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J Am Oil Chem Soc (2015) 92:409–421
DOI 10.1007/s11746-015-2599-2
1 3
ORIGINAL PAPER
Oxidative Stability and Changes in Chemical Composition
of Extra Virgin Olive Oils After Short‑Term Deep‑Frying
of French Fries
Emília Akil · Vanessa Naciuk Castelo‑Branco · André Mesquita Magalhães Costa ·
Ana Lúcia do Amaral Vendramini · Verônica Calado · Alexandre Guedes Torres
Received: 31 January 2014 / Accepted: 15 January 2015 / Published online: 6 February 2015
© AOCS 2015
carbonyl compounds, and were highly stable after deep-
frying. In addition, oleic acid, tocopherols, and flavor com-
pounds were transferred from EVOO into the French fries.
In conclusion, EVOOs were more stable than refined seed
oils during short-term deep-frying of French fries and also
contributed to enhance the nutritional value, and possibly
improve the flavor, of the fries prepared in EVOO.
Keywords Olive oil quality · Thermal oxidation · DSC ·
Deep-frying · Stability · Tocopherols
Abbreviations
AV Acid value
DAG Diacylglycerol
DSC Differential scanning calorimetry
EVOO Extra virgin olive oil
FAMEs Fatty acid methyl esters
FFA Free fatty acid
GC Gas chromatography
HPLC High performance liquid chromatography
IP Induction period
IV Iodine value
MAG Monoacylglicerol
MUFA Monounsatured fatty acids
PUFA Polyunsaturated fatty acid
p-Av p-Anisidine value
PV Peroxide value
TAG Triacylglycerol
Total-Toc Total tocopherols
Introduction
Deep-frying is an attractive food preparation method and
has been in use for centuries. It is a very popular culinary
Abstract We aimed at investigating oxidative stability
and changes in fatty acid and tocopherol composition of
extra virgin olive oil (EVOO) in comparison with refined
seed oils during short-term deep-frying of French fries, and
changes in the composition of the French fries deep-fried in
EVOO. EVOO samples from Spain, Brazil, and Portugal,
and refined seed oils of soybean and sunflower were stud-
ied. Oil samples were used for deep-frying of French fries
at 180 °C, for up to 75 min of successive frying. Tocoph-
erol and fatty acid composition were determined in fresh
and spent vegetable oils. Tocopherol, fatty acid, and vola-
tile composition (by SPME–GC–MS) were also determined
in French fries deep-fried in EVOO. Oil oxidation was
monitored by peroxide, acid, and p-anisidine values, and by
Rancimat after deep-frying. Differential scanning calorime-
try (DSC) analysis was used as a proxy of the quality of the
spent oils. EVOOs presented the lowest degree of oleic and
linoleic acids losses, low formation of free fatty acids and
E. Akil · A. L. do Amaral Vendramini · V. Calado
Escola de Química, Universidade Federal do Rio de Janeiro,
Avenida Athos da Silveira Ramos, No 149, Bloco E, Centro
de Tecnologia, Cidade Universitária, CEP 21941-909 Rio de
Janeiro, RJ, Brazil
E. Akil · V. N. Castelo-Branco · A. M. M. Costa ·
A. G. Torres (*)
Laboratório de Bioquímica Nutricional e de Alimentos,
Instituto de Química, Universidade Federal do Rio de Janeiro,
Avenida Athos da Silveira Ramos, No 149, Bloco A, Centro de
Tecnologia, sala 528A, Cidade Universitária, CEP 21941-909 Rio
de Janeiro, RJ, Brazil
e-mail: torres@iq.ufrj.br
V. N. Castelo-Branco
Laboratório de Biotecnologia de Alimentos, Faculdade de
Farmácia, Universidade Federal Fluminense, Rua Dr. Mário
Viana, 523, Santa Rosa, CEP 24241-000 Niterói, RJ, Brazil
410 J Am Oil Chem Soc (2015) 92:409–421
1 3
practice worldwide and can be used both industrially and
domestically. Deep-fried foods have unique sensorial prop-
erties, such as flavor, texture, and appearance, which are
generally highly appreciated by consumers. French fries
are a convenient product, highly consumed in industrialized
countries [1]. However, the chemical stability of the frying
oil can be affected by the high temperatures normally used
in deep-frying. Thermal oxidation is an important cause
of oil deterioration, especially in the food industry, after
consecutive frying. High temperatures promote chemical
decomposition resulting in the loss of sensory and nutri-
tional quality. Thermal oxidation of oils leads to the for-
mation of hydroperoxides, known as primary oxidation
products, which degrade into hydrocarbons, aldehydes, and
ketones, among other classes of compounds, known as the
secondary oxidation products. Secondary products tend to
be volatile and are responsible for the rancid flavor of oxi-
dized oils. The oxidative degradation indices assess these
primary and secondary products and are used as surrogate
measures for oil quality and oxidative stability [2].
