Casein micelles changes

Milk is a natural colloidal dispersion that contains casein micelles, self-assembled protein associates with a diameter of about 200 nm [20]. The casein micelles are protected against flocculation by an assembly of dense hairs (often called a brush ) at their surfaces. Polymer brushes can thus provide steric stabilization of colloids. For millennia, man used the fact that milk flocculates and gels when it is acidified, as in yogurt production. Below pH = 5 macroscopic flocculation of the casein micelles in milk is observed [21]. This means that the interactions between casein micelles change from repulsive to attractive. The explanation is that acidification leads to collapse of the casein brushes [22]. In cheese-making the steric stabilization is removed by enzymes, which induce gelation into cheese curd. 

These changes in the concentration of Ca2 + and pH lead to destabilization of the casein micelles. 

Figure 7.15 Equilibrium water content of (a) casein micelles and (b) sodium caseinate and casein hydrochloride as a function of pH and changing water activities (isopsychric curves)...

Changes in hydration. As would be expected from many of the changes discussed above, the hydration of the casein micelles decreases with the duration of heating at 140CC. The decrease appears to be due mainly to the fall in pH - if samples are adjusted to pH 6.7 after heating, there is an apparent increase in hydration on heating. 

Surface (zeta) potential. It is not possible to measure the zeta potential of casein micelles at the assay temperature but measurements on heated micelles after cooling suggest no change in zeta potential, which is rather surprising since many of the changes discussed above would be expected to reduce surface charge. 

All the heat-induced changes discussed would be expected to cause major alterations in the casein micelles, but the most significant change with respect to heat coagulation appears to be the decrease in pH - if the pH is readjusted occasionally to pH 6.7, milk can be heated for several hours at 140°C without coagulation. The stabilizing effect of urea is, at least partially,... 

Figure 10.3 Schematic representation of the rennet coagulation of milk, (a) Casein micelles with intact K-casein layer being attacked by chymosin (Q (b) micelles partially denuded of K-casein (c) extensively denuded micelles in the process of aggregation (d) release of macropeptides ( ) and changes in relative viscosity (0) during the course of rennet coagulation.

Quick cooling of either skim milk or whole milk that had been heated to 65 °C results in a temporary decrease of a few hundredths of a centi-poise in the viscosity. The data of Whitnah, et al. (1956) and Eilers et al. (1947) agree in indicating that this effect is due to a reversible change in the caseinate micelles. The relaxation of this change was exponential with time. This change could be related to the migration of /3-casein from the micelle. 

Beeby, R. and Kumetat, K. J. 1959. Viscosity changes in concentrated skim milk treated with alkali, urea, and calcium complexing agents. I. The importance of the casein micelle. J. Dairy Res. 26, 248-257. 

K-casein is physically inaccessible to enzyme. This theory is also supported by Creamer et al (1978), who, with an electron microscope, observed protein complexes formed by heating skim milk at 100°C for 30 min. These complexes are large, containing hundreds of individual protein molecules attached to casein micelles. At higher pH levels the complexes change, becoming more filamentous and associating less with micelles. 

Because of these chemical and physicochemical changes in the casein micelles upon cooling of milk, the milk becomes more viscous and displays an increased tendency to foam. Also, casein micelles in cold milk commonly exhibit incomplete coagulation upon acidification and treatment with rennet (Harper 1976 Muller 1982A Marshall 1982 Morr 1982). 

Casein exists in milk as a calcium caseinate-calcium phosphate complex the ratio of these components is approximately 95.2 to 4.8. The dispersed casein particles appear to be spherical m shape and of various sizes. The size distribution of the casein micelles is nol constant, hut varies with aging, heating, concentration, and other processing treatments. Processing alters ihe water-binding of casein and this in turn affects the apparent viscosity of products that contain casein Changes in hydration have not been measured quantitatively although the casein panicles of raw milk... 

Keenan, 1975 Neville et al., 1981 Watters, 1984 Virk et al., 1985), the presumption is that the formation of casein micelles is orchestrated with the transport of ions, the phosphorylation and glycosy-lation of the caseins, and lactose synthesis, such that the intravesicular ionic environment and casein concentration change continuously during the 20 min or so required for micelle assembly. Patton and Jensen (1975) observed, in electron micrographs, the same density of micellar particles in the alveolus as in mature vesicles, suggesting that, by this stage, the vesicular concentrations are virtually identical to those in the aqueous phase of milk. 

Protein Composition of Milk. Skim milk is a colloidal suspension of extreme complexity. The particulate phase, the casein micelles, consists primarily of a mixture of asi, as2, / , and x-caseins combined with calcium ions and an amorphous calcium-phosphate-citrate complex. The soluble phase contains lactose, a fraction of the caseins and calcium, and, in raw milk, the whey proteins, which are predominantly /3-lacto-globulin and a-lactalbumin. When milk is centrifuged at high speed (in our experiments, 30 min at 110,000 X gravity), the casein micelles sediment. This permits one to separate the two physical phases of skim milk and to measure changes in composition of the phases resulting from... 

In Situ Observation ofUHP-Induced Changes in Casein Micelles... 

Bulca, S., and Kulozik, U. (2004). Eleat-induced changes in native casein micelles obtained by microfiltration. In Advances in Fractionation and Separation Processes for Novel Dairy Applications. Bull. 389, International Dairy federation (IDE), Brussels, pp. 36-39. 

Generally a decrease in the pH of milk systems prior to heating results in more association of the denatured whey proteins to the casein micelle (Corredig and Dalgleish, 1996 Oldfield et al., 2000 Vasbinder and de Kruif, 2003). Even small changes in pH can shift the distribution of the association of the denatured whey protein with the casein micelle. For example, at a level of 95% whey protein denaturation, there is 70% of the denatured whey proteins associated with the casein micelle at pH 6.55 and this is decreased to 30% when the pH of milk prior to heating was... 

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