Summaryβ-Casein is highly resistant to proteolysis in Cheddar cheese. A decrease in NaCl concentration reduced its resistance, but even in the absence of salt the amount of proteolysis of β-casein was slight. Proteolysis in Cheddar cheese increased when the moisture levels were raised by adding water. The relative susceptibility of β-casein to proteolysis by rennin was reduced considerably when the concentration of a sodium caseinate solution was raised from 10 to 20%. Sequestering the Ca2+by means of EDTA had no significant effect on proteolysis of β-casein. It would appear that the resistance of β-casein to proteolysis is due to the substrate rather than the enzyme and it is suggested that the reduced relative susceptibility to proteolysis is due to some concentration-dependent physical change in the casein molecule which renders the β-casein inaccessible. The salt concentration would also appear to influence this change.Cheddar-cheese flavour is largely independent of rennet concentration and it is possible to manufacture cheese of satisfactory quality using half-normal rennet levels.
A method for the partial fractionation of the a s -casein complex is described. It involves removal of K-casein from iso-electric casein by the method of Zittle & Ouster (1963) followed by fractionating the residue (a s -and /?-casein) by the urea procedure of Hipp et al. (1952). a s0 -and a sl -caseins precipitate from 3-3 M urea at pH 4-6, but much of the a s2 -, a^-and a s4 -casein remains in solution and co-precipitates with /tf-casein from 1-7 M urea at pH 4-9. Chromatography on DEAE cellulose clearly separates ^-casein from the a s2 -, a s3 -and a g4 -caseins, which are not resolved by the conditions employed. The method is suitable for the fractionation of isoelectric casein into all its major components (i) /c-casein; (ii) a s0 -and a sl -caseins; (iii) a s2 -, a s3 -and a s4 -caseins; (iv) /?-casein; (v) y-casein.Wake & Baldwin (1961) demonstrated the heterogeneity of the a s -casein fraction by electrophoresis on starch-urea gels. The proteins comprising this group had relative electrophoretic mobilities in starch or acrylamide gels of 1*16, 1-04, 1-00 and 0-86. In accordance with the recommendations of the Milk Protein Nomenclature Committee (Thompson et al. 1965) these proteins are now called a gl -, a^-, a g4 -and a g5 -casein respectively. Two additional members of the group, with relative electrophoretic mobilities of 1-19 (a s0 -) and 1-11 (a g2 -) have since been identified (ElNegoumy, 1967;Annan & Manson, 1969). The major constituent of the group, a sl -casein, was purified by and others, and has been characterized in considerable detail (Thompson et al. 1965;Rose et al. 1970). Very little work has been done on the minor a s -casein components and only recently have methods for their partial isolation been described. Annan & Manson (1969) fractionated as-casein on columns of Sulphoethyl Sephadex C-50 at pH 4-0 in the presence of 8 M urea into 3 fractions containing (i) a s0 -casein, (ii) a sl -casein and (iii) a s2 -, a s3 -and a g4 -caseins. They showed that a s3 -and a s4 -caseins were derived from a s5 -casein on reduction. Starting with the precipitate from the 50% ethanolammonium acetate purification of a sl -casein ), Hoagland et al. (1971 isolated a^-, a 84 -and a s5 -caseins by DEAE chromatography (0-01 M imidazole-HCl, 3-3 M urea, pH 7-0 buffer) and preparative gel electrophoresis. These latter authors confirmed the findings of Annan & Manson (1969) as to the origin of a g3 -and a g4 -caseins, and carried out some additional characterization studies.The present communication describes an alternative procedure for the fractionation of
The susceptibility of the components of various casemate systems (skimmilk, yff-casein-depleted milks, colloidal phosphate-free (CPF) milk, sodium caseinate and isolated y?-casein) to proteolysis was investigated. Isolated a si -and /?-caseins were quite susceptible to proteolysis, but their susceptibility decreased in heterogeneous soluble systems and even more so in heterogeneous aggregated systems. In skim-milk and /?-casein-depleted milks only about 50 % of both a 81 -and /?-casein was hydrolysable by high levels of rennin, and in CPF milk all a si -and 70 % of the /S-casein was hydrolysable. It is suggested that about 50 % of micellar /?-casein is firmly fixed within the micelle and is unavailable for proteolysis, while the remainder can dissociate from the micelle on cooling and is then readily hydrolysable.The compatibility of the data with the various published models of the casein micelle is discussed, and a modification of Rose's (1969) model is proposed.
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