The heat transferred from oil to food during frying pro-
cesses promotes water loss from the food and this water is
partially replaced by oil that is absorbed from the frying
medium by the food. Water availability and the food–oil
interface form a favorable medium for chemical decom-
position through pathways different from those occurring
in heated bulk oil, such as hydrolysis. Deep-frying foods
promotes a complex pattern of thermolytic reactions in the
frying oils, resulting in increased free fatty acids (FFA),
which accelerate the formation of primary and secondary
oxidation products, modifying nutritional and sensorial
properties of the oils [3]. Changes in chemical composition
of deep-frying oils occur even during short-term frying [4],
which is of concern for the domestic preparation of French
fries, and other deep-fried foods.
Extra virgin olive oil (EVOO) stands out among plant
oils because of its nutritional value, especially the high con-
tents of oleic acid (18:1n-9) and of minor components with
strong antioxidant activity, and also because of its appreci-
ated flavor [2]. EVOO is typically the major lipid source in
the Mediterranean diet, where it is added raw to prepared
food products and is also used for cooking, such as by deep-
frying. The habitual consumption of EVOO might help pre-
vent cardiovascular diseases, even after deep-frying. Food
fried in EVOO reduced insulin and C-peptide responses
after intake of a mixed meal, improving postprandial insu-
lin response in adult obese women [5]. Although stability
of seed oils and EVOO has been thoroughly studied during
long-term deep-frying, short-term deep-frying in EVOO
has been scarcely studied [6]. Sánchez-Gimeno et al. [6]
studied the formation of polar compounds, and changes
in color and viscosity during short-term deep-frying with
EVOO. The amounts of primary and secondary oxidation
products formed in refined olive oil during short-term deep-
frying were lower than those of seed oils [6]; however, the
behavior EVOO was not studied. To our knowledge, there
is still a lack of information concerning chemical transfor-
mations of deep-fried EVOO for short time periods, which
is of concern for domestic food preparation.
In the present study we aimed to evaluate changes in
the chemical quality of EVOOs, from different olive varie-
ties, during short-term deep-frying of French fries. We also
aimed to compare oxidation stability between EVOOs and
refined seed oils commonly used for deep-frying foods,
and to investigate the composition of tocopherols, fatty
acids, and volatile compounds in French fries after frying
in EVOOs.
Materials and Methods
Samples
Three commercial samples of European EVOOs were pur-
chased in local markets in Spain (monovarietal Arbequina
and Picual) and Portugal. One non-commercial sample
of EVOO produced from olives produced in the city of
Maria da Fé (Minas Gerais, Brazil) was acquired directly
from EPAMIG (Minas Gerais Enterprise for Agricultural
Research, Brazil). Two samples of refined oils of sun-
flower and soybean were purchased at local markets (Rio
de Janeiro, Brazil) and used for comparisons with EVOO.
Oil samples were aliquoted and stored at −18 °C protected
from light and under nitrogen until analysis.
Industrially prefried and frozen French fries (Sadia®,
Brazil) were purchased in local markets in Rio de Janeiro,
Brazil. Before analysis and experiments, the prefried
French fries were completely defrosted and excess water
was dried with a clean cloth. Before analysis, the French
fries were dried in a convection oven with forced air circu-
lation (65 °C, 24 h) and milled in an analytical mill (A11
Basic mill, IKA®, Brazil) before lipid extraction using
petroleum ether in a Soxhlet apparatus ([7], method Ba
3-38). This ether extract, which was the oil absorbed by
French fries, was stored at −20 °C until analysis.
