Sugars and organic acids of the apple skin of six cultivars were analyzed as trimethyl-silyl derivatives by gas chromatography (GC) with a capillary column, from beginning of coloring to the later stages of coloring. Differences in composition were compared between the red cultivars: ‘Jonathan’, ‘Starking Delicious (S. D.)’, ‘Tsugaru’ and ‘McIntosh’; and the yellow cultivars: ‘Golden Delicious (G. D.)’ and ‘Mutsu’.
In all of the cultivars, the main sugars were glucose, fructose, sorbitol and sucrose, and the main organic acids were malic and quinic acids. All the cultivars showed a great increase in sucrose and a decrease in malic and quinic acids over the ripening stage. But no difference in the changes in these substances was found between red and yellow cultivars.
Differences in other minor organic acids between red and yellow cultivars were studied using GC and gas chromatography-mass spectrometry, and citramalic acid was identified as having a difference. In the yellow cultivars, at the biginning of coloring, the acid was scarcely detected and increased very little at later stages of coloring. In the red cultivars, at the beginning of coloring, the acid was also scarcely detected, but at the later stages, it increased remarkably.
When commercial citramalic acid (sodium salt) was applied to disks of unripe skin which had not shown red color under the light of a discharge lamp, using red cultivars (‘Jonathan’, ‘S. D.’, ‘Tsugaru’, ‘Fuji’ and ‘Ralls Janet’), and yellow cultivars (‘G. D.’ and ‘Mutsu’), the anthocyanin content of the skin in all cultivars increased more than in the control. But the optimum concentration of the acid was different among the cultivars.
It was concluded that citramalic acid is related to the development of anthocyanin in apple skin.