Heat stability of milk: influence of colloidal calcium phosphate and β-lactoglobulin

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Abstract
Summary Reduction of the level of colloidal calcium phosphate (CCP) progressively increased the heat stability of milk at pH values <~7·0 and increased the pH of maximum stability. Removal of 40% CCP also stabilized the system at the pH of minimum stability, but removal of ≥60% CCP rendered milk very unstable at pH values >7·2, an effect not offset by a 4-fold increase in κ-casein concentration. Doubling CCP had a slight destabilizing effect in the pH range 6·5–7·5. Addition of β-lactoglobulin to serum protein-free casein micelles had a marked destabilizing effect at pH values > ~6·8, but increased stability in the pH range 6·4–6·8. β-Lactoglobulin had a similar and more apparent effect on the heat stability of Na caseinate dissolved in milk diffusate. It is suggested that rather than being a stabilizing factor responsible for the maximum in the heat stability-pH curve, the true effect of β-lactoglobulin is to shift the curve to more acid pH values (reason unknown) and to sensitize the caseinate system to heat-induced Ca phosphate precipitation at pH values > ~7·0. Low stability at ~pH 7·0 introduces an apparent maximum in the heat stability-pH curve at ~pH 6·8, but this has no independent existence. At pH values >7·2, increased protein charge more than off-sets the influence of heat-precipitated CCP and stability again increases in micellar but not in soluble casein systems.