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

Table 7
Inhibition zone diameter (mm, mean ± SD) of red onion skin extracts against C. albicans 
Values in a row with same lower case letters and values in columns with same upper case letters are not statistically different (p > 0.05)
**Shows a significant difference at 1% probability level
Extract
Concentrations (μg/mL)
6.25
12.5
25
50
100
200
400
Aqueous
6.33 ± 0.58d (C)
7.33 ± 0.58c (B)
7.33 ± 0.58c (B)
7.00 ± 1.00bc (B)
7.00 ± 0.00bc (B)
7.00 ± 0.00bc (B)
7.67 ± 0.58b (B)
Ethanol
7.67 ± 0.58b (B)
7.67 ± 0.58b (B)
7.67 ± 0.58b (B)
8.33 ± 0.58b (B)
8.00 ± 0.00b (B)
8.33 ± 0.58b (B)
8.67 ± 0.58b (B)
Methanol
7.00 ± 0.00c (BC)
7.00 ± 0.00c (B)
7.00 ± 0.00c (B)
7.33 ± 0.58c (B)
8.00 ± 1.00bc (B)
8.00 ± 1.00bc (B)
8.67 ± 0.58b (B)
n-butanol
7.33 ± 0.58b (B)
7.67 ± 0.58b (B)
7.33 ± 0.58b (B)
7.67 ± 0.58b (B)
7.67 ± 0.58b (B)
7.67 ± 0.58b (B)
7.67 ± 0.58b (B)
Ethyl acetate
9.00 ± 0.00e (A)
9.33 ± 0.58de(A)
10.00 ± 0.00cde(A)
9.67 ± 0.58cde (A)
10.33 ± 0.58cd (A)
10.67 ± 0.58c (A)
11.67 ± 0.58b (A)
F tests
14.67**
9.13**
22.17**
9.13**
10.70**
11.08**
20.70**
Table 8
MIC and MBC 
concentrations of red onion 
skin extracts against the studied 
microorganism
MIC minimum inhibitory concentration (μg/mL), MBC minimum bactericidal concentration (μg/mL), 
MFC minimum fungicidal concentration (μg/mL)
Extract
Aqueous
Ethanol
Methanol
n-butanol
Ethyl acetate
Staphylococcus aureus
MIC
400
200
200
100
100
MBC
800
400
400
200
200
Escherichia coli
MIC
400
400
400
200
400
MBC
800
800
800
400
800
Salmonella Typhi
MIC
400
400
400
400
400
MBC
800
800
800
800
800
Aspergillus niger
MIC
200
100
100
100
100
MFC
400
200
200
200
200
Candida albicans
MIC
800
800
800
800
400
MFC
1600
1600
1600
1600
800

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Comparative study on the effect of extraction solvent on total phenol, flavonoid content,…
1 3
effects could be attributed to the quantity, quality and solu-
bility ratios of bioactive compounds in solvents [
38
].
The studies extracts also showed good antifungal activ-
ity as antimicrobial. Especially, ethyl acetate and n-butanol 
extracts with concentration of 400 μg/mL showed remark-
able inhibitory effect against A. niger with the same inhibi-
tion zone diameter to that of Miconazole (23.00 ± 2.65 mm) 
as positive control. In all concentration of the extracts, the 
water extract of red onion skin had the lowest antifungal 
effect. Moreover, C. albicans was more resistant to the 
extracts as compared to A. niger.
Similar results were reported by Alseeni et al. [
39
] who 
shown methanol extract of onion was more effective in 
inhibiting the growth of A. niger compared to the aqueous 
extract. In fact, according to their study, the aqueous extract 
had no inhibitory effect. Higher inhibitory effect of alcoholic 
extract of onion against C. albicans with compared to water 
extract have been also reported previously by some authors 
which is consistent with the results of the present study [
40
].
In general, the results showed that the solvents, which 
resulted to higher TPC, caused higher antioxidant and anti-
microbial activity of the extracts. A relationship between 
TPC and antimicrobial activity was also observed. Actu-
ally, the position of hydroxyl groups of polyphenols in the 
aromatic ring and the length of the saturated side chain of 
phenolic acids can cause an increase the antimicrobial activ-
ity of the extracts [
41
].
Considering MIC, MBC and MFC values, ethyl acetate 
and n-butanol extract was the most and water extract was the 
least effective against tested microorganism. In addition, C. 
albicans was the most resistant and A. niger was equally sen-
sitive to all extracts except the water extract. Thus, A. niger 
was less sensitive to the water extract than other extracts.