Food and Nutrition Sciences, 2013, 4, 1260-1265
Published Online December 2013 (http://www.scirp.org/journal/fns)
http://dx.doi.org/10.4236/fns.2013.412161
Open Access FNS
Evaluation of the Antioxidan t Activity of Ethanolic
Extracts of Some Varieties of Onion
María del Carmen Gutiérrez, Patricia Della Rocca, Elizabeth Graciela De Seta, Fernando Reina
Chemical Engineering Department, Buenos Aires Regional Faculty, National Technological University, Ciudad Autónoma de Bue-
nos Aires, Argentina.
Received September 4th, 2013; revised October 4th, 2013; accepted October 11th, 2013
Copyright © 2013 María del Carmen Gutiérrez et al. This is an open access article distributed under the Creative Commons Attribu-
tion License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly
cited. In accordance of the Creative Commons Attribution License all Copyrights © 2013 are reserved for SCIRP and the owner of
the intellectual property María del Carmen Gutiérrez et al. All Copyright © 2013 are guarded by law and by SCIRP as a guardian.
ABSTRACT
The content of polyphenolic in commercial onions has been determined. The antioxidant activity of their ethanolic ex-
tracts, as well as their effects on the oxidation of edible corn oil during accelerated ageing, was studied. Maceration of
taxonomically identified commercial vegetable samples, previously peeled and thinly sliced, was carried out at ambient
temperature, out of direct light, with occasional agitation and ultrasound, employing 95% ethyl alcohol as the extraction
solvent, allowing them to stand for 7 days. The total polyphenolic contents were determined on the filtrated extracts
using the Folin-Ciocalteau method. The antioxidant activity was evaluated on emulsions of ethanolic extracts of onion
prepared in edible commercial corn oil, using sorbitan monooleate as emulsifying agent. The peroxide values were ana-
lyzed using the iodometric method; oxidation induction times were obtained from the peroxide evolution graphs, using
the tangent method. Oil samples emulsified with ethanolic onion extracts showed an extension of the induction period.
A 7 day ageing study at 45˚C was additionally performed to determine the conjugated dienes on pure commercial corn
oil and its emulsions by visible spectrophotometry. The spectral analysis showed an increase of the measured absorb-
ances in oil samples without additives and no change for the oils emulsified with onion extract. An increasing of diene
values was observed for corn oil without additives during ageing; no changes in the value were observed in oils emulsi-
fied with onion extracts.
Keywords: Onion; Antioxidant Activity; Polyphenolic; Ethanolic Extracts; Emulsified Oil
1. Introduction
Epidemiologic studies have determined that the con-
sumption of vegetables and fruit is related to the reduc-
tion of the risks of contracting a cardiovascular disease or
cancer.
Foods containing significant amounts of bioactive
components may provide desirable health benefits and
play important roles in the prevention of chronic diseases
[1].
At present we pay the interest in the biological effects
of phenolic compounds as it was found that diets rich in
fruits and vegetables appear to protect against cardio-
vascular disease [2,3] and some forms of cancer [4].
Currently there is an increasing preference for the use of
natural antioxidants [5] although many of them have
been used since antiquity. Many plant extracts have
demonstrated considerable stabilizing effect against lipid
oxidation reactions and, consequently, they can have
significant commercial potential as source of nutraceuti-
cal or functional food ingredients [6]. Antioxidants with
an important activity have been found in berries [7],
cherries [8], citrics [9], kiwis [10], olives [11], cocoa [12],
potatoes [13], tomatoes [14], garlics [15], onions [16]
and soybeans [17]. Most of the spices, e.g. red pepper
[18], ginger [19] and rosemary [20] also have relevant
antioxidant properties.
Onion is proposed as a viable source of phenolic
compounds and flavonoids. Studies conducted over dif-
ferent cultivars demonstrate that total oxidant activity is
lower in white onion varieties; red varieties have a 100
mg/100g average content of gallic acid equivalents [21].
The essential oil reveals interesting properties, such as

Evaluation of the Antioxidant Activity of Ethanolic Extracts of Some Varieties of Onion 1261
antimicrobial agent and moderate reducing power feasi-
ble to implement in food [22]. Also, phenolic extracts
obtained from wastes of onion were used to evaluate the
capacity inhibiting of processes inflammatory and oxida-
tion of low-density lipoprotein (LDL) [23].
