A study on protein nitrogen and total nitrogen values of various food extracts

Abstract

I N ACCORDANCE with the standardization procedure proposed by Cooke and Stulll> 2 for pollen extracts, all pollen and inhalant extracts prepared in this laboratory are standardized on their proetin nitrogen basis.” In the case of food extracts, it was formerly believed that protein represented as a rule at least 90 per cent of the total nitrogen value and, therefore, the standardization of food extracts was practiced in previous years on the basis of total nitrogen, because of the greater convenience of this procedure. Doubts as to the validity of this belief arose; when 143 food extracts were analyzed for both protein nitrogen and total nitrogen, these doubts were confirmed. The results of these 143 analyses are included in a mimeographed list for the benefit of laboratories that may desire to perform protein nitrogen determinations in standardizing food extracts.+ Although the protein nitrogen values of some 30 extracts fell within the range of 90 per cent of their total nitrogen values,$ this was not the case in the vast majority, where the ratios of these two va,lues were found in a lower range, even as low as 17.9 per cent (rhubarb). Moreover, in some instances, these ratios varied considerably in different preparations of the same extract. Examples in this regard together with some other representative result,s are given in Table I. Considering such significant variations, which do not allow any constant correlation of the two nitrogen values, it seems quite possible that even other extracts, which in this study appeared to adhere to the “rule of 90 per cent,” would fail to do so with any degree of regularity.