C24:0 and negatively with C10:0, C11:0, C12:0, C13:0, C14:0, C16:1, C17:0 anteiso, C18:1 cis 11, C18:1 cis 12. When milk samples contained higher levels of milk fat globules with diameter smaller than 6_m, a positive correlation was observed with saturated fatty acids, that can explain also the positive correlations between globule size and atherogenic index of milk. In order to better understand the different results regarding the relationship between milk fatty acid composition and milk fat globule size, more knowledge are needed regarding both source and synthesis pathway of fatty acids and possible different control mechanism in various species of ruminant. However, as previously observed for cow’s milk, the diameter of 6_m of fat globules may be considered a threshold value for the definition of same nutritional parameters of milk.