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Probing the location of casein fractions in the casein micelle using enzymes and enzyme–dextran conjugates

Published online by Cambridge University Press:  01 June 2009

Brian Chaplin  and
Margaret L. Green
Brian Chaplin
Affiliation:
National Institute for Research in Dairying, Shinfield, Reading, RG2 9 AT, UK
Margaret L. Green
Affiliation:
National Institute for Research in Dairying, Shinfield, Reading, RG2 9 AT, UK
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Summary

The rates of milk clotting and of formation of para-κ-casein in milk, colloidal phosphate-free milk and isolated κ-casein by pepsin and by its soluble size-fractionated conjugates with dextran were determined. Milk clotting with all the enzyme derivatives was dependent on the rate of the enzymic phase and required essentially complete κ-casein hydrolysis at 30 °C and throughout the range of pH 5·6–6·7. κ-Casein hydrolysis by pepsin at pH 6·6 was fastest in milk and slowest in isolated κ-casein, but the rate decreased as the enzyme size increased, especially with milk. When corrected for the changes in pepsin activity, the rates of κ-casein hydrolysis in all substrates were identical at 30 and 5 °C, but increased with decrease in pH, especially with the larger enzyme conjugates. Hydrolysis of the C-terminal bonds of β-and κ-casein in native and disrupted casein micelles by carboxypeptidase A and soluble conjugates of it were also investigated. κ-Casein was hydrolysed much faster, and β-casein slightly faster, in native than in disrupted micelles by the native enzyme. Increase in the size of carboxypeptidase A increased the rate of hydrolysis of κ-casein in disrupted micelles and also induced lag periods before hydrolysis commenced, especially with disrupted micelles. The results are compatible with a model for the casein micelle in which the κcasein is on the outside and the casein components are in a more ordered arrangement than in the casein complexes formed on micelle disruption. They also indicate that immobilized coagulants would be unable to clot milk.

Type
Original Articles
Information
Journal of Dairy Research , Volume 49 , Issue 4 , November 1982 , pp. 631 - 643
Copyright
Copyright © Proprietors of Journal of Dairy Research 1982

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