Deep-Frying Experiments at 180 °C
EVOOs and seed oils (soybean and sunflower) were used
in short-term deep-frying experiments at 180 ± 1 °C with
the industrially prefried French fries. After equilibrating
500 mL of each vegetable oil for 10 min at 180 °C in an
electric deep-fryer with 850 mL capacity (Vicini®, Bra-
zil), French fries were deep-fried for 5 min in batches of
200 g. Each batch of fries presented acceptable appearance
and texture after 5 min. After three consecutive batches of
411J Am Oil Chem Soc (2015) 92:409–421
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French fries, 50-mL aliquots of the oils were taken at 25,
50, and 75 min of deep-frying tests (Fig. 1). Subsequently,
the oil volume was refilled to 500 mL with 60 ± 1 mL, fol-
lowed by 10 min for reheating the oil back to 180 °C. This
sequence was repeated for three times, consisting of frying
tests with a total time of 75 min.
A subsample of French fries, deep-fried in Portuguese
and Spanish Arbequina EVOOs, was analyzed in order to
investigate absorption of oil and chemical compounds from
the EVOOs. In these prepared fries, the chemical compo-
sition (moisture, and fatty acid, tocopherols, and volatile
composition) and oxidative quality (peroxide value) and
stability (Rancimat®) were determined. Total lipids in the
prepared French fries were extracted from dried and milled
samples, as described above. Prefried French fries (non-
prepared) were used as controls. All samples were stored at
−20 °C under nitrogen until analysis.
Oxidative Stability During Deep-Frying at 180 °C
Quality indices of the oils were determined in fresh sam-
ples and during deep-frying at 180 °C. The acid value (AV)
of the samples was determined by titrating 1–2 g of sam-
ple in ethyl ether/ethanol (2:1, v/v) with 0.01 N NaOH, and
the results were expressed as percentage of oleic acid ([7],
method Ca 5-40). The peroxide value (PV) was determined
by iodometric titration ([7], method Cd 8-53). Briefly,
0.5 mL of saturated KI solution in water was added to
0.5 g of sample dissolved in 10 mL of acetic acid/chloro-
form (3:2, v:v). After incubation in the dark for 5 min, and
addition of 10 mL of distilled water, the I2 liberated was
titrated with 0.01 N Na2S2O3. A starch solution was used to
help visualize the titration endpoint. PV was expressed as
mEq O2/kg. The p-anisidine value (p-Av) was determined
in approximately 2–4 g of oil, depending on the samples’
degree of oxidation, and analyzed as described previously
([7], method Cd 18–90). Briefly, 5 mL of oil solution in
n-hexane was mixed with the p-anisidine reagent in gla-
cial acetic acid and incubated in the dark for 10 min. The
absorbance of samples and blank solutions were measured
at 350 nm in a double beam spectrophotometer (Shimadzu
1800, Kyoto, Japan).
In a subsample of olive oils, Portuguese and Span-
ish Arbequina EVOOs, oxidative stability was monitored
through the induction period (hours) determined by Ranci-
mat® equipment (Metrohm 743; Metrohm Co., Basel, Swit-
zerland), both in the bulk oil from the fryer and in the oil
absorbed by the French fries. Oil samples (3 g) were heated
at 110 °C with a 20 L/h air flow rate and the time required
for a sharp increase in water conductivity was calculated
by the instrument’s software package and corresponds to
the induction period, in hours. All analyses were done in
duplicate.
Analysis of Fatty Acids and Calculation of Iodine Value
(IV)
Fatty acid methyl esters (FAMEs) from EVOOs and the oil
absorbed by French fries during deep-frying were prepared
as described previously [8]. The FAMEs were analyzed in
a Varian CP 3800 GC (Varian Co., USA) equipped with a
split/splitless injector, a fused silica polyethylene glycol
capillary column (30 m × 0.53 mm ID and 1.0 µm film;
Carbowax 20 M; Supelco, USA), and a flame ionization
detector. The column oven temperature was programmed as
follows: 150 °C kept for 1 min, followed by gradient heat-
ing of 10 °C/min up to 180 °C through a second gradient of
3 °C/min to reach the final column temperature of 230 °C.
The FAMEs were dissolved in n-hexane, and 1.0 μL was
injected at a 1:10 split ratio. The injector and detector tem-
peratures were 220 and 260 °C, respectively. Peaks were
identified on the basis of the retention times of FAME
commercial standards (Sigma-Aldrich, Brazil). Fatty acid
contents were calculated by internal normalization and
the results expressed as grams per 100 g of fatty acids,
and presented for the fatty acids with contents higher than
0.5 g/100 g. Iodine value (IV) was calculated on the basis
of the samples’ fatty acid composition, for all EVOOs and
seed oils, and all time intervals of deep-frying, as described
in AOCS method 1c-85 [7].