Likewise, different methods of extraction of the active
components have been used for their assessment, result-
ing in variations of the reported activities [24,25]. When
extraction is carried out by the method of microwave-
assisted greater efficiency is obtained with higher anti-
oxidant activities [26].
Antioxidant capacity scores were reported [27] from
the oxygen radical absorption capacity measurements
(ORAC).
Most investigations have been dedicated to quantify-
ing polyphenols and its antioxidant capacity. Despite the
results achieved, further studies should be performed in
order to enhance the knowledge about extraction and
identification of active components present in less stud-
ied vegetables.
The efficiency of extraction of natural antioxidants
(NAO) depends on the fraction, the type of vegetable
used, and the ability of the active components to provide
sufficient antioxidant capacity. Therefore a simple me-
thod of extraction of polyphenolic components from
commercial onions has been developed by the authors, in
order to evaluate their performance as an alternative
source of natural antioxidants.
2. Materials and Methods
2.1. Plant Material
White onion taxonomically identified as Allium cepa,
provided by the National Institute of Agricultural Tech-
nology (I.N.T.A. Mendoza) and commercial white onion
purchased in local grocery store. Bulbs from each variety
were stored at 4˚C until sampling.
2.2. Sample Preparation
Bulbs were randomly selected for extraction. Onions
were peeled, eliminating the skin and the first and second
layers, and chopped in fine pieces.
2.3. Extraction
Maceration was performed at room temperature, out of
direct light, with occasional agitation and ultrasound (to
facilitate extraction) of an exactly weighed sample quan-
tity, using 99.5%-absolute-pro analysis ACS ethyl alco-
hol as extraction solvent, in a 50% concentration (in
masses), allowing them to stand for 7 days. Ethanolic
extracts were filtered using glass wool and diluted 10:1
with 80% ethanol to a total of 5 cm3.
2.4. Spectrophotometric Analysis
Absorbance (AU) readings were made in triplicate using:
Shimadzu series UV1700 Spectrophotometer
Quartz Cuvettes of 1.0 cm thickness.
Wavelengths (λ):
 Determination of Total Polyphenol at 765 nm.
 Determination of Conjugated Dienes at 233 nm.
 Determination of Conjugated Trienes at 268 nm and
278 nm.
2.5. Determination of Total Polyphenols
Spectrophotometric determination of Total Polyphenol
(PFT), at 765 nm, in the extracts obtained was performed
using the Folin-Ciocalteu method, based on the Singleton
and Rossi (1965) procedures and modified by Water-
house (2001).
Determinations were carried out, both for the classi-
fied and for the commercial onion samples. Concentra-
tions of polyphenols in sample extract were calculated by
linear regression onto the standard curve of monohy-
drated gallic acid, ACS analytic reagent, as standard.
Three parallel determinations of each sample were
performed. The average values and their standard devia-
tions were calculated
2.6. Determination of Peroxide Value
The peroxide values (PV) were analyzed by the iodomet-
ric method, employing p.a. ACS potassium iodide solu-
tion, analytic reagent ACS sodium thiosulfate pentahy-
drate for the preparation of the titrating solution, pro
analysis trichloromethane (chloroform) and soluble starch
as indicator, according to the American Oil Chemists’
Society AOAC Official Method 942.27 [28].
Iodine is liberated by hydroperoxides in the oil in
presence of excess iodide in a stoichiometric ratio. The
amount of iodine present is determined by titration with a
standard sodium thiosulfate solution using a starch indi-
cator, thereby reflecting how much peroxide is present in
the oil or lipid extract.
Three parallel determinations of each sample were
carried out. The average values and their standard devia-
tions were calculated.
2.7. Antioxidant Capacity
Emulsions of ethanolic extract of onion were prepared in
10% concentration (in masses) in edible commercial corn
oil with composition of 51 g/100g polyunsaturated fat
and vitamin E (25 mg/100g of oil) using sorbitan mono-
oleate (SPAN 80) in a concentration of 1% m/m as emul-
sifying agent.
A sample of the same commercial corn oil without ad-
ditives was used as blank; a corn oil sample with a syn-
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Evaluation of the Antioxidant Activity of Ethanolic Extracts of Some Varieties of Onion 1262
thetic antioxidant (butylhydroxytoluene) added, in 0.01%
concentration (in masses) was also prepared for com-
parison.
The samples were stored for 45 days on a heater at
45˚C temperature and protected from direct light, with
occasional agitation. With the PV results, the evolution
of the mEq O2/kg generated in the oxidation process was
recorded as function of the storage time. For each of the
samples under study, oxidation induction times were
determined from the charts by the tangents method.
