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Heat stability of milk: influence of colloidal and soluble salts and protein modification on the pH-dependent dissociation of micellar κ-casein

Published online by Cambridge University Press:  01 June 2009

Harjinder Singh  and
Patrick F. Fox
Harjinder Singh
Affiliation:
Department of Food Chemistry, University College, Cork, Irish Republic
Patrick F. Fox
Affiliation:
Department of Food Chemistry, University College, Cork, Irish Republic
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Summary

Reducing the colloidal calcium phosphate (CCP) content of milk by 40% or increasing it by 20% did not significantly affect the heat-induced pH-dependent dissociation of micellar κ-casein. However, changes in soluble Ca and phosphate affected the dissociation of κ-casein markedly; decreasing the phosphate concentration or increasing the Ca concentration reduced the formation of non-sedimentable N (NSN) and non-sedimentable 12% TCA-insoluble N-acetyl-neuraminic acid (NANA). Dialysis of milk against water for short periods (∼ 5 h) reduced the formation of both NSN and non-sedimentable 12% TCA-insoluble NANA, as did NaCl at concentrations above 0·05 M. Modification of protein amino groups by succinylation promoted the release of κ-casein while amidation of carboxyl groups had the opposite effect. It appears that the pH-dependent dissociation of κ-casein produced on heating milk above 90°C is controlled by electrostatic interactions. The effects of soluble ions such as Ca2+ or Na+ appear to be due to shielding of such negatively-charged groups as seryl phosphate and carboxyl on the protein, thus reducing the release of κ-casein.

Type
Original Articles
Information
Journal of Dairy Research , Volume 54 , Issue 4 , November 1987 , pp. 523 - 534
Copyright
Copyright © Proprietors of Journal of Dairy Research 1987

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