Phenolics and lipid-soluble antioxidants in fruit cuticle of apples and their antioxidant activities in model systems

Phenolics were extracted from fruit cuticles of 'Delicious', 'Golden Delicious', 'Empire' and 'Cortland' apples, using either cuticular wax scraped from fruit peel or enzyme-isolated cuticles. The concentrations of free phenolics in fruit cuticle ranged from 8 to 45 mu g g(-1) and bound phenolics ranged from 50 to 110 mu g g(-1) in these cultivars. Free cuticular phenolic concentrations in the four cultivars were in the order:'Golden Delicious' > 'Delicious' > 'Empire' > 'Cortland'. In a linoleate emulsion (oil-in-water) system, diphenylamine (DPA, lipophilic) displayed higher antioxidant activity than methanol-extracted cuticular phenolics (hydrophilic). Antioxidant activity of quercetin (flavonoids) was higher than that of gallic acids (phenolic acids). Antioxidant activity of free phenolics in fruit cuticle was equivalent to that of quercetin while bound phenolics, when released by chemical methods, displayed activity similar to that of gallic acid. In an alpha-farnesene-hexane (bulk oil) system, however, antioxidant activities of methanol-extracted cuticular phenolics were higher than that of DPA. Lipid-soluble antioxidants (LSAs) from cuticle displayed higher activity in the linoleate emulsion system than in alpha-farnesene-hexane system. Only about 10-15% of the total LSA activity in fruit peel was detected in isolated fruit cuticle. Among the four cultivars, LSA activity in epidermal and hypodermal cells was similar in 'Golden Delicious','Empire' and 'Cortland' apples, while 'Delicious' had lower activity. There was no clear correlation between LSA activity and scald resistance among the four cultivars. However, the total antioxidant activities (activity of free phenolics in cuticle plus activity of LSAs in fruit peel) of the four cultivars were in the order:'Golden Delicious' > 'Empire' > 'Delicious' > 'Cortland', which was similar to their scald resistance potentials. (C) 1999 Elsevier Science B.V. All rights reserved.