The main goal of this experiment was to study the effect of milk fat depression, induced by supplementing diet with plant oils, on the bovine fat metabolism, with special interest in cholesterol levels. For this purpose 39 cows were divided in three groups and fed different rations: a control group (C) without any oil supplementation and two groups with soybean oil (SO) or rapeseed oil (RO) added to the partial mixed ration (PMR). A decrease in milk fat percentage was observed in both oil feedings with a higher decrease of -1·14 % with SO than RO with -0·98 % compared with the physiological (-0·15 %) decline in the C group. There was no significant change in protein and lactose yield. The daily milk cholesterol yield was lower in both oil rations than in control ration, while the blood cholesterol level showed an opposite variation. The milk fatty acid pattern showed a highly significant decrease of over 10 % in the amount of saturated fatty acids (SFA) in both oil feedings and a highly significant increase in mono (MUFA) and poly (PUFA) unsaturated fatty acids, conjugated linoleic acids (CLA) included. The results of this experiment suggest that the feeding of oil supplements has a high impact on milk fat composition and its significance for human health, by decreasing fats with a potentially negative effect (SFA and cholesterol) while simultaneously increasing others with positive (MUFA, PUFA, CLA).
It was possible to show a connection between the temporal variation of milk fat globule diameter, fat and cholesterol content in milk and the expression of candidate genes in the mammary gland epithelial cells in milk. The beginning of lactation corresponded with higher levels of fat and cholesterol in the milk as a result of a higher expression of key enzymes in the purified bovine milk epithelial cells, paralleled with an increase in milk fat globule mean size.
This study was performed to determine the effects of long-term freezing on milk composition and to evaluate the possible effects on analysis of four preservative substances (Azidiol, Kathon CG, NaOH and Thiomersal) on the determination of milk cholesterol, progesterone and lactoferrin concentration. Collected milk was separated in aliquots, stored at 20°C in 10mL plastic vials and analysed after 1, 6 and 12 months. It could be shown that the preservatives are not equally appropriate for all analysing methods used in this experiment. Preservatives and storage conditions for milk samples have to be carefully selected during the study design depending on the parameters to be measured.
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