INFLUENCE OF PROCESSING AND STORAGE ON THE PHENOLIC COMPOSITION OF THOMPSON SEEDLESS GRAPE JUICE

Methodology for measurement of phenolics in fruit juices using HPLC separation and diode array detection is presented. Quantitation of phenolic acids and flavonol glycosides was achieved with minimum sample preparation. Procyanidin quantitation, however, required removal of interfering compounds with Sepha-dex LH-20 minicolumn chromatography. Good reproducibility and high recoveries (up to 92.3 %) were achieved in procyanidin isolation. The methods were used to study the effect of SO2, enzymatic clarification, fining, bottling, concentration, and storage on the phenolic composition of Thompson Seedless grape juice. SO2 addition during processing resulted in higher levels of phenolic acids and procyanidins, but it had no apparent effect on the quercetin glycoside composition. Oxidation of caftaric acid to 2-S-glutathionylcaftaric acid was evident in juices processed both with and without SO2-Enzymatic clarification caused hydrolysis of caftaric, coutaric, and quercetin derivatives, but it showed no effect on the 2-S-glutathionylcaftaric acid. Procyanidins demonstrated sensitivity to the heat applied during bottling and concentration. Storage of concentrates for 9 months at 25 °C led to the formation of (hydroxymethyl)furfural (HMF) (up to 33.5 mg/L), extensive oxidation of cinnamics, and total loss of procyanidins and quercetin glycosides. Colorimetric measurement of phenolics showed no correlation with the HPLC quantitation. © 1990, American Chemical Society. All rights reserved.