The amino-acid composition of potato protein and of cooked potato

Abstract

The potato's nitrogenous constituents have a nutritive value as high as do those of whole wheat (Chick & Cutting, 1943). This has long seemed surprising, for the pro- portion of protein nitrogen in the potato may be as low as 37% and appears not to exceed about 60% (Neuberger & Sanger, 1942; Mulder & Bakema, 1956) of the total nitrogen. The non-protein nitrogen, which consists largely of asparagine and glut- amine together with other nitrogenous bases (Neuberger & Sanger, 1942), is of low nutritive value with no growth-promoting properties on its own (Chick & Cutting, 1943; Chick & Slack, 1949), but nevertheless appears to have a supplementary action when mixed with wheat gluten (Chick & Slack, unpublished, reported by Chick, 1950). The existing amino-acid analyses of the heat-coagulable protein that forms the bulk of the potato protein (Neuberger & Sanger, 1942; Mulder & Bakema, 1956), of the non-protein nitrogen and of the whole raw potato have so far provided only in part an explanation of these facts ; moreover, there are considerable discrepancies between the results of different authors. For these reasons it seemed worth while to repeat and extend some of the analyses by the method of ion-exchange chromatography of Moore & Stein (1951). The iso- lated, crude potato protein has been analysed and also the whole potato after being cooked by boiling, because this is how potatoes are generally prepared for con- sumption. Moreover, losses of nitrogenous material are likely to be greatest with this form of cooking (McCance, Widdowson & Shackleton, 1936) and might cause differences in composition from that of whole raw potato, for which some information does exist (Slack, 1948; Lyman & Kuiken, 1949; Agren, 1949).