Stem- and bud-end tissue from 41 samples of potatoes representing various degrees of after-cooking discoloration were analyzed for organic acids content. The individual acids determined were glutamic, aspartic, pyroglutamic, malic, citric, orthophosphoric, oxalic, and one unidentified acid. The stem-end tissue contained a lower concentration of all the acids except the unidentified one. The difference between the stem- and bud-end was very large in some cases, notably for malic acid and citric acid.
Examination of the data indicated a strong tendency for degree of blackening to be associated with low organic acid content. A statistical analysis of the data showed highly significant correlation of low citric acid, orthophosphoric acid, and oxalic acid content with blackening. Citric acid exhibited the highest degree of correlation, having an r value of 0.768.
The significant correlation between low citric acid content and after-cooking blackening was maintained in all but one case when subgroups of the samples were formed according to variety, crop year, and location grown. Of the three varieties studied statistically, Ontario and Katahdin showed correlation, whereas Kennebec did not.
The interrelationships of iron, citric acid, and potassium contents were studied. Since there is always a large excess of citric acid over iron, it must be assumed that something prevents the citric acid from chelating the iron in blackening potatoes. The data indicate that potassium may be the main constituent playing this role. In the final analysis, the primary factor in the blackening phenomenon is probably the amount of free organic acid present.