Influence of Shape on the Chemical Composition of Potato Tubers

Abstract

Decided differences in the shape of potato tubers of different varieties are a matter of common knowledge. It has been suggested that these variations in shape may affect the composition of the tubers. The evidence on this, however, is conflicting. GLYNNE and JACKSON (3), investigating the distribution of dry matter and of nitrogen in potato tubers, dissected out the different zones and analyzed them. The main regions are the medulla, or inner, and the cortex, or outer region. They found that the percentage of dry matter is lowest in the skin, increases in the cortex, and then decreases in the medulla. The nitrogen content in these three is in the reverse order to that of the dry matter. COUDON and BUSSARD (1) obtained practically the same results.EAST (2) states that the flatter tubers have a higher starch content, because of a greater proportion of cortex. Thus, if changing the shape changes the ratios of the zones, and if the latter differ in composition, it is fair to predict that the composition of the tuber as a whole will vary with the shape. On the above lines of reasoning WILLAMAN and WEST (8,9) concluded that the more spherical tubers would have the higher protein content.RENSKI (6), however, found a relation opposite to that above. He reports that the longer tubers have the lower starch content. His material included many varieties over a period of several years. More recently NEUMAN (5) also reports that the longer tubers have the lower starch content. He states that high nitrogen fertilization causes an elongation of the tubers, but he does not give any data on the nitrogen content of these tubers.During 1922 and 1923 several lots of potatoes, including several varieties, were measured as to shape, and analyzed for dry matter and nitrogen. Although the data are somewhat scanty, they point rather definitely to the conclusion that the more spherical tubers have the higher protein content. Since it is improbable that the writers will have further data in the near future, it was thought best to publish the information so far obtained." PLANT PHYSIOLOGYMethods and materials From 12 to 20 tubers were taken for a sample for analysis. The dimensions were taken in millimeters, and averaged for each lot of tubers. The tubers were scrubbed, dried, and weighed on a special balance in air and in water for determining specific gravity. Then they were shredded for analysis. Dry matter was determined on a small sample by drying at 1000 C. Protein was determined on the dried and finely ground material by the Kjeldahl-Gunning method.The five groups of samples involved were as follows, all of them grown in Minnesota:A. Among the shape ratios, the length-width bears a very marked relation to the length-depth; and the latter bears a significant relation to the widthdepth ratio. Thus the length is the dominating dimension. Furthermore, in comparing the weight of the tubers with the shape, it is evident that as the tubers increase in size it is largely because of increased length. This is to be expected from the fact tha...