Analysis of Tocopherols
Tocopherols (α, β, γ, and δ) were analyzed by normal-
phase HPLC [9] with fluorescence detection [10] in an LC-
10AVp chromatograph equipped with an RF-10AXL detec-
tor, a photodiode array detector (PDA), and a CBM-20A
communication module (all from Shimadzu, Tokyo, Japan).
Fig. 1 Experimental design of the deep-frying tests with the French fries. Vertical arrows indicate the end of each 5-min batch of short-term
deep-frying of French fries
412 J Am Oil Chem Soc (2015) 92:409–421
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Commercial standards (Sigma-Aldrich, Brazil) of α-, β-, γ-,
and δ-tocopherols were used to identify chromatographic
peaks and for quantitative analyses of tocopherols in the
samples. Analytes were eluted from a normal-phase silica
column (ZORBAX Rx-Sil; Agilent Technologies, USA)
with an isocratic binary solvent system of n-hexane/2-
propanol (99:1, v/v) at 1.0 mL/min. Samples were injected
through a Rheodyne valve with a 20-μL volumetric loop.
Peak assignment in the samples’ chromatograms was based
on relative retention time, standard spiking, and UV spec-
tra (240–400 nm) from the PDA. Quantitative analysis was
based on peak areas from the fluorescence detector oper-
ated at 290 nm for excitation and 330 nm for emission.
Analyses were performed in duplicate and average values
were reported.
Differential Scanning Calorimetry (DSC) Analysis
The DSC analyses were performed according to Che Man
and Tan [11] with adaptations. Before analysis, the equipment
was purged with dry nitrogen at a flow rate of 30 mL/min.
Samples of oil (4–7 mg) were weighed in an specific analyti-
cal balance AD6® (Perkin Elmer, USA) into aluminum pans
and analyzed in a Pyris Diamond® DSC equipment (Per-
kin Elmer, USA) from −50 °C, and heated at 10 °C/min to
250 °C, and holding for 2 min at the final temperature. An
inert atmosphere was maintained during analysis, with dry
nitrogen at 20 mL/min. Thermograms were analyzed with the
Pyris Diamond® DSC software (Perkin Elmer, USA), and the
transition enthalpies (ΔH, J g−1) were calculated.
Determination of Moisture in French Fries
Moisture was determined by heating 2-g samples of French
fries in an oven at 110 °C until constant weight [12]. Non-
prepared prefried French fries were also analyzed as controls.
Analysis of Volatile Compounds
Volatile compounds were analyzed in a subsample of
EVOO, Portuguese and Spanish Arbequina, both in fresh
oils and in oils absorbed by French fries. Oils extracted
from non-prepared French fries were used as control.
Qualitative Analysis of Volatile Compounds
Volatile compounds from EVOOs and French fries were
extracted by solid phase microextraction (SPME) using a
divinylbenzene/carboxen/polydimethylsiloxane (DVB/CAR/
PDMS) fiber (Supelco, PA, USA) [13]. Extraction fiber was
conditioned for 60 min in the GC injection port at 260 °C.
An aliquot (1 g) of dehydrated French fries or EVOO was
weighed in a headspace vial. Vials were sealed with a
PTFE-lined septum and placed in a glycerol bath (40 °C)
until equilibrium (30 min). The septum was pierced and the
fiber was exposed to the sample headspace for 10 min.
Qualitative analysis was performed by GC–MS on a
GC-17A gas chromatograph coupled to a QP5050A mass
spectrometer (Shimadzu, Japan) equipped with a split/split-
less injector and a fused silica column 5 % phenyl/95 %
methylpolysiloxane (30 m × 0.32 mm ID, 3 µm film thick-
ness; 007-5; Quadrex, USA). Volatile compounds were des-
orbed from the SPME fiber in the injection port for 5 min
at 260 °C, in splitless mode, and after 5 min sampling the
split purge valve was open at 3.0 mL/min. Helium was used
as the carrier gas and the column pressure was set to attain
a carrier speed of 25.0 cm/s. The column oven temperature
was held at 30 °C for 10 min, then increased at 3 °C/min
to 200 °C and held for 10 min. The mass spectrometer was
operated in electron impact mode at 70 eV. The interface
and ion source temperatures were 260 °C. Analyses were
performed in full scan acquisition mode, in the mass range
40–500 m/z at 0.5 scan/s. A mixture of C7–C30 hydrocar-
bons was run under the same conditions to allow calcula-
tion of linear retention index (LRI) values for the volatile
compounds [14]. Data were collected by Lab Solutions
GC–MS software package (Shimadzu, Japan). Compounds
were identified first by comparison of mass spectra with
those of the National Institute of Standards (NIST) library
and calculation of similarity indices by the instrument’s
software (Lab Solutions GC–MS; Shimadzu, Japan), and
also by comparing LRI values with published data [15].
