Changes in the Pectic Constituents of Apples in Relation to Softening ^

One of the principal changes that takes place in apples as they ripen on the tree and in storage is the softening of the flesh of the fruit. In recent years many studies have been made on the ripening and storage of apples (iO),^ and in these the softening of the fruit has been given major consideration. In general, it has been found that there is a gradual, though somewhat irregular, softening of the fruit on the tree as it becomes mature. This softening continues in storage and is the outstanding change that takes place during the process of ripening, the fruit changing from the hard unripe condition to the relatively soft, eating-ripe condition, and finally to the very soft, mealy, overripe condition. The rate at which softening takes place in storage depends primarily upon the temperature at which the apples are kept. For a number of varieties the rate of softening at 40° F. was found to be slightly more than double that at 32°, whereas that at 50° was slightly less than double that at 40°, and that at 60° was nearly double the rate at 50°. In these studies (10) a fruit pressure tester (9) was used to measure the firmness of the apples. This tester records the pressure in pounds necessary to force a plunger seven-sixteenths of an inch in diameter into the fruit a distance of five-sixteenths of an inch. In penetrating the fruit the tester crushes some of the cells and separates and pushes aside others. The firmness of the apples as measured by the pressure tester is probably due principally to three factors—the size of the cells and intercellular spaces, the thickness of the cell walls, and the ease with which the cells separate. . The last two factors are of particular interest in relation to the studies herein reported, as the thickness of the cell walls depends on the thickness of the cellulose or substance of which they are composed and upon the thickness of the layer of cementing substance or pectic material of the middle lamella, while the ease with which the cells separate depends upon the amount of pectic material cementing them together. All three factors might play a part in the softening of the fruit on the tree. In storage, however, there would be no further increase in the size of the cells and intercellular spaces, and softening should be due to changes in the thickness of the cell wall and the ease with which the cells separate. The studies reported in this paper were undertaken to obtain a better understanding of the underlying chemical changes that take place in apples during the softening process.