Pectic Content of Apples in Relation to Thiocyanate Sprays.

An earlier report (5) has described the effect of thiocyanate sprays on sugars and certain other constituents in apples. The present paper is a continuation of the general problem with attention directed to the pectic constituents. In previous work it was noted occasionally that apples spray.ed with thiocyanates were found to keep better in cold storage at 30 to 50 C. than control samples not sprayed with thiocyanates. Since changes in pectic substances are known to occur in connection with the ripening and softening of fruit, it was considered worth while to examine suitable samples of sprayed and unsprayed material to find out if the improved keeping quality might be related to differences in pectic composition. Titratable acidity was determined on most of the samples. Several investigators (1, 2, 3, 4) have made a study of the pectic changes occurring in apples during ripening. In general, the results show a gradual change from insoluble protopectin to soluble pectin accompanied by a decrease in the thickness of the cell wall. Materials and methods The apples used for this investigation were of similar origin to those used for the earlier work with sugars (5). They were grown, sprayed, harvested in season and stored at 30 C. in the summer and fall of the years 1942, 1943, and 1944. Fruit grown in 1942 and 1944 was held in storage for a period of five to eight months before analyses were made. In 1943 analyses were started before the thiocyanate sprays were applied and continued until after the apples were harvested and stored. A total of fifteen varieties was used in the study and included both early and late maturing varieties as follows: Duchess, Red Duchess, Maiden Blush, Melba, Gravenstein, Wealthy, McIntosh, Golden Delicious, Grimes Golden, Jonathan, Delicious, Stayman Winesap, Rome Beauty, Wagener, and York Imperial. Each sample was prepared from ten to twenty individual apples depending upon the number available. They were cored with a cork borer and suitable slices were frozen immediately for comminution in the Waring Blendor. All samples of sprayed and unsprayed fruit were paired to the extent that for any given variety they were taken on the same day from nearby limbs of the same tree, stored for the same period under the same conditions, sampled in similar number, and analyzed at the same time. Analyses were made by the methods reported previously (7). All pectic determinations were made by the carbon dioxide method and calculated to pectic acid (milligrams of CO2 x 5.67). Titratable acidity was calculated as percentage of malic acid.