Caseina

5008

J. Agric. Food Chem. 1998, 46, 5008−5016

Effects of Acidification and Storage of Milk on Dissociation of Bovine Casein Micelles
Andrew J. R. Law* and Jeffrey Leaver
Molecular Recognition Group, Hannah Research Institute, Ayr KA6 5HL, Scotland, U.K.

The effects of acidification and storage of skim-milk on the rate and extent of solubilization of colloidal calcium phosphate anddissociation of caseins from the micelles were examined. As the pH was reduced, calcium phosphate was progressively removed from the micelles, and after each pH adjustment there was an initial rapid increase in serum concentrations of Ca and inorganic phosphate, but only a slight further increase on storage. The extent of disssociation of the caseins from the micelles on acidification wastemperature-dependent. At 30 °C, concentrations of serum casein remained low, but at 20 °C, and especially at 4 °C, the levels of all caseins increased in the serum as the pH was reduced to about pH 5.5 and 5.2, respectively. After each pH adjustment, there was an initial rapid increase in the serum concentrations of the caseins and a more gradual increase on storage up to 24 h. Keywords: Calciumphosphate; casein; dissociation; pH
INTRODUCTION

Above 30 °C, most of the casein in milk is in micellar form, closely associated with calcium phosphate and smaller amounts of Mg and citrate. The stability of the casein micelles and the processing characteristics of milk are affected by changes in temperature and pH. When milk is refrigerated and stored at 4 °C, a small amount of the colloidal calciumphosphate dissolves (Davies and White, 1960; Pierre and Brule, 1981), and ´ up to about half of the -casein dissociates from the micelles due to reduced hydrophobic interaction at the low temperature (Rose, 1968; Downey and Murphy, 1970). The remainder of the -casein is more tightly bound within the micelles together with the κ-, Rs1-, and Rs2-caseins and most of the calcium phosphate, and if themilk is rewarmed the changes in the composition and distribution of micellar and serum caseins can be reversed (Davies and Law, 1983). On reducing the pH of milk during the manufacture of products such as cheese, fermented products, or acid casein, the colloidal calcium phosphate and small amounts of Mg and citrate are dissolved, and below pH 5.0 removal of the Ca and inorganic phosphate (Pi)from the micelles is almost complete (Van Hooydonk et al., 1986; Dalgleish and Law, 1989; Le Graet and Brule, ´ 1993). The caseins also dissociate from the micelles on acidification (Snoeren et al., 1984; Van Hooydonk et al., 1986), but the extent of dissociation is temperaturedependent (Rose, 1968). Dalgleish and Law (1988) found that at 30 °C, even when most of the calcium phosphate is removedfrom the micelles, only a small amount of casein dissociates into the serum. At 20 °C, and even more so at 4 °C, the amount of serum casein increases considerably as the pH is reduced, reaching maximum levels at about pH 5.4 and 5.2, respectively. The combined effect of low temperature and low pH is more
* Author to whom correspondence should be addressed [fax, +44 (0)1292 671 052; e-mail,lawa@hri.sari.ac.uk].

than additive in causing dissociation of caseins from the micelles. As the pH is further reduced the amount of casein in the serum decreases due to isoelectric precipitation. Although the changes in the micellar-serum equilibria of the minerals and caseins on acidification of milk at various temperatures are well-documented, there is comparatively little information about therelative rates of solubilization of the colloidal calcium phosphate and dissociation of the caseins from the micelles. In most of the above studies milks were acidified, and changes in the serum concentrations of minerals and proteins determined after a fixed time interval. In this study we have measured the changes in the levels of Ca, Pi, and individual caseins in the serum immediately after...