Phenolic acids in selected scab‑resistant and mildew‑tolerant apple cultivars

Phenolic compounds play important roles in plant defences against both biotic and abiotic stressors, and many in crop plants are highly beneficial for human health, but their quantities and profiles are influenced by numerous factors. Thus, the purpose of this study was to quantify free and glycosylated phenolic acids in apples (important sources of beneficial phenolics) in the course of fruit development, at harvest and during 5 months of storage. We selected three scab-resistant and powdery mildew-tolerant varicoloured cultivars for testing, and found they had qualitatively similar spectra of phenolic acids. The major free phenolic acid in both peel and flesh of the apples was chlorogenic acid, followed by (at much lower concentrations) three hydroxybenzoic acid derivatives (protocatechuic, vanillic and gallic acids) and three hydroxycinnamic acid derivatives (p-coumaric, ferulic and caffeic acids). 
Spectra of phenolic glycosides were broader, and included glycosylated forms of protocatechuic acid (most abundantly) and four other hydroxybenzoic acids (gallic, p-hydroxybenzoic, gentisic and syringic acids). Dynamic changes in phenolic acid contents during apple development and storage were also observed. 
Contents of free chlorogenic and glycosylated protocatechuic acids were highest at the beginning of apple fruit growth and markedly declined during ripening. Levels of chlorogenic acid in both peel and flesh continually increased during storage, accompanied by marked increases in contents of glycosylated protocatechuic acid in all studied cultivars. Both of these acids have strong radical scavenging activity and contribute considerably to apples’ antioxidant capacities.