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

Effect of solvents on nitric oxide (NO) scavenging activity
NO is an important chemical mediator generated by 
endothelial cells, macrophages, neurons, and are involved 
in the regulation of various physiological processes, includ-
ing inflammation. Excessive production and release of NO 
are associated with several diseases [
31
]. Excess NO, which 
accumulate in the acidic environment of the stomach, reacts 
with oxygen to form nitrite ions and induce mutagenic reac-
tions. Thus, the produced NO must be scavenged from the 
human body and previous studies have shown that phenolic 
and flavonoid compounds have a great nitrite scavenging 
activity.
NO inhibition of red onion skin extracts obtained using 
different solvents is depicted in Fig. 
3
. Various concentra-
tions of extracts ranged from 100 to 1600 μg/mL indicated 
NO inhibition of 17.56 to 40.18%. The results revealed that 
ethyl acetate and n-butanol had the highest activity to inhibit 
nitric oxide production. The water extract showed the lowest 
activity to inhibit nitric oxide production. The potential of 
NO inhibition of the extracts varied in a following manner 
with some irregularities: Ethyl acetate > n-butanol > metha-
nol > ethanol > distilled water. Furthermore, increasing the 
concentration of onion skin extracts resulted to an increase 
in the radical scavenging activities. The results confirmed 
that the extracts were effective scavengers of nitric oxide 
and could be serve as free radical scavengers.
Antibacterial activity of the extracts
The red onion skin extracts were tested against Gram-pos-
itive (S. aureus) and Gram-negative (E. coli and S. Typhi) 
bacteria and fungi (A. niger and C. albicans). The results 
of inhibition zone diameters of studied microorganism are 
presented in Tables 
3
, 
4
, 
5
, 
6
, 
7
. The MIC, MBC and MFC 
results are also reported in Table 
8
. According to the tables, 
the antimicrobial activity of the extracts increased with an 
increase in the concentrations. Salehi et al. [
32
] and Nelson 
et al. [
33
] reported similar results for the evaluation of anti-
microbial activity of Allium akaka and Allium cepa, respec-
tively. Those authors stated the direct relationship between a 
decrease in a concentration of plant extract and reduction in 
inhibition zone diameter and antimicrobial activity.
All tested extracts revealed capacity to inhibit S. aureus 
and among them, ethyl acetate extract with concentration of 
400 μg/mL showed the most close inhibition zone diameter 
(17.67 ± 0.57 mm) to that of Gentamycin (19.67 ± 0.58 mm). 
In addition, a comparison of the effect of solvent type on 
antibacterial activity of red onion skin extract against S. 
aureus shows that there is a significant difference between 
different extracts. Hafez Quran et al. [
34
] showed that ethyl 
Fig. 3
NO inhibition of red 
onion skin extracts obtained 
using different solvents. (Differ-
ent letters for each solvent show 
the significant difference of NO 
inhibition for various concentra-
tions of extract at p < 0.05)

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Comparative study on the effect of extraction solvent on total phenol, flavonoid content,…
1 3
acetate extract of Scillapersica onion at different con-
centrations resulted in larger diameter of inhibition zone 
compared to chloroform and hydroalcoholic extracts on S. 
aureus, which is consistent with the results of the present 
study on the greater effect of ethyl acetate extract. How-
ever, E. coli and S. Typhi were more resistant to the extract 
showing lower antibacterial activity of the extracts against 
Gram-negative bacteria. Higher inhibitory activity of the 
extracts against S. aureus in comparison with E. coli and S. 
Typhi may cause by the existence of the outer membrane 
of the Gram-negative bacteria as an additional barrier to 
the penetration of some molecules. This membrane is an 
asymmetric bilayer of lipopolysaccharides and phospholip-
ids, into which nonspecific porins and specific uptake chan-
nels are embedded [
35
, 
36
]. Inhibition zone diameter of red 
onion skin extracts against Gram-negative bacteria were 
also lower than the inhibition zone diameters obtained by 
Gentamycin against E. coli (17.33 ± 0.58 mm) and S. Typhi 
(21.00 ± 1.00 mm) confirming the extracts were less active 
against these bacteria.
Some pervious study reported the inhibitory effect of 
onion-hydroalcoholic extract against E. coli, S. aureus and 
S. Typhi [
2
, 
7
, 
16
, 
37
]. The results obtained by Abdel-
Salam et al. [
37
] showed that a concentration of 60 mg/