Factors associated with spontaneous oxidized flavour in cow´s milk

Sammanfattning: Spontaneous oxidized flavour (SOF) is an off-flavour in milk that has received attention in only a few countries, primarily due to lack of monitoring. The poly unsaturated fatty acids in milk are considered to make it more prone to oxidize, with effects like shorter shelf life of dairy products and a reduction in consumer acceptance of milk. On the other hand, unsaturated fatty acids have a reputation of being healthier than the saturated fatty acids. Means to optimize the fatty acid composition of milk include breeding measures, why acyl-CoA:diacylglycerol acyltransferase1 (DGAT1), a major gene for milk fat content, has lately attracted a lot of research efforts. The risk of milk to develop SOF showed a strong and positive association with milk concentrations of both the well known prooxidant copper and its substrate the poly unsaturated milk fatty acids. The observed statistical interaction between copper and fatty acid composition suggests that SOF will not develop as easily in milk with high copper content unless sufficient amounts of substrate is available to promote the oxidation process. Breed and selection for fat content had an effect on the composition of milk fat where milk from Swedish Holstein cows generally had larger amounts of poly unsaturated fatty acids compared to the Swedish Red cows in the two selection lines in the SLU research herd. Selection for high milk fat content was associated with lower concentration of conjugated linoleic acid (CLA). The selection for fat content created differences in allele frequencies of the DGAT1 gene between selection lines, with a higher frequency of the 232A allele within the low fat line. DGAT1 genotype was associated with both composition of milk fat and occurrence of SOF, with the A allele being associated with a higher proportion of the polyunsaturated fatty acid CLA and an increased susceptibility of milk to develop SOF. The observed genetic variation in occurrence of SOF indicates that it may become a milk quality issue should bulls become selected that happen to carry genotypes predisposing for milk sensitive to oxidation.