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 reducing power
Reducing power is based on the reduction of Fe
3+
to Fe
2+
in the presence of reductants which present in the extracts, 
whereby the yellow color of the test solution changes to 
green and blue, depending on the reducing power of the 
sample. The presence of antioxidants in the sample causes 
the reduction of Fe
3+
/ferricyanide complex to the Fe
2+
form, 
which is monitored by measuring the formation of Perl’s 
Prussian blueat 700 nm. Figure 
2
presents the effect of the 
extraction solvent on the reducing power of the extract. 
Ethyl acetate extract exhibited the highest reducing power 
at concentrations ranging from 6.25 to 400 μg/mL with 
Table 2
TPC and TFC for various extraction solvent
Different letters in each column show the significant difference 
between solvents at p < 0.05
SE standard error
Extraction solvent
Mean ± SE
TPC (mg GAE/g of extract)
TFC (mg 
QE/g of 
extract)
Distilled water
204.56 ± 10.22d
18.46 ± 1.42d
Ethanol
330.51 ± 7.88c
25.97 ± 0.96c
Methanol
332.05 ± 2.97c
25.32 ± 0.25c
n-butanol
385.06 ± 2.55b
27.72 ± 1.13b
Ethyl acetate
423.95 ± 11.39a
29.59 ± 1.71a

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3583
Comparative study on the effect of extraction solvent on total phenol, flavonoid content,…
1 3
mean absorbance of 0.085 to 0.387, respectively, followed 
by n-butanol with absorbance of 0.06 to 0.361.
Ethyl acetate and n-butanol extracts had also higher 
amount of phenolic and flavonoid content indicating that 
the reducing power of all the extracts is highly related to the 
amount of flavonoid and phenolic compounds in the extracts. 
Thus, the extracts can serve as electron donor to terminate 
the radical chain reaction. At lower doses, ranging 6.25 to 
100 μg/mL, the reduction effect was not comparable with 
that of ascorbic acid (positive control). However, in doses 
of 200 and 400 μg/mL, ethyl acetate extract showed better 
activity than ascorbic acid.
In all concentrations, water extract exhibited low reduc-
tion power compared with the other extractions. The results 
also showed that the reducing power increased with concen-
tration. The obtained results depicted a correlation with the 
amount of phenolic and flavonoid constituents and reducing 
capacity. The extracts with higher levels of TPC and TFC 
Fig. 1
DPPH radical scavenging 
activity of red onion skin extract 
obtained using different sol-
vents. (Different letters for each 
solvent show the significant 
difference of DPPH scavenging 
activity for various concentra-
tions of extract at p < 0.05)
Fig. 2
Reducing power of red 
onion-skin extracts obtained 
using different solvents. (Dif-
ferent letters for ascorbic acid 
and each solvent show the 
significant difference of reduc-
ing power for various concen-
trations of ascorbic acid and 
extract at p < 0.05)

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3584
S. Khalili et al.
1 3
exhibited greater ferric reducing power showing reducing 
power assay to be an important parameter used in evaluat-
ing antioxidant activities of natural extracts. It inhibits lipid 
peroxidation by donating a hydrogen atom, resulting in 
termination of free radical chain reaction. Our results were 
consistent with the outcomes reported by many research 
groups, confirming that extraction of phenolic compounds 
greatly depends on the solvent and its polarity and phenols 
and flavonoids directly contribute to antioxidant activity of 
extracts [
30
].