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With 4 Figures)WHILE the ultimate composition of the mineral matter of milk has been known for many years, the precise mode of combination of the separate acid and basic constituents is a subject which still requires elucidation. Apart from its great academic interest, the solution of the problem becomes urgent in view of the rapid development of the dairy industry, and the increasing tendency to subject milk to processes which are known to disturb the delicate balance of its mineral constituents. The problem is complicated by the fact that some are found in both the soluble and colloidal phases, and may also exist in both organic and inorganic combination. Thus Na, K, Cl are almost exclusively confined to the soluble phase, while Ca, Mg, P 2 O 5 occur in the soluble and colloidal states; and further, the elements Ca and P are found in combination with caseinogen.Although it is generally agreed that the major part of the colloidal inorganic phase consists of calcium phosphate, opinions are divided as to whether it is the di-or tricalcium salt. It is clearly a matter of utmost importance to discover which form is present in milk since, if dicalcium phosphate is present, it is necessary to allocate a high proportion of calcium to caseinogen, as opposed to a lower proportion if the tricalcium salt is present. The behaviour of these two alternative complexes cannot be assumed identical.The distribution of phosphorus is very much more complex than that of calcium because its soluble compounds are not entirely inorganic, but include considerable amounts of phosphoric esters(i, 2, 3). Van Slyke & Bosworth(4), Palmer(5), Grimmer (6), and Wright (7), maintain that the colloidal inorganic phosphate is in the form of the dicalcium salt; Porcher(8) and Soldner(9) consider that it is composed of a mixture of di-and tricalcium phosphates; while Lehman(iO), Lindet(ii), and Pyne(i2) contend that tricalcium phosphate accounts for the major portion of the colloidal phosphate of milk. A study of these works also reveals a great diversity of opinion on the question of the supposed association of the colloid phosphate with caseinogen. Some of these workers(10,11,12), as well as Hammarsten(i3) and Gyorgy(i4), consider that the colloid phosphate is combined to caseinogen, most probably through the calcium salt of this protein; while others (9) either reject the association entirely or concede only a protective role to the protein(4,8). Richmond(i5) advances a formula for the caseinogen phosphate complex: CayNagH casein + 2Ca3P 2 O 8 .Jour, of Dairy Research VII 10

References1 Russell, E. J., et al., Rothamsted Memoirs, 9 et seq.A rapid method for the determination of lactic acid in milk is described. After precipitation with NaOH-ZnSO, in the presence of BaCl,, the filtrate is treated with FeCI,-HCl reagent, and the colour is measured by one of the three common methods. Reference to standard curves prepared from similar observations on lactic-treated fresh mixed milk enables lactic acid to be determined with an accuracy which compares favourably with the 9-hydroxydiphenyl method.The approximate lactic content may be determined by adding to the filtrate a mixture of ferric chloride, potassium ferrocyanide and salicylic acid solutions.Although the apparent lactic content of fresh mixed milk is 0.003?& advanced lactation may cause considerable increases in this value. J.Sci. Food Agric., 2, June, 1951 65 ml. 1% ferric chloride reagent, 7.5 ml. 1% gelatin, 8 ml. 0.1277~ potassium ferrocyanide, 8 ml. saturated salicylic acid, made up to IOO ml. 2 0 ml. 2% ferric chloride reagent, 10 ml. 1% gelatin, 30 ml. each of o.127y0 ferrocyanide and saturated salicylic acid, per IOO ml. B. C. Buflerd chromate colour standards.-Potassium chromate was dissolved in potassium hydrogen phthalate buffered to pH 4.4 (20.41 g. potassium hydrogen phthalate plus 147 ml.

With 15 Figures)THE discovery by Pyne(i) that the colloidal phosphate of milk is largely, if not entirely, tricalcium phosphate was a valuable link in the chain of evidence required for the complete elucidation of the mineral equilibria of milk. A method for estimating tricalcium phosphate in milk was examined by the writer (2) in a previous paper, and the suggestion was made that it should prove an effective means of exploring the hitherto obscure partition of calcium and phosphorus. The present paper gives the results of work undertaken with this end in view.Fifty-four samples of mixed milk (alternatively a.m. and p.m.), from the College herd of thirty-six to forty-eight Dairy Shorthorns, were taken at regular intervals during the period October 1935 to November 1936. All samples were kept in cold storage at 45° F. until required in the laboratory. METHODS OF ANALYSISOn arrival in the laboratory, the samples were placed in a water-bath at 30° C. and then cooled to room temperature. Quantities required for the various determinations were measured with a pipette, and the weight of milk or whey so discharged was subsequently ascertained. All estimations were made in duplicate. Soluble constituentsFor this purpose the analyses were conducted on the rennet whey, correcting for the removal of fat and protein by the use of the factor 100-(F + P-0-4) 100where F = percentage fat, and P = percentage nitrogen x 6-38. This factor has been found suitable for converting whey percentages to soluble milk constituents (2). The whey was prepared by adding 225 ml. of milk at 30° C. to 1-5 ml. of rennet extract, maintaining at 30° C. in a water-bath, cutting with a stainless steel knife when of the requisite firmness, and then filtering through a battery of four Whatman No. 40 11 cm. filter papers. The time of coagulation was

1. 670 samples of the mixed milk from 15 herds were analysed, and the average percentages of total ash, soluble ash, insoluble ash, lime and phosphoric acid are given.2. Tables showing frequency distributions are also given, with the standard deviation, mean and probable error of mean for each constituent determined.3. Various correlations of these constituents with solids not fat and protein have been prepared, and these correlations are illustrated by graphs.It is observed that the total ash falls with the solids not fat until low values of solids not fat are reached, when the ash content appears to rise. This variation is confirmed by a curve illustrating the variation in ash content of samples of individual cow’s milk. Soluble ash rises as the solids not fat falls, but the insoluble ash shows a reverse variation. Lime and phosphoric acid both fall with the solids not fat.