beta-Lactoglobulin isolated from milk of cow, sheep, and goat had about 0.5 mol of fatty acids bound per mol of monomer protein. Fatty acids, mainly palmitic and oleic acids, were the major components (about 75% of total lipids). Albumin isolated from the same samples had about 4.5 mol of fatty acids bound per mol of protein. These two proteins were the only whey proteins able to bind labeled fatty acids in vitro. Interaction of beta-lactoglobulin and albumin with insolubilized fatty acids showed some differences, suggesting different structures of the respective fatty acid binding sites.
The changes in insulin concentration in bovine milk in the first period of lactation and its association with other milk proteins were studied. Highest concentration was found in the first milking (327 ng/ml). This concentration fell within the first 24 h postpartum to about 50% of its initial value. By d 3, the level was about 25%, and, on d 7, a stable concentration was reached at approximately 46 ng/ml (about 14% of its initial value). This concentration is about 100 times higher than that in serum, which suggests a specific mechanism of transfer from blood to milk. Colostral whey obtained by ultrafiltration or ultracentrifugation contains much less insulin than colostrum. When colostrum or milk was incubated with [125I]insulin and whey and casein fractions were separated by precipitation, it was observed that most insulin remained with the casein. However, when colostrum was incubated with [125I]insulin and subjected to gel filtration, most of the radioactivity corresponded to free insulin, indicating that insulin is associated with the precipitated casein but not with the casein micelles in solution.
Adulteration of ewes' milk with cows' milk is a wellknown fraud. It can become a serious problem in cheese manufacture as the composition of the coagulated milk influences the organoleptic characteristics of the final product. Analytical methods for detecting cows' milk in ewes' milk or cheese are based on the composition of milk fat or milk protein (Ramos & Juarez, 1984). Gas liquid chromatography has been used to analyse the fatty acid composition of fat from milk mixture and cheese (Benassi, 1963), but this method is useless when adulteration is with skim milk. Moreover, the sensitivity of the method is low, only allowing detection of the addition of > 10% cows' milk (Ramos etal. 1977). Methods of milk protein analysis include mainly immunological and electrophoretic techniques. The latter are based on the different electrophoretic mobility of ewes' and cows' milk caseins (Assenat, 1967). However, adulteration in aged cheese is difficult to detect because of the complexity of the electropherograms, due to proteolysis occurring during the ripening process (Grappin et al. 1985). Most of the immunological methods currently used in these analyses are based on precipitation reactions between whey proteins, mainly immunoglobulins, and specific antibodies (Bernhauer et al. 1983; Elbertzhageng & Wenzel, 1982). While these methods are suitable for distinguishing milk from different animal species, the immunochemical reactivity of whey proteins is affected by heat treatment (Ramos & Juarez, 1984), while the antigenicity of the heat stable caseins is poor. Antisera against caseins have low titres, so precipitation techniques are often unsuitable for detecting cows' milk in ewes' milk or cheese. This paper describes a method based on immunodotting techniques, which does not require high titre antisera, and its application to the detection of cows' milk in ewes' milk cheese. MATERIALS AND METHODS Preparation of whole casein Bovine or ovine milk was defatted by centrifugation at 2000 g for 15 min at 4 °C. Skim milk was acidified to pH 4-6 with 1 M-HC1. Whole casein was recovered by centrifugation for 10 min at 2000 g, washed and neutralized with 0-5 M-NaOH to pH 70. The above operations were repeated twice to eliminate any remaining whey proteins.
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