Casein Micelle Structure, Functions and Interactions
In this chapter, we aim to present a consistent description of the structure and functions of caseins and the casein micelle. Function, it must be stressed, is interpreted here to mean both the biological functions of casein in the mammary gland and milk
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DE KRUIF AND
5.1
C.
HOLT
INTRODUCTION
In this chapter, we aim to present a consistent description of the structure and functions of caseins and the casein micelle. Function, it must be stressed, is interpreted here to mean both the biological functions of casein in the mammary gland and milk and the function in dairy foods. Thus, we tread a variation of a well-worn path in biology by exploring a structurefunction trichotomy. We do not accept the proposition that casein is a random coil protein with only a nutritional function, but argue that casein is likely to be as highly adapted to its biological function as any enzyme or structural protein. Accordingly, we derive a structure for the casein micelle from its perceived function in the sequestration of calcium phosphate in the secretory vesicles of the mammary gland. Intrinsic to this structure are the twin concepts of the rheomorphic (from the greek rheos, meaning stream and morphe, form; Holt and Sawyer, 1993; Blanch et al., 2002) protein and the calcium phosphate nanocluster that make the structure of the casein micelle as fascinating to us as that of any other protein assembly known to science. Other biological functions for the micelle as a transporter of calcium and phosphate and its gel-forming abilities relate more or less directly to its function in foods. The marked stability of the casein micelle to heat is a less direct consequence of the biological functions of casein for three main reasons. First, the rheomorphic caseins do not denature and aggregate on heating. Second, Ca-sensitive caseins are cemented into the micelle structure by strong linkages to the calcium phosphate and so the micellar structure is preserved on heating. Thirdly, lC-casein contributes to a diffuse, hydrophilic, hairy layer conferring steric stability on the micelle, even at elevated Advanced Dairy Chemistry Volume 1: Proteins, 3rd edn. Edited by P.F. Fox and P.L.H. McSweeney, Kluwer Academic/Plenum Publishers, 2003.
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STRUCTURE, FUNCTIONS AND INTERACTIONS
temperatures. In a similar way, the stability of micelles to ethanol, pH adjustment and renneting depends primarily on the properties of the hairy layer and its preservation. In treating each of these areas of stability we have pursued the goal of consistency provided by the theory of the salted brush to describe the interactions of micelles under good or poor solvent conditions. The dynamic aspects of the casein micelle are an intrinsic part of its structure and hence of its biological and food functions. The rheomorphic proteins and ions in the micelle respond to their environment and exchange with it on time scales that extend from the shortest times up to > 104 s. This aspect is of extreme importance for foods because casein micelles are association proteins that continue to react to changes in their environment over days and weeks and affect structure on every length scale from the Ato macroscopic gel structures. Our drive for consistency has meant some neglect of alternative and sometimes useful approaches to t
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