Quantitative Analysis of Volatile Compounds
Volatiles were analyzed with a GC-2010 (Shimadzu, Japan)
gas chromatograph equipped with a FID detector, split/
splitless injector, and the same capillary column used for
qualitative analysis. An aliquot (1 g) of dehydrated French
fries or EVOOs was weighed in a headspace vial and
20 µL of internal standard (0.1 mg/mL of bromobenzene
in methanol) was added. Chromatographic conditions used
were similar to those described in the section “Qualitative
analysis of volatile compounds”. During analysis, injector
and detector temperatures were 260 and 280 °C, respec-
tively. A mixture of C7–C30 hydrocarbons was run under
the same conditions to allow calculation of LRI values
for the volatile compounds, and comparison with GC–MS
data. Exclusively volatile compounds from EVOO trans-
ferred to the French fries were targeted in this analysis. To
aid the comparison of the contents of these volatile com-
pounds between the fresh EVOOs and the French fries,
results were expressed in micrograms of volatile compound
per gram of oil. The contents of volatiles in the French fries
absorbed oil was corrected by their respective contents
in the non-prepared fries and expressed as fold-increase
413J Am Oil Chem Soc (2015) 92:409–421
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relative to contents in the respective fresh EVOO used for
deep-frying.
Statistical Analysis
Descriptive statistics was performed for all variables in
order to calculate means, medians, standard deviation,
and to estimate data normality. Results were expressed
as mean ± standard deviation considering at least two
independent replicates. Multifactor analysis of variance
(MANOVA) was used for comparisons between means;
significant differences between pairs of means were deter-
mined by the Fisher’s test. Paired t test was used for com-
parisons of volatile compounds in French fries. P values
less than 0.05 were considered statistically significant. All
statistical analyses were performed using the Statistica 8.0
software (StatSoft®, Oklahoma, USA).
Results and Discussion
Fresh Oils: Fatty Acids, Tocopherols, and Quality Indices
of EVOOs and Seed Oils
Olive oil production is highly concentrated in the Mediter-
ranean region, and Spain contributes 50 % of the world’s
olive oil production. However, olive tree cultivation and
olive oil consumption are expanding in other countries,
such as in the USA, Argentina, Chile, Asia, and Australia
[1]; only recently EVOO started to be produced industri-
ally in Brazil, although its quality was not yet reported.
The quality of EVOOs is primarily determined by genetic
and climatic factors, as well as by agricultural practices.
Olives of good quality are used for obtaining monovarie-
tal EVOOs, especially of the Picual, Arbequina, and Hoji-
blanca varieties. Monovarietal EVOOs stand out not only
for their chemical, sensory, and nutritional properties, but
also for their oxidative stability [16]. In this section, the
composition and quality indices of oil samples are com-
pared to the corresponding legislation limits and Codex
Alimentarius recommendations. Comparisons between
samples were not performed (Table 1) because this was not
our objective in the present work, although in some cases
there were significant differences between oils.
All EVOO samples studied presented high contents of
oleic acid (18:1n-9) and of total monounsatured fatty acids
(MUFA; Table 1), which might provide protection against
cardiovascular diseases [17]. Fatty acid compositions were
within the ranges presented by the Codex Alimentarius
Commission [18].
