Note on the formaldehyde titration of milk-protein, and its application to the estimation of caseinogen.

THE writer described in this Journal [1932] an improved technique for the determination of milk-proteins by formaldehyde titration. Very satisfactory results were obtained for mixed milks using a factor, 1 cc. N NaOH = 1-74 g. milk-protein, which however for milks of individual cows showed a somewhat wide variation, ranging from 1-63 to 1F87. It was tentatively suggested that this might arise from variations in the relative amounts of caseinogen and albumin from sample to sample and be susceptible perhaps of easy correction; and the further work now described has been carried out primarily to test this point. For this purpose the formaldehyde value, and protein content (N x 6.38) of a number of individual milks and of the corresponding caseinogen-free acid wheys were determined. Most of the samples were taken during two main periods-March to May 1932, and October to November, 1932-the entire herd of some 30 animals (Dairy Shorthorn breed) being tested on each occasion so as to obtain values representative of different stages of lactation. The aldehyde values of the milks were obtained by the method referred to above and those of the acid whey somewhat similarly. The latter was prepared by a modification of the isoelectric precipitation method for caseinogen, recommended by Moir [1931], the principal changes being that the milk was not diluted with more than about 80 % of its volume of water before adding the acetic acid and sodium acetate solutions, and that these were employed in somewhat higher concentrations, viz. 2-5 N and M respectively. In this way too great dilution was avoided, the final volume before filtration not being greater than twice that of the original milk. For the determination of its aldehyde value 20 cc. of the filtrate (corresponding approximately to 10 cc. milk) was roughly neutralised with 2N NaOH, 0-4 cc. saturated aqueous potassium oxalate and 1F0 cc. 0-5 % phenolphthalein were added, and the liquid was accurately neutralised with N/10 NaOH, the end-point being a faint pink which was matched with that of a similar quantity of whey and oxalate tinted with magenta. 2 cc. 40 % formaldehyde were then run in and the titration continued to the same tint as before. Allowance must of course be made for the acidity of the formaldehyde under similar conditions of dilution, etc. The results are summarised mainly as group averages in Table I. The numbers in the column headed "g. caseinogen calc." were obtained by dividing the difference in the protein contents of the milks and their corresponding quantities of acid whey, by the difference in their respective aldehyde titrations, and are seen to display a marked uniformity over the whole period, the extreme variation in even individual samples being largely attributable to