737. The heat coagulation of milk: II. variations in sensitivity of casein to calcium ions

Abstract

1. The sensitivity to calcium of the caseinate-phosphate complex of milk heated at 120°C., increases to an early maximum and thereafter steadily declines, apparently in consequence of parallel changes in the sensitivity of its caseinate constituent.2. A simultaneously developed capacity of the heated complex to bind additional colloidal phosphate appears to be unrelated to these changes in casein sensitivity.3. Colloidal phosphate content is confirmed as a factor in the heat coagulation of milk.4. Heat-developed acidity contributes to the heat coagulation of milk primarily by increasing hydrogen-ion concentration and only slightly, if at all, through release of calcium ions from insoluble combinations.5. Neither serum proteins nor protein-lactose combinations play any significant part in the heat coagulation of milk.6. The bearing of these results on the heat coagulation of milk in general and on the operation of certain high-temperature stabilization treatments is discussed.

Effect of selected amides on heat-induced changes in milk

“…The only effect which the 5 stabilizing amides had in common and which the other compounds did not share was a significant pH-buffering capacity in synthetic systems and in milk. It is suggested that urea exercises its stabilizing influence in milk principally through its ability to buffer the pH of the system during heating.Addition of 002 % (3-6 mM) or 0-05 % (8-4 mM) urea to milk before heating increased its coagulation time from 35 to 62 and 104 min respectively (Pyne, 1958). A significant positive correlation between the heat stability at 120 °C of bulk milk at its natural pH and the level of non-protein nitrogen (NPN)…”

Milk Coagulation and Protein Denaturation

“…Because of its association with casein micelles, heat-precipitated calcium phosphate in milk does not sediment (Evenhuis and de Vries 1956). Colloidal calcium phosphate slowly dissolves to restore equilibrium (Fox et al 1967;Kannan and Jenness 1961;Pyne 1958).…”

“…Pyne (1958) and stabilized milk by adding urea to it before heating. {3-Lactoglobulin can be completely denatured in urea (Kauzmann and Simpson 1953).…”

“…Early studies were prompted by the need to ensure sufficient heat stability for evaporated milk to withstand heat sterilization Hart 1919, 1922). Between 1919 and1960, most attention was directed to the influence of milk salts on heat stability (Miller and Sommer 1940;Pyne 1958;Pyne and McHenry 1955). It was not until the early 1960s that the importance of heating time and pH on coagulation of milk was appreciated (Rose 1961A,B).…”

Characteristics of a calcium–milk coagulum

“…Extensive research has been carried out on the effect of ionic calcium in the heat coagulation of milk (Jeurnink and de Kruif, 1995;Morrissey, 1969;Omoarukhe et al, 2010;Pyne, 1958;Seivanen et al, 2008;Van Boekel et al, 1989), but the role of ionic calcium in causing coagulation of milk proteins during heat treatment is still unclear. Further, most studies have investigated the effects of low levels of calcium supplementation (<30 mM) and at high temperatures, particularly 140 and 120°C (Omoarukhe et al, 2010;Seivanen et al, 2008).…”