Tocopherols protect oil from oxidation and consequently
prevent the formation of free radicals from unsaturated
Table 1 Major fatty acids (g/100 g total fatty acids), α-, β-, γ-, δ-tocopherols (mg/100 g) total tocopherols (mg/100 g), acid value (% oleic acid),
peroxide value (mEq O2/kg), and p-anisidine value in fresh oil samples
Oil components
and quality indices
Extra virgin olive oils Seed oils
Brazilian Portuguese Spanish Soybean Sunflower
Arbequina Picual
Major fatty acids (g/100 g)
16:0 14.8 ± 0.04 11.3 ± 0.04 10.9 ± 0.04 10.5 ± 0.03 11.9 ± 1.75 5.49 ± 0.07
16:1n-7 1.88 ± 0.05 0.95 ± 0.01 0.83 ± 0.07 1.08 ± 0.01 0.13 ± 0.07 0.08 ± 0.00
18:0 1.38 ± 0.01 3.27 ± 0.01 3.04 ± 0.01 3.13 ± 0.01 4.10 ± 0.08 3.47 ± 0.01
18:1n-9 72.2 ± 0.09 76.6 ± 0.04 77.2 ± 0.20 75.0 ± 0.06 24.9 ± 0.12 32.5 ± 0.04
18:2n-6 7.18 ± 0.04 6.63 ± 0.01 7.10 ± 0.10 8.50 ± 1.87 54.7 ± 2.20 57.1 ± 0.04
18:3n-3 0.56 ± 0.00 0.63 ± 0.00 0.56 ± 0.05 0.91 ± 0.02 6.59 ± 0.98 0.13 ± 0.00
Tocopherols (mg/100 g)
α-Toc 17.4 ± 1.39 17.0 ± 0.35 14.0 ± 0.18 16.5 ± 4.91 6.94 ± 0.94 79.2 ± 6.80
β-Toc 0.30 ± 0.02 0.11 ± 0.08 0.04 ± 0.00 0.08 ± 0.01 0.53 ± 0.13 0.81 ± 0.09
γ-Toc 0.06 ± 0.00 0.50 ± 0.08 0.14 ± 0.01 0.44 ± 0.49 30.6 ± 0.49 0.81 ± 0.01
δ-Toc ND ND ND ND 11.7 ± 0.14 ND
Total-Toc 17.8 ± 0.01 17.6 ± 0.12 14.2 ± 0.03 17.0 ± 0.02 49.7 ± 0.02 80.8 ± 0.01
Quality indices
Iodine value 79.8 ± 0.03 79.9 ± 0.04 80.9 ± 0.06 81.3 ± 3.43 133.6 ± 2.42 127.3 ± 0.10
Acid value (% oleic acid) 0.86 ± 0.01 0.71 ± 0.03 0.28 ± 0.03 0.17 ± 0.02 0.09 ± 0.02 0.24 ± 0.02
Peroxide value (mEq O2/kg) 5.80 ± 0.01 8.90 ± 0.01 4.00 ± 0.22 4.30 ± 0.03 4.80 ± 0.02 3.70 ± 0.10
p-Anisidine value 5.02 ± 0.04 5.04 ± 0.06 5.06 ± 0.09 5.26 ± 0.08 1.48 ± 0.36 5.66 ± 0.09
414 J Am Oil Chem Soc (2015) 92:409–421
1 3
lipids. Contents of total tocopherols and the distribution
of tocopherol homologues were consistent with previous
reports [19]. α-Tocopherol was the most concentrated tocol
in the EVOOs, and the contents of β, γ, and δ-tocopherol
were low (Table 1). Sunflower and soybean oils showed the
highest contents of α-tocopherol and γ-tocopherol, respec-
tively. Concerning the action of vitamin E, α-tocopherol,
the major tocol of EVOOs, is the most effective, because
of the specificity of absorption and transport systems in the
human body [20].
The acid value is a quality index associated with the
contents of FFAs, indicating the degree of hydrolysis in
a triacylglycerol (TAG) mixture. Therefore, high acid
values indicate elevated FFA contents. FFAs accelerate
oil oxidation and therefore promote oil fuming and rapid
flavor depreciation [2]. Besides, AV is the main criterion
for classification of olive oils as extra virgin, and EVOOs
should present AV below 0.8 % [18]. Higher AV for cold-
pressed olive oils might indicate poor agricultural, post-
harvesting, and/or processing practices. All fresh sam-
ples of EVOO were in accordance with this upper limit
of AV, except the Brazilian EVOO (Table 1), indicating
that in this case the adopted production practices need
improvement.
