Comparative study on the effect of extraction solvent on total phenol, flavonoid content, antioxidant and antimicrobial properties of red onion (Allium cepa)

Keywords
Red onion skin · Phenol · Flavonoid · Antioxidant · Antimicrobial
Introduction
Since ancient times, onions have been used as food and for 
the treatment of many diseases [
1
]. They are an important 
source of phenolic and flavonoid compounds in the diet [
2
, 
3
]. 
Onions have different types, such as white, purple, red, or a 
mixture of red and purple [
4
]. Red onion (Allium cepa L.) skin 
extract is rich in phenolic compounds with anti-cancer and 
antimicrobial activities. One of the most effective compounds 
among flavonoids of onion skin is a substance called querce-
tin. In pharmacological studies, the onion skin has the highest 
amount of quercetin among 28 vegetables and 9 fruits [
5
]. 
Besides, it has been reported that there are ferulic acid and pro-
tocatechuic acid in different layers of onions [
6
]. Ferulic acid 
is an antioxidant used in the manufacture of aromatic com-
pounds. Antibacterial properties of ferulic acid have been also 
reported. According to previous researches, levels of phenolic 
compounds of red onion skin classified as antimicrobial agents 
and antioxidant compounds are about three to five times more 
than ones extracted from edible parts [
7
]. This is also true for 
flavonoids of onion skin. In general, red onion skin extract has 
stronger inhibitory effects compared to the extract of edible 
parts of it against bacteria and fungi [
7
]. Bagherloo et al. [
8
] 
reported that the amount of total phenolic content (TPC) in dif-
ferent varieties of onion was different and the maximum TPC 
* Maryam Khavarpour 
[e-mail]
1
Department of Food Science & Technology, Islamic Azad 
University, Sabzevar Branch, P. O. Box 678, Sabzevar, 
Khorasan Razavi, Iran
2
Department of Chemical Engineering, Ayatollah Amoli 
Branch, Islamic Azad University, Amol, Iran
3
Department of Chemistry, Ayatollah Amoli Branch, Islamic 
Azad University, Amol, Iran
4
Biotechnology Research Laboratory, Faculty of Chemical 
Engineering, Babol Noushirvani University of Technology, 
Babol, Iran

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Comparative study on the effect of extraction solvent on total phenol, flavonoid content,…
1 3
was observed in the red onion. In another study, Bagherloo 
et al. [
1
] stated that the antioxidant activity of red onion was 
higher than that of white onion. Also, scavenging capacity for 
radical nitrite, hydrogen peroxide and superoxide was found 
to vary significantly (p < 0.05) among outer and inner layers 
of red onion, and the outer layer had higher scavenging capac-
ity compared to the inner layer. The same results were also 
obtained by some other researches [
9
–
12
].
Overall, doing research on enhancement of the extract 
yields and phenolic and flavonoid contents of red onion skin 
to acquire potent antimicrobial and antioxidant agents is desir-
able. Extraction is the first important step to recover the bioac-
tive compounds from natural sources. As reported in the litera-
ture, the solvent polarity is one of the important factors which 
has influenced on variations in the yields and phenolic and 
flavonoid contents of various extracts [
13
]. Thus, the choice 
of solvent is a critical step due to the different solubility of 
various classes of antioxidant and antimicrobial compounds 
in solvents.
Abdul Qadir et al. [
14
] showed that TPC and TFC of etha-
nol extract of onion were higher than methanol, acetone and 
aqueous extracts. Also, DPPH radical scavenging activity of 
onion aqueous extracts was higher than those obtained using 
other solvents. Singh et al. [
15
] reported that 70% methanol 
extract of onion had the maximum TPC, TFC and DPPH scav-
enging activity compared to methanol, distilled water and 70% 
ethanol extracts. Generally, polar solvents like aqueous mix-
tures of methanol, ethanol and acetone more than non-polar 
solvents are confirmed to extract of plant bioactive.
Some authors have also evaluated the antimicrobial effects 
of onions on various microorganism [
2
, 
16
]. They suggested 
use of onions as antibacterial agents against foodborne patho-
gens. Thus, this study was designed to evaluate the effect of 
different solvents on the extraction of TPC, TFC, antioxidant 
and the antimicrobial effects of red onion skin extract. Based 
on our knowledge, it is the first report on the phenolic and 
flavonoid contents and antioxidant and antimicrobial activity 
of the extract obtained from red onion skin of Mazandaran 
province, Iran. The objective of the present study was to 
examine the effects of solvent polarity on the extraction yield 
and bioactive content of red onion skin [the outermost layer 
(dry)] extract. Antioxidant and antimicrobial properties of the 
acquired extract is evaluated using DPPH radical scavenging, 
reducing power and nitric oxide scavenging activity to propose 
possible use as food nutritive and medicinal purposes.