الفهرس 
Evaluation of Avocado and Red Onion By-products Extract Rich in Polyphenols as Natural Antioxidants
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Abstract
Avocado (Persea americana Mill.) peel (cv. Hass) and red onion skin
(Allium cepa L.) are rich sources of natural antioxidants. The present work aims
to evaluate ultrasound (U) and enzyme (E)-assisted extraction methods of
polyphenols with antioxidant properties from avocado peel and red onion skin .
The polyphenolic compounds were identified by HPLC. The major
polyphenolic compounds identified in the avocado peel extracts were benzoic
acid, vanillic acid, resveratrol, and syringic acid. Benzoic acid, rutin, naringin,
rosmarinic acid and quercetin were the major polyphenols in red onion skin
extracts. The highest yield of polyphenols of avocado peel (35.4 ±1.08 mg
GAE/g dried peel) was obtained with a solid-to-solvent ratio of 1:20 (w/v) using
20% ultrasonic intensity for 30 min or by treatment with viscozyme at a 1%
level for 60 min. The highest yield of polyphenols of red onion skin (83.2 ±3.03
mg GAE/g dried skin) was obtained with a solid-to-solvent ratio of 1:20 (w/v)
using 20% ultrasonic intensity for 45 min, while the maximum yield obtained
by the enzyme extraction method was 41.77 ±2.27 mg GAE/g DM after 60 min
at 0.5% enzyme concentration with solid-to-buffer ratio 1:20 (w/v). The DPPH
and FRAP IC50 values of investigated U extracts were statistically lower than
those of the enzyme-assisted extracts. The oxidative stability of sunflower oil
was measured by Rancimat method. Enriching sunflower oil with U and E
avocado peel extracts at 200 mg GAE/kg oil or BHT (200 mg/kg oil) increased
the oxidative stability of oil by 13% and 15%, and 20%, respectively. The
addition of U red onion skin extract at 200 mg GAE/kg oil did not increase the
oxidative stability of the oil, while the incorporation of E red onion skin extract
at the same level increased its stability by 7%. The avocado peel and red onion
extracts are promising sources of antioxidants. The extraction with U proved to
be more efficient than enzyme-aided extraction in terms of the yield of phenolic
compounds.