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Discipline: composition; Key words: conjugated linoleic acid, dietary protein, fatty acid, milk, cheese
Reducing the environmental impact of dairy farming and producing milk and cheese with improved nutritional characteristics are among the most important challenges for the industry today. Feeding dairy cows excess protein contributes to environmental nitrogen (N) pollution and could lead to unnecessary feeding expenditure due to the high costs of protein sources. However, the use of low-protein rations as a means of reducing N loss in the environment may cause changes in the composition of milk which should also be transferred to the cheese. It has been shown that a reduction in dietary crude protein (CP) content caused some alteration to the milk fatty acid (FA) profile, notably a decrease in the proportion of medium-chain FA and an increase in some trans isomers and some conjugated linoleic acid (CLA) isomers (bio-active FA). CLA isomers are associated with improved human health and when feeding rumen-protected CLA to cows CLA isomers do increase, but total milk fat may be depressed. Because dietary CP reduction has also been related to an increase in these bioactive milk FA, it is possible that dietary protein and CLA supplementation interact with respect to milk FA composition and FA yield. The aim of the study of Dr S. Schiavon and co-workers was therefore to investigate the consequences of reducing the dietary crude protein content, with or without a supply of protected CLA, on the milk FA yield and recovery in cheese ripened for 90 days. Their work was published in the Journal of Dairy Science Volume 99, pages 8759 to 8778, the title being: The influence of dietary nitrogen reduction and conjugated linoleic acid supply to dairy cows on fatty acids in milk and their transfer to ripened cheese.
In their study, 20 mid-lactation Friesian dairy cows were reared for four periods of three weeks each in groups of five, the design being a 4 × 4 Latin square. The cows were fed four different rations, consisting of a combination of two dietary CP levels, respectively 15% and 12% on a DM basis, with or without rumen-protected CLA. The CLA was fed at 80 gram per cow per day. Milk yield was recorded. Twice in each period, milk samples were analyzed for protein, fat and lactose and 10 litre milk samples (pooled by group) were processed to produce 96 cheeses. The cheeses were ripened for 90 days. Milk and cheese fat were analyzed for their FA profiles.
Dietary CP reduction had small or no effect on the yield and relative presence of FA in milk and cheese, except for a small increase in mid-chain branched saturated fatty acids. The CLA supply strongly reduced the yield of various categories of FA, and had major effects on the synthesis of short-chain FA, resulting in changes to the relative proportions of the various FA in milk and cheese. The addition of CLA uniformly reduced the recovery of all milk constituents and of short-, medium-, and long-chain FA groups, but large differences occurred among individual FA with apparent recoveries ranging between 640 and 1710 gram per kilogram. The highest recoveries were found for polyunsaturated long-chain FA and the lowest for saturated or mono-unsaturated short- or medium-chain FA. A notable rearrangement of these FA components, particularly the minor ones, took place during ripening. There was no interaction between CP level and the addition of CLA, suggesting a consistent outcome in the FA profile in the cheese.
Bottom line: The composition of nutritional attributes in cheese may change notably if alterations in cow diets are made or feed additives included. This may or may not influence the physico-chemical characteristics of the cheese which is not always realized in the manufacturing process.