Heat stability of milk: role of β-lactoglobulin in the pH-dependent dissociation of micellar κ-casein

Abstract

On heating casein micelle systems containing /Mactoglobulin (/Mg) at 90 °C for 10 min, /Mg complexed with casein micelles at pH < 6-9, probably as a result of interaction with K-casein via sulphydryl-disulphide interchange, and co-sedimented with the micelles on ultracentrifugation. Complex formation with /Mg appeared to prevent the dissociation of micellar /c-casein on heating. However, at pH ^ 69, /ccasein//Mg complexes dissociated from the micelles on heating, thus enhancing the release of micellar /c-casein. High concentrations of /Mg (^0-8%) induced coagulation at pH 7-3, essentially by promoting the dissociation of micellar /c-casein. It appeared that a sl -, a s2 -, /?-and /c-caseins dissociated from serum protein-free casein micelles to equal extents, but the presence of /Mg specifically enhanced the dissociation of /c-casein at pH values ^ 69. Micelle hydration increased slightly when casein micelles were heated in the presence of /Mg at pH 67, while at pH 7-3 /Mg decreased the degree of hydration of casein micelles. Formation of a complex between /Mg and /c-casein appeared to stabilize the micelles in the pH range 6-5-6-7, possibly via increased micellar charge or degree of hydration or by preventing the dissociation of /c-casein.It has been recognized since the work of Tessier & Rose (1964) that the pHdependence of the heat stability of milk is dependent on the formation of a disulphide-linked complex between /Mactoglobulin (/Mg) and /c-casein. Although a critical level of soluble calcium and phosphate is also necessary for the occurrence of a minimum in the heat coagulation time (HCT)-pH profile of milk, the presence of /Mg or a-lactalbumin (a-la) is essential. The HOT of serum protein-free casein micelle (SPFCM) systems is low at pH < 6-9, but increases progressively with increasing pH in the range.6-4-7-4 (Rose 1961a, b). Addition of /Mg or a-la to SPFCM systems increases heat stability in the pH range 6-4-6-8, but decreases it at pH

Formation of heat-induced protein aggregates in milk as a means to recover the whey protein fraction in cheese manufacture, and potential of heat-treating milk at alkaline pH values in order to keep its rennet coagulation properties. A review

“…Early studies on the stability of heattreated or UHT-treated skim milk showed that κ-casein dissociated more extensively from casein micelles during heating at pH > 6.9, than at lower pH values [140,141]. Because they interact with κ-casein upon heating, the denatured whey proteins remain in the serum phase when the heattreatment is performed at pH > 6.9, whereas a high proportion of them can be sedimented with the casein micelles by ultracentrifugation when the heat-treatment is performed at lower pH values [57,117,[139][140][141].…”

“…Because they interact with κ-casein upon heating, the denatured whey proteins remain in the serum phase when the heattreatment is performed at pH > 6.9, whereas a high proportion of them can be sedimented with the casein micelles by ultracentrifugation when the heat-treatment is performed at lower pH values [57,117,[139][140][141]. Virtually no aggregates could be found in the serum phase of milk at pH < 6.7 [156].…”

“…Rennet coagulation of milk heated at alkaline pH 7 and the dissociation rate of κ-casein, to the extent that blocking the free thiol group of β-lactoglobulin with N-ethylmaleimide (NEM) prevented the extensive dissociation of κ-casein upon heating [3,141]. Furthermore, a positive relationship has been demonstrated between the pH of heat-treatment, and the heat-induced dissociation of the caseins, especially κ-casein [2,139].…”

Proteolytic degradation of ewe milk proteins during fermentation of yoghurts and storage

“…Caseins are not denatured by heat as are the whey proteins, mainly b-lactoglobulin and a-lactalbumin, which are denatured at the temperatures used during the yoghurt production. Upon denaturation, b-lactoglobulin reacts with other milk components being also able to oligomerize itself while a-lactalbumin undergoes heat-induced interactions only after severe heat treatment [7][8][9][10][11]. The observed improvements in the rate of gel formation are mainly due to interactions between b-lactoglobulin and caseins, since heating milk at 80 8C for 30 min denatures more than 90% of b-lactoglobulin as compared with only 60% of a-lactalbumin.…”

Acid gelation of colloidal calcium phosphate-depleted preheated milk

“…Dissociation of κ-casein on heating was higher in Ca phosphate depleted milk, possibly as a result of its higher pH value, as the pH of heat treatment strongly affects casein dissociation [2,38]. On itself, the increase in mineral depletion from 20% to 40% did not steeply change the κ-casein dissociation after heating.…”