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Keywords:

  • cloudy apple juice;
  • clear apple juice;
  • procyanidins;
  • thiolysis;
  • EPR;
  • DPPH;
  • HPLC;
  • radical scavenging

Abstract

Clear and cloudy apple juices from Idared and Champion varieties were studied for their radical-scavenging effects. The polyphenolic content and composition of the juices before and after thiolysis were determined by high-performance liquid chromatography with diode array detection. Cloudy juices, especially that prepared from Champion variety, had a higher content of procyanidins than clear juices. Radical-scavenging activity was measured by electron paramagnetic resonance (EPR) spectroscopy using the stable 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical. All juices showed long-lasting radical-scavenging activity, and EPR spectra were recorded over time to follow the reaction kinetics. Scavenging of DPPH showed pseudo-first-order kinetics, which might be expected in the presence of polymerised antioxidants that prevent closer contact between the DPPH radical and hydroxyl groups. The content of polymeric procyanidins showed a linear dependence on the rate constant, suggesting that these compounds are mainly responsible for time-extended radical-scavenging activity. The antioxidant properties of apple juices were much better reproduced by EPR spectroscopy than by UV–visible measurements. The former method is especially sensitive to the concentration of polymerised or bound procyanidins, whilst the latter method requires transparent (clear) samples. Apple juices, especially cloudy ones, are a rich source of natural antioxidants that may be used in the pharmaceutical or food industry. Copyright © 2006 Society of Chemical Industry