A new method for determining the composition of the colloidal phosphate of milk has been developed, based on analysis of milk free from colloidal phosphate. Preparation of this material is described.The results suggest that the so-called colloidal calcium phosphate of milk should be more properly described as a colloidal phosphate-citrate.This colloidal phosphate-citrate shows considerable analytical resemblance to the precipitate formed on neutralizing acidified milk serum. Both can be represented approximately by the empirical formula of a hypothetical citrate apatite.

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...

Removal of colloidal phosphate leads to changes in the properties of milk of which the increased viscosity and greatly increased sensitivity to calcium salts, and the apparently diminished degree of mutual integration of the various casein fractions are the most striking. Re-introduction of colloidal phosphate restores incompletely and only in certain instances the original properties of the milk.These observations are interpreted as favouring the conception of chemical links between colloidal phosphate and casein in milk.* For brevity the term colloidal phosphate will be used in this paper.

1. A study of the compositional factors which affect the heat coagulation of milk has been undertaken.2. Calcium-ion concentration and colloidal phosphate content appear to be the chief factors determining the tendency of a milk to coagulate on heating.3. Acidity (mainly derived from thermal decomposition of lactose and casein) and heat denaturation of casein are supplementary coagulation factors which develop during the heating process.4. Lactose, as the main source of heat-developed acidity, is an important secondary factor in heat coagulation, but not an essential one. Coagulation can proceed, though more slowly, in its absence. The serum proteins play no part in the phenomenon.5. A provisional theory of the heat coagulation of milk based on these findings is put forward.

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.