Distribution and nutritive value of the nitrogenous substances in the potato

Abstract

In the King Edward potato grown in the Fenlands,total N represented 1.7-2% of the dry weight; 40 to 50% of the total N was present as protein and this decreased slightly with storage. In expressed juice 30% of the N was protein N; of the non-protein N about 20% was present as free amino acids, 40% as amides and 20% as nitrogenous bases. Of the total N, 7-10% was unextracted from the residual pulp with salt soln. and alkali. At least 2 soluble proteins were detected, one (alpha-globulin) precipitated from the diluted sap on standing at pH 4, the other (beta-globulin) on boiling the filtrate therefrom. In a specimen of freshly dug potatoes the alpha- to beta-globulin ratio was about 1/2; in a sample of stored potatoes, about 2/1. Weanling rats grew less well on the N of the whole steamed potato than on the N of skinned potatoes and still less well than on the N in potatoes from which a 2 mm. layer had been peeled. The growth-promoting value of the N was usually less in stored potatoes, the deterioration depending upon the conditions, but being least after storage at 5[degree]. The ratio of protein to non-protein N was also lowered after storage. The nutritive value of tuberin prepared from the sap by heating at 80[degree] at pH 4 was not superior per unit of N to that of the nitrogenous material in the whole potato. The non-protein N remaining in the press juice, after removing the tuberin, was unable to support growth alone, but feeding expts. showed that it complemented the nutritive value of the N present as protein. The coefficient of (apparent) digestibility of the N was about 76 for the intact potato, 65 for the tuberin, 83 for the press juice and 69 for the artificially reconstituted potato. The capacity of the protein-free fraction of the press juice to supplement the tuberin could not be explained on the basis of amino acid content. Nor should the whole potato furnish the essential amino acids as abundantly as tuberin, unless it contains an insoluble protein even richer in them. The somewhat inferior value of the tuberin, may possibly be ascribed to chemical degradation during its separation.