2.8. Conjugated Dienes Assay
A 7 day ageing study, at a temperature of 45˚C, was ad-
ditionally performed to determine primary lipid oxidation
products: Conjugated Dienes were determined on both
the pure commercial corn oil and the samples of the
emulsions of ethanolic onion extract in edible corn oil,
by UV-Visible spectrophotometry, measuring the ab-
sorbances at 233 nm, 268 nm and 278 nm and 2.2.4-
trimethylpentane (Isooctane) for chromatographic use, as
a sample preparation solvent, as described in Current
Protocols in Food Analytical Chemistry (2001) D21.1-
D2.1.15 [29].
The absorbance, measured at 233 and 268 nm, was
employed to monitor the formation of conjugated dienes
(CDs) and trienes (CTs) of polyunsaturated fatty acids
(PUFAs), respectively. Increasing absorption values are
an indication that oxidation is proceeding. The CD moi-
ety is a strong UV-absorbing chromophore that can be
spectrophotometrically detected. When present in fatty
acids, the CD moiety shows an absorption in the UV re-
gion at 233 nm and stands out as a distinct peak. When
PUFAs containing three or more double bonds (e.g., li-
nolenic acid) undergo oxidation, the conjugation of CD
moieties can be extended to include another double bond
resulting in the formation of a conjugated triene (CT).
CTs absorb radiation in the UV region like CDs, but
show three signature absorption bands. The main peak is
at 268 nm; there is also a secondary peak at 278 nm.
From the results obtained the concentration of conju-
gated dienes [CD] was calculated as follows:

CD
A
l



(1)
where
A = Absorbance at 233 nm.
ε = 2.525 × 104 M−1·cm−1 = Molar absorptivity of Li-
noleic Acid Hydroperoxide.
l = Optical path of the cuvette 1 cm.
And the conjugated diene value (CD value):
4
value
[CD]2.510
CD m

 (2)
where:
2.5 × 104 is the correction factor.
m = mass of the sample (g).
3. Results and Discussion
3.1. Total Polyphenols
The calibration curve obtained, using gallic acid as ex-
ternal standard, corresponds to a first-order equation, and
0.006 0.027yx


with correlation coefficient R2 = 0.9975.
The taxonomically identified onion extract has a total
polyphenol concentration of (323 ± 18) mg/dm3, being
slightly higher than that corresponding to the commercial
onion extract (310 ± 18) mg/dm3.
3.2. Peroxide Values
The values obtained for peroxide values (PV in mEq
O2/kg) at different storage times for the samples analyzed
are shown in Table 1.
Figure 1 shows the evolution of the Peroxide Values.
Table 1. Peroxide values of the samples analyzed at differ-
ent storage times.
Time
(days)
PV (1)
(mEq O2/kg)
PV (2)
(mEq O2/kg)
PV (3)
(mEq O2/kg)
PV (4)
(mEq O2/kg)
5 1.7 ± 0.1 1.5 ± 0.1 1.9 ± 0.1 1.6 ± 0.1
12 4.2 ± 0.1 2.1 ± 0.1 1.6 ± 0.1 3.4 ± 0.1
19 3.5 ± 0.1 3.1 ± 0.1 2.9 ± 0.1 2.0 ± 0.1
24 12.4 ± 0.1 12.7 ± 0.1 18.1 ± 0.1 22.7 ± 0.1
33 14.9 ± 0.1 18.2 ± 0.1 34.1 ± 0.2 53.6 ± 0.2
40 17.4 ± 0.1 22.9 ± 0.2 46.9 ± 0.2 92.0 ± 0.2
47 27.0 ± 0.2 26.9 ± 0.2 48.9 ± 0.2 100.1 ± 0.2
References: (1) Edible oil with aggregated ethanolic extract of commercial
Allium cepa. (2) Edible oil with aggregated ethanolic extract of Allium cepa
classified. (3) Edible oil with aggregated Butylhydroxytoluene. (4) Edible
oil without antioxidant aggregate.
Figure 1. Evolution of the peroxide values.
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Evaluation of the Antioxidant Activity of Ethanolic Extracts of Some Varieties of Onion 1263
It can be seen that the peroxide value for emulsified
oils with ethanol extracts of onion, after forty-five days
of ageing, is 25 mEq O2/kg. In comparison, the oil
samples without additive and with butylhydroxytoluene
reached that value in less time.
