Casein micelles of different size, larger casein micelle and smaller casein micelle, were prepared from skim milk by differential centrifugation and treated with rennet.The firming rate of rennet curd of smaller micelle was found to be higher than that of larger micelle.The firmness of rennet curd of smaller micelle was higher than that of larger micelle. The curd firmness of both samples was found to increase with the micellar concentration: roughly proportional to the square of the concentration.The microstructure of rennet curd was examined using scanning electron microscopy. Renneted smaller micelle were more strongly clustered or fused with one another than larger micelle.Jpn. J. Zootech. Sci., 55 (6): [409][410][411][412][413][414][415] 1984
The temperature and concentration dependent association of /?-casein was studied by means of viscometry, gel filtration chromatography, electron microscopy, analytical ultracentrifugation and UV difference spectrophotometry. Degrees of polymerization of 12, 22 and 49 and free energies of association of -21, -23 and -25 kj/mole monomer were found at temperatures of 10, 15 and 20 °C respectively in 0-2 M Na phosphate buffer pH 6-7.Monomeric yff-casein was not a completely random coil but became more compact with increasing temperature, due to hydrophobic interactions.One of the most interesting properties of /?-casein is that it forms polymers at room temperature while it exists as monomers at low temperatures. Numerous investigations on the temperature-dependent association behaviour of /^-casein have been conducted but details about the shape and size of /?-casein are still not clear. The present study is designed to clarify this problem.yS-Casein was prepared by the method of Hipp et al. (1952) and further purified by DEAE-cellulose chromatography. /9-Casein was then dissolved in a 0-2 M-phosphate buffer, pH 6-7, containing 0-02% (w/v) Na azide.Viscosities were measured with an Ostwald viscometer. The intrinsic viscosity was 22-3 ml/g at 4-9 °C. This value is similar to that of a denatured protein and so /?-casein exists in a nearly random form at 4-9 °C. Huggins 1 constants ranged from 1-08 to 1-78 between 4-9 and 34-6 °C. The reduced specific viscosity displayed a definite downward curvature at /?-casein concentrations lower than 0-3% (w/v) at 30 °C. The intrinsic viscosity was 4-6 ml/g as found by extrapolation to infinite dilution from concentrations between 0-1 and 0-3% (w/v). This value is near to that of a typical globular protein.Gel filtration, to study the concentration-dependent association of /?-casein at concentrations between 015 and 3-19 % (w/v) at 30 °C, was performed on a Sepharose CL-6B column (2-17 x 62 cm) in 0-2 M-phosphate buffer, pH 6-7. The elution patterns showed that both monomers and polymers existed at concentrations between 1 and 2 % (w/v), but only monomers existed at a concentration of 045 % (w/v).Electron micrographs of polymerized /?-casein at 30 °C showed the polymers to be spherical in shape, with 10-20 nm diam.Association-dissociation behaviour of /?-casein was analysed thermodynamically by the sedimentation equilibrium method at 2, 10, 15 and 20 °C in a 0-2 M-phosphate buffer, pH 6-7. One curve, independent of the initial concentration, was obtained at each temperature when the apparent weight-average molecular weight of /?-casein was plotted against concentration. This suggests that a reversible associationdissociation equilibrium exists. The results also show that association of /?-casein IO-2
Composition and properties of glyco macropeptides (GMP) released from k-casein A by rennin, were studied. GMP soluble in 2% or 12% trichloro-acetic acid were fractio nated to five portions on DEAE-cellulose column chromatography. Each fraction was rechromatographed, lyophilized, and analyzed for contents of sugars and amino acid compositions. Contents of sugars in the fractions increased with the increasing concen tration of buffer and molar ratio of galactose : galactosamine: sialic acid in each fraction was approximately 2: 1 : 1. The first fraction consisted of only four kinds of amino acids and other four fractions consisted of eleven kinds of amino acids, but molar ratios of amino acid residues in each fraction were different.
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