Oxidation of fatty acids leads to formation of hydroper-
oxides, which are often assessed by PV, another important
oil quality index. The PV and p-Av are commonly used to
estimate the degree of oxidative degradation in heated oils
and indicate the levels of primary and secondary oxidation
products, respectively. All samples showed initial PV
(Table 1) below their respective maximum limits allowed
in legislation for EVOOs (20 mEq O2/kg) [18]. The p-
Av is a proxy of secondary oxidation products formed by
decomposition of the so-called primary oxidation products
(hydroperoxides) and therefore it is a valuable measure of
oxidation in heated oils [21]. For all samples of fresh oils
(Table 1), the p-Av was lower than the recommended upper
limit of 10.0 [22].
Deep-Fried EVOOs: Time Course of Quality Indices
The AV increased significantly from 0 to 75 min of short-
term deep-frying in all samples of EVOOs and refined oils
(Fig. 2a). The Brazilian EVOO presented the smallest AV
increase (19 %) after 75 min of short-term deep-frying.
Conversely, the Spanish Picual EVOO presented the high-
est AV increase (147 %) after 75 min of short-term deep-
frying, half of which (76 %) occurred from 0 to 50 min.
Similarly, AV of Spanish Arbequina EVOO increased by
30 % after deep-frying for 25 min. In contrast, it has been
reported that the AV of olive oils increased roughly by
50 % in the first 3 h of a deep-frying test at 170 °C with
potato slices [23]. The earlier changes in AV observed in
the present study were possibly caused by temperature
differences in the adopted frying protocols, differences in
the oils’ composition, residual oil in the French fries of
the present study, and differences in the surface area and
in the rate of water transfer into the frying oil, because of
Fig. 2 Evolution of acid value (a), peroxide value (b), and p-anisi-
dine value (c) in the EVOOs and seed oil samples after short-term
deep-frying of French fries (180 °C) for 25, 50, and 75 min. Symbols:
Spanish Arbequina, Spanish Picual, Portuguese,
Brazilian, soybean, sunflower. Quality indices changed
significantly (P < 0.05; repeated-measures ANOVA, with Fisher’s
post-test) during oxidation for all samples, with the exception of a
acid value, deep-frying 50 min vs. 75 min; b peroxide value,
deep-frying 50 min vs. 75 min, deep-frying 50 min vs. 75 min;
c p-anisidine value, deep-frying 25 min vs. 50 min, deep-
frying 25 min vs. 50 min, deep-frying 25 min vs. 50 min
415J Am Oil Chem Soc (2015) 92:409–421
1 3
the potato cuts used. Although there is not a rejection limit
of AV for frying oils, short-term deep-frying in conditions
similar to domestic preparation of French fries appears to
possibly influence oil quality. All EVOOs presented lower
rates of increase in AV compared to refined soybean and
sunflower oils, indicating higher stability of EVOO during
deep-frying and suggesting better nutritional and sensory
quality of food deep-fried in EVOO.
The PV and p-Av increased significantly in all samples
during the short-term deep-frying tests (Fig. 2b, c). Perox-
ide value increased sharply for all EVOO during the first
25 min of deep-frying and plateaued after 50 min, except
for Brazilian EVOO. The two varieties of Spanish EVOO
showed the steepest increase in this time interval. In con-
trast, Brazilian EVOO showed a linear increase (r2 = 0.99,
P < 0.0001) in PV, between 0 and 75 min. These results
suggest that this was the only sample in which the rate
of formation of hydroperoxides was higher than the rate
of decomposition for the whole time interval tested. The
Portuguese EVOO sample was the only one that showed
a high rate of degradation of hydroperoxides from 50 min
of deep-frying, suggesting lower oxidative stability for this
sample. Although EVOOs presented an average increase
of 90 % in PV from 0 to 25 min of oxidation, these oils
remained more stable than both the soybean and sunflower
oils which showed the highest PV increase (0–50 min)
and degradation (50–75 min), respectively. Formation of
hydroperoxides varied among oils even between EVOO,
because Spanish Arbequina and Picual behaved similarly
to the seed oils.
EVOOs behaved similarly concerning p-Av during the
short-term deep-frying tests, with increasing values until
75 min, indicating that on average the rate of formation of
carbonyls exceeded the rate of loss of these secondary oxi-
dation products. Additionally, all EVOOs presented p-Av
lower than the seed oils, for all time intervals of the fry-
ing tests (25–75 min; Fig. 2c). These results indicate that
formation of rancid flavor by deep-frying in EVOO might
occur later than for seed oils commonly used for frying
foods. For the Spanish Arbequina and Portuguese EVOOs,
p-Av plateaued between 25 and 50 min of deep-frying.
