Resin-Contact-Time Method for Determination of Protein-Bound Calcium in Milk and Model Systems

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Abstract
A theory for exchange of Ca between milk or model milk-protein system and an appropriately buffered weak cation-exchange resin was developed, which could be used for determination of protein-bound or colloidal calcium. With suitable preconditioning of the resin, the resin-contact-time method yielded results for ionic and protein-bound calcium in a model .beta.-casein system which not only conformed to the proposed theory but also agreed with those by the murexide method of Sundararajan and Whitney. .beta.-Casein dispersions (3%) in calcium-potassium chloride solutions at pH 7.0, .GAMMA./2 0.14, and 2.degree. C bound calcium as if there were 11.20 binding sites/molecule, each with an intrinsic association constant of 76.62 l/mol. Data from skim milk samples at 23 and 2.degree. C by the resin-contact-time method also were consistent with the proposed theory and yielded protein-bound or colloidal calcium in reasonable agreement with reported results.