The determination of milk-proteins by formaldehyde titration

THE formaldehyde method for the determination of milk-proteins, originally proposed by Steineggar [1905], has been much employed since its inception on account of its ease, rapidity, and reasonable accuracy. In the original method, sodium hydroxide appears to have been the alkali employed in the titration. This difference he attributed primarily to the differing degrees of hydrolysis of the strontium and sodium salts of the milk-proteins and of the products formed from them by the action of formaldehyde, and, to a slight extent, to similar differences in the degrees of dissociation of the strontium and sodium salts of the acids usually present in commercial formaldehyde. It appears however that there is an additional and probably more important factor which plays a part in the titration differences mentioned, namely, the formation of tricalcium phosphate during the formaldehyde titration. It has long been recognised that this precipitation occurs in the determination of milk-acidity in the usual way, and that the extent to which it occurs, and therefore the resulting titratable acidity of the milk, depends greatly on the conditions of titration. Thus the diminished acidity resulting from the dilution of milk has been ascribed by Sommer and Menos [1931] to the greater solubility, i.e. diminished precipitation, of tricalcium phosphate in the diluted medium. Similarly the very different milk-acidities found by Roeder and Radoi [1925] according to whether sodium or calcium hydroxide had been used for the neutralisation have been attributed to the different degrees of phosphate precipitation in the two cases, the higher acidities occurring, as was to be expected, when the alkaline earth was employed, and conditions were more favourable to increased precipitation.