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

Antioxidant activity
Effect of solvents on DPPH radical scavenging activity
DPPH free-radical scavenging method is based on the 
exchange of hydrogen atoms between the antioxidant and 
the stable DPPH free radical. The used synthetic-free radi-
cals are DPPH. This compound reacts with antioxidant 
compounds by taking hydrogen atoms from antioxidant 
compounds and getting electron pairs. When an antioxi-
dant encounters the DPPH solution, the color changes from 
purple to yellow due to its scavenging action on free radi-
cal. The color change of DPPH solution in the presence of 
extracts was measured at 517 nm spectrophotometrically 
[
29
]. A lower absorption spectrum intensity shows a higher 
DPPH radical-scavenging activity. Figure 
1
illustrates 
DPPH radical scavenging of various extracts obtained by 
extraction of powdered onion skin at concentrations rang-
ing from 6.25 to 100 μg/mL. The results depict the DPPH 
scavenging activities of the extracts in a concentration-
dependent manner. The IC
50
values of the extracts were 
20.926 ± 0.41, 21.738 ± 0.43, 22.447 ± 0.59, 24.48 ± 0.64 
and 36.111 ± 0.86 μg/mL for ethyl acetate, n-butanol, etha-
nol, methanol and distilled water, respectively. The extract 
obtained by ethyl acetate yielded the highest DPPH radical 
scavenging activity. IC
50
values of the extracts demonstrated 
the noticeable antioxidant activity, comparable to that of 
BHA (51.43 ± 0.70 μg/mL). In general, the results showed 
that all the prepared extracts have the ability to scavenge 
free radicals. This finding is in agreement with the results 
reported by Singh et al. [
15
] and Skerget et al. [
25
] which 
showed that the antioxidant and radical scavenging activities 
of red onion skin extracts were commonly high, compara-
ble to that of BHA. However, extracts acquired by organic 
solvents provided stronger radical scavenging capacity than 
that of distilled water.
Moreover, our results show that DPPH radical scaveng-
ing activity has high correlation with total flavonoid content 
(R = 0.9468). Lower correlation was observed between this 
activity and total phenolic contents (R = 0.8908). Solvents 
polarity and its effect on extraction of phenol and flavonoids 
are mainly responsible for differences between the DPPH 
radical scavenging capacities of the extracts. The capacity of 
phenolic and flavonoid compounds to scavenge free radicals 
may be due to the presence of the hydroxyl groups in their 
structure and their electron donating ability which transfer 
hydrogen to radicals and produce phenoxide radical to sta-
bilize products.