Studier av koppar i mjölk : en prooxidant med negativa effekter på mjölkkvaliteten

Abstract: The taste of the dairy product is probably the main factor determining consumer's dairy products choice. Consequently it is of great importance that the product tastes good and as expected every time. Off-flavors in milk can be caused by for example oxidation of the milk fat. Since copper is a prooxidant, a high copper content in the milk can cause oxidation. The aim of this project was to investigate if there are differences between individual cows regarding the proportion of the copper totally excreted that was excreted through the milk compared to that excreted in faeces and urine. If there are differences between individual cows, this indicates a genetic influence and thus it may be possible to decrease the incidence of oxidized flavor in milk through breeding. Off-flavors in milk are defined as unpleasant smell and/or taste of the milk. Most of the off-flavors in milk are related to the fat component of the milk. Milk fat may oxidize spontaneously and thereby cause oxidized flavor in milk. The fatty acid composition, or more specifically the degree of unsaturation of the milk fat, influences the milk fat's sensitivity to oxidation. The stability of unsaturated milk fat is also influenced by the copper content in the milk, due to the pro-oxidative characteristics of copper. The problems with spontaneous oxidation of milk fat have, according to the literature, been found to increase during the winter and spring seasons. This increase is thought to partly depend on that at that time the feed have been stored for quite some time and thereby may have lost some of its content of antioxidants. Important antioxidants in this respect may be first and foremost vitamin E, but also beta-carotene (precursor to vitamin A) and vitamin C (ascorbic acid). The present study included eight cows of the Swedish Red and White dairy breed. The cows belonged to either of two selection lines, both selected for an equally high total production of milk energy. But one selection line was selected for a high milk fat content in the milk and the other line was selected for a low milk fat content. The cows were fed four different rations, containing 30 % or 50 % of hay or silage, respectively. Each ration was complemented with grain and concentrates to become comparable with respect to content of metabolizable energy and amino acids absorbed in the small intestine (AAT). All the cows were given all of the rations, but in a different order. Each ration was given during five weeks. The feed consumption was measured individually for each cow and analyses were made on milk, feces, urine and feeds (Gustafson, 2000). This study showed significant differences between the different cows regarding how much of the total excreted copper that was excreted in the milk, which indicates individual differences in copper metabolism. This can have a partly genetic background, which is supported by observations on a number of other species. The results indicate that it may be possible to reduce the problem with oxidized flavour in milk through selection and thereby exceed the shelf life of dairy products.