Content area
Full text
RECEIVED: March 8, 2017 * REVISED: May 5, 2017 * ACCEPTED: May 17, 2017
THIS PAPER IS DEDICATED TO PROF. MIRJANA METIKOŠ-HUKOVIĆ ON THE OCCASION OF HER 80TH BIRTHDAY
Abstract: Oxidation mechanism of quercetin in three different electrolytes (KCl, NaCl and LiCl) was studied by cyclic and differential pulse voltammetry. The results have shown that quercetin oxidation is reversible and diffusion controlled process in all three investigated electrolytes. Calibration diagram of quercetin was obtained in KCl, so this electrolyte was selected for total polyphenol content determination in apple peel extracts. Total polyphenol content was analysed by differential pulse voltammetry (dpv) and spectrophotometric Folin-Ciocalteu method (FC) in several ancient Croatian apple varieties and a wild apple variety grown in Croatia by using quercetin as a standard. Very high correlation for TP content obtained by two methods was observed since the TPdpv/TPFC ratio was around 1 and a correlation coefficient (R2) for TPdpv vs. TPFC was 0.98. The highest total polyphenol content was found in Slavonska srčika variety.
Keywords: apple extracts, differential pulse voltammetry, spectrophotometry, quercetin, polyphenol determination.
INTRODUCTION
PHENOLIC compounds or polyphenols are a group of compounds that have more than one phenolic hydroxyl group attached to one or more aromatic rings. In plants, they occur in both free and bound form as esters or glycosides. Polyphenols are called natural antioxidants and they have a potential to show anti-inflammatory, antiallergic, anticancer and antihemorrhagic properties displaying some of their prominent roles in vivo as antioxidants. Many epidemiological studies suggest that regular consumption of fruits and vegetables rich in polyphenols may reduce the risk of cancer and cardiovascular diseases. The beneficial effects of polyphenols may be based on several factors, however, quenching free radicals and inhibition of lipid peroxidation (antioxidant properties) are one of the important biological activities of phenolic compounds.[1,2] Recent studies have shown that the ancient apple varieties are a rich source of phenolic compoundsA4] Polyphenols most often found in apples are flavan-3 ols in monomeric ((+)-catechin, (-)-epicatechin) and in oligomeric form (proanthocyanidins), flavonols, phenolic acids, dihydrochalcones, and anthocyanidins.[4,5] Apple peel extracts mostly contain quercetin derivatives which belong to the flavonol subgroup of polyphenols.[6] Individual polyphenols together with quercetin derivatives from the peel were identified and quantified in...