Figure 2 shows the time in which the maximum limit
accepted by the Argentinean Food Code (CAA), 10 mEq
O2/kg is reached.
An increase of the induction period can be observed in
the oil samples emulsified with ethanolic onion extract,
including those classified as commercial, as well as a
delay of the time in which peroxides reach the limit es-
tablished by the Argentinean Food Code (CAA), 10 mEq
O2/kg, in the kinetic curves of peroxide accumulation,
thus demonstrating a net antioxidant effect produced by
the polyphenolic contents present in the onion extracts.
Figures 3-5 show the kinetics curves peroxide accu-
mulation, which were used to determine the induction
times. The results were: 17, 17.5 and 19.5 days, respec-
tively, for pure corn oil, oil with addition of butylhy-
droxytoluene and oil emulsified with ethanolic onion
extract.
Oxidation induction times were determined from the
charts by the tangents method.
Figure 2. Evolution of the peroxide values.
Figure 3. Evolution of the peroxide value for corn oil with-
out aggregates.
3.3. Spectral Analysis by Means of UV-Visible
Spectrophotometry and Determination of
Conjugated Dienes
Figures 6 and 7 show the spectral analysis, which re-
veals an increase of the absorbancies measured for the oil
samples without additives and no modification for the
oils emulsified with onion extracts. The information ob-
tained is presented on Tables 2 and 3.
Results obtained for the concentration of conjugated
dienes were 9.02 × 10−6 mmol/cm3 for the corn oil with-
Figure 4. Evolution of the peroxide value for corn oil with
BHT.
Figure 5. Peroxide value evolution over time for emulsion
obtained from the Allium cepa extracts in corn oil.
Table 2. Sample absorbancies obtained at day 1.
Samples Mass (g)Abs 233 nm Abs 268 nm Abs 278 nm
Pure oil 0.0180.228 ± 0.001 0.052 ± 0.001 0.046 ± 0.001
Oil + extract0.0190.294 ± 0.001 0.078 ± 0.001 0.071 ± 0.001
Table 3. Sample absorbancies obtained at day 7.
Samples Mass (g)Abs 233 nm Abs 268 nm Abs 278 nm
Pure oil 0.01550.347 ± 0.001 0.066 ± 0.001 0.059 ± 0.001
Oil + extract0.01860.288 ± 0.001 0.059 ± 0.001 0.053 ± 0.001
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Evaluation of the Antioxidant Activity of Ethanolic Extracts of Some Varieties of Onion 1264
Figure 6. UV-Visible spectrophotometry for corn oil with-
out antioxidant aggregates in its initial state (dotted line
spectrophotometry) and after 7 days at a temperature of
45˚C (continuous line spectrophotometry).
Figure 7. UV-visible spectrophotometry for the emulsions of
ethanolic extracts of onion (10% m/m) in commercial corn
oil, in its initial state (dotted line spectrophotometry) and
after 7 days elapsed at a temperature of 45˚C (continuous
line spectrophotometry).
out additives at the beginning of the experiment, rising to
1.37 × 10−5 mmol/cm3 on the 7th day of ageing, corre-
sponding to a dienes index of 12.5 mmol/g and 22.2
mmol/g respectively.
On the other hand, the oils emulsified with onion ex-
tracts presented a concentration of conjugated dienes of
1.16 × 10−5 mmol/cm3 in the initial level and 1.14 × 10−5
mmol/cm3 on the 7th day. The conjugated dienes indexes
were 14.9 µmol/g and 15.3 µmol/g respectively.
Data shows the increase of concentration of conju-
gated dienes in the oil without antioxidants. Likewise no
increase was found on oil samples with ethanolic onion
extracts. Thus was demonstrated the antioxidant capacity
of polyphenols extracted from some varieties of onion.
4. Conclusions
The results obtained suggest that commercial onions
have a considerable content of polyphenols. These facts
lead to the conclusion that the addition of ethanol ex-
tracts of onion commercial to corn oil prolongs oxidation
induction time and also generates a decrease in the con-
centration of conjugated dienes in ageing period, which
show the antioxidant power of the polyphenols present.
The use of commercial onions as sources to obtain
polyphenolic substances and their applications as anti-
oxidant additives can be considered entirely viable.
5. Acknowledgements
This research was funded by National Technological
University, Buenos Aires Regional Faculty.
The authors thank Dr. Isaac Marcos Cohen for his
valuable contribution.
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