Milk fat globule stability

The implementation of automatic milking systems (AMS) initially caused a lowering of the milk quality regarding free fatty acids (FFA). A high level of FFA increases the risk of rancid flavour in dairy products. The objective of the present thesis was to evaluate different factors in AMS which could potentially cause an increased level of FFA. The stability of the MFG was studied by using model systems. A correlation (r2=0.54) was found between the average diameter of the milk fat globule (MFG) and the diurnal fat yield of cows. The activity of a MFG membrane enzyme was found to decline with increasing average MFG size. The results of this work indicate that the MFGs grow larger when the fat synthesis increases, probably because of a limitation in the production of MFG membrane. This new, obtained knowledge was used to produce milk with various average diameters of MFGs. Three groups of cows were fed concentrates with different fatty acid compositions; one high in saturated fat, another high in unsaturated fat and the last one stimulating high de novo synthesis. The feedings resulted in milk with fat contents of 5.0, 3.7 and 4.0%, respectively. The MFGs were significantly larger in the milk with the highest fat content. All three types of milk were pumped at various shear rates and temperatures. Afterwards, measurement of particle size distribution showed that the highest coalescence of MFGs in the milk occurred with the largest MFGs. Moreover, the MFGs were more unstable at a pumping temperature of 31°C compared with lower temperatures. Likewise, an increase was found in the FFA content for milk with the largest MFGs, indicating that milk with a high fat content is more unstable when subjected to mechanical stress. The results recommended to cool raw milk to 5°C before pumping it from the milking unit to the milk bulk tank. The effect of milking frequency on FFA was studied because cows are more frequently milked in AMS. The level of FFA was significantly higher (1.49 meq./100g fat) in milk from the udder half milked four times daily compared with the milk from the udder half milked twice daily (1.14 meq./100g fat). This is ascribed to the fact that milk from the udder half milked four times daily contained MFGs with a significantly larger average diameter. The results are of great importance for further understanding of the mechanisms behind the increased content of FFA which is frequently observed in AMS