Section C. Dairy Chemistry: Some aspects of the physical chemistry of the salts of milk

Abstract

Rennet coagulation of milk Heat coagulation of milk Effect of preheating on the heat cot tion of milk Evaporated milk. Gelation of concentrated Dried milk Instant milk. Frozen milk products Curd tension. milks igula PACK 113 113 117 102 Reviews of the progress of dairy science pH of milk, exist partly in dissolved and partly in insoluble or rather colloidal form, and, in fact, in close association with casein. As the influence of this type of salt on the properties of milk differs considerably according to the phase in which it is situated, a correct assessment of salt partition between serum and milk colloids is of primary importance. The dividing line between soluble and colloidal is admittedly tenuous and somewhat arbitrary, its exact position depending very much on the means employed to achieve the separation. For the milk salts, however, a fairly sharp separation is not unduly difficult chiefly because the insoluble or colloidal salts occur mainly in association with the relatively coarse colloidal micelles of casein. All modes of separation used must of course meet the requirement of not bringing about any alteration in the equilibrium between the dissolved and colloidal states; quite a variety of methods is available and in common use which fulfils this condition approximately. These include dialysis, ultrafiltration, high-speed centrifugation, and the use of caseincoagulating enzymes such as rennin. All of these methods have been used almost indiscriminately for the purpose both in earlier and more recent work. From a recent comparative study of these methods Davies & White (3) conclude that dialysis of milk (to which a little chloroform has been added as preservative) at 20 °C against a relatively small volume of water furnishes the most satisfactory procedure of separation, and that there is no reason for supposing that the diffusate obtained in this way does not truly represent the aqueous phase of milk. Dialysis at 3°C or so, commonly employed in earlier investigations, is less suitable, for reasons to be discussed later. Ultrafiltration of milk through cellophane at 20 °C and under a pressure of 38 cmHg is also satisfactory and gives a serum of very similar composition to the diffusate, though slightly lower in calcium, citrate and lactose. Use of higher pressures accentuates this disparity, presumably through a 'sieving' effect. Both 'centrifuge serum', prepared by centrifuging separated milk for 3 h at about 20°C and 50000g, and rennet whey differ from diffusate and ultrafiltrates in containing the serum proteins of the milk ('centrifuge serum' contains in addition a small amount of the colloidal calcium caseinate-phosphate complex, and rennet whey some products of casein proteolysis), but nevertheless they correspond well with the diffusate in salt composition except for a natural tendency to be slightly high in calcium. The basic similarity in composition of all these sera, so differently prepared, is strong presumptive evidence, say the authors, of the identity of composition of the diffusate pr...

Etude des relations entre les caractéristiques physico-chimiques des laits de chèvre et leur aptitude à la coagulation par la présure

“…Toutefois, Pearse et al (1986) notent une influence de la proportion de caséine~sur la fermeté de gel mais non sur la vitesse de raffermissement. L'influence de la concentration en calcium total sur la fermeté du gel a été bien ' /établie dans le cas du lait de vache (~ quot et al, 1954;Auriol, 1961;Pyne, 1962;Storry et al, 1983;Tervala & Antila, 1985) et nos résultats confirment sur lait de chèvre l'importance de ce paramètre.…”

Magnesium in milk

“…Another difference between magnesium and calcium in the serum phase is that some during heating, they have "a much inferior role" in rennet coagulation (Pyne, 1962 (> 90%) at the isoelectric point (~5.5); at pH ≤ 4.5, both aggregation and denaturation were < 367 80% while at pH values of 6.5-9.5, aggregation was ≤ 90% but denaturation was 30-80%.…”

Study of the dissociation of β-casein from native phosphocaseinate

“…It is known that a decrease in free calcium can cause solubilization of colloidal phosphate with loss in micellar integrity (Pyne, 1962;Morr, 1967) leading to the solubilization of~-casein, x-casein (Ono et al, 1978) and ultimately to that of as caseins (Lin et al, 1972). It is known (Payens, 1982) that the sensitivity of the casein fractions to calcium is related to their phosphoserine (Ser-P) content and it increases in the following order: a s 2 (10-13 Ser-P/molecule) > a s 1(8-9 Ser-P/molecule) >~(4-5 Ser P/molecule) > lC(1 Ser-P/molecule).…”

“…At pH 4.57, Oavies and White (1960) reported that in milk at 20°C, 96% of the magnesium, 97% of the calcium and 73% of the total phosphorus was soluble. Pyne (1962) reported that between 0 and 2°C at pH 4.6 there was complete demineralisation of the caseins that leads to the solubiIization of the caseins.…”

“…Creamer et al (1977) noted a maximum of soluble caseins at O°C. The decrease in hydrophobic interactions (Oavies and Law, 1983) and the solubilization of calcium phosphate at low tempe rature (Pyne, 1962) cou Id be responsible for the solubilization of caseins. Pierre and Brulé (1981) showed that the solubilization of p-casein from micelles in milk had an optimum at a protein concentration of 30 gII of total milk proteins.…”