During this interval p-Av was probably at a steady state,
and the rate of formation of carbonyls from degradation of
hydroperoxides was similar to the rate of loss of carbonyls
by volatilization or chemical reactions [20]. Higher rates
of increase in p-Av were observed in oven-heated oils than
in fried oils at 185 °C, consistent with accelerated partial
removal of aldehydes by steaming during frying [24]. p-Av
was elevated (>10) for virtually all EVOO samples after
75 min of deep-frying French fries. In contrast to previous
results with deep-fried olive oil [25], we show that short-
term deep-frying similar to domestic preparation promotes
oil deterioration.
Deep-Fried EVOOs: Time Course of Losses in Unsaturated
Fatty Acids and Tocopherols
Oleic acid (18:1n-9) was the major fatty acid in EVOOs
(Table 1) and was relatively stable during the short-term
deep-frying tests in all olive oil samples. The contents of
18:1n-9 in EVOOs did not change significantly or changed
only slightly after deep-frying for 75 min (from −0.71
to −3.5 %; Fig. 3). However, the contents of 18:1n-9
changed significantly in soybean oil. The apparent stability
of 18:1n-9 was probably due to the low rate of oxidation
of monounsaturated fatty acids, which is roughly 100-fold
lower than that of polyunsaturated fatty acids (PUFA) [21].
According to fatty acid composition, EVOOs might be
considered more stable than seed oils to oxidation during
deep-frying. However, susceptibility to oxidation depends
on antioxidants and other minor components, on chemical
micro-environments in the medium, and on the positional
distribution of the glycerol backbone of fatty acids [26], as
well as on oxidizable fatty acids [20].
In contrast to 18:1n-9, the contents of polyunsatu-
rated linoleic (18:2n-6) and α-linolenic (18:3n-3) acids
reduced appreciably, especially at the end (75 min) of the
short-term deep-frying, reaching the maximum losses of
22 % for 18:2n-6 and 26 % for 18:3n-3 in Spanish Picual
(Fig. 3). Soybean and sunflower oils lost, respectively, 62
and 38 % of the original 18:3n-3 contents after 75 min of
deep-frying showing the highest losses of PUFA. Losses of
double bonds in unsaturated lipids might also be observed
through changes in IV (Fig. 3), which might be a sensitive
measure of oxidative changes in PUFA during thermal oxi-
dation [25]. Accordingly, seed oils presented the highest
changes in IV from 0 to 75 min, indicating highest losses
in the degree of unsaturation of fatty acids. Consequently, it
seems that the losses of unsaturated lipids due to oxidation
during short-term deep-frying were higher in refined seed
oils than in EVOO.
Cumulative α-tocopherol degradation at the end of
deep-frying experiments varied from 18 % (sunflower oil)
to 74 % (Spanish Picual EVOO). The degradation rate of
tocopherols was higher in EVOO compared to seed oils
(Fig. 3g). In heated oils with low PUFA contents, such as
EVOO, tocopherols react faster than PUFA with oxidiz-
ing species [27]. Additionally, hydroperoxides formed in
the highly unsaturated vegetable oils decompose rapidly
before reacting with tocopherols [21]. Despite these differ-
ences in the degradation rate of tocopherols, α-tocopherol
contents in oil after deep-frying for 75 min were similar
between EVOO and soybean oil, because the former pre-
sented higher initial levels of α-tocopherol. Losses of total
tocopherol strongly correlated (r2 = 0.99; P < 0.05) with
α-tocopherol losses, except in soybean oil that correlated
strongly (r2 = 0.99; P < 0.05) with γ-tocopherol losses.
416 J Am Oil Chem Soc (2015) 92:409–421
1 3
Fig. 3 Percent changes in the contents of polyunsaturated fatty acids
(18:2n-6 and 18:3n-3), iodine value (IV), α-tocopherol (α-Toc), and
total tocopherols (Total-Toc) in each EVOO and in refined seed oils
after 25, 50, and 75 min of short-term deep-frying (180 °C), and
degradation rate of Total-Toc (µmol/h) after 75 min. Legend: white
bars frying (25 min), gray bars frying (50 min), black bars frying
(75 min); a Brazilian EVOO, b Portuguese EVOO, c Spanish Arbe-
quina EVOO, d Spanish Picual EVOO, e soybean oil, f sunflower oil,
g degradation rate of Total-tocopherols
417J Am Oil Chem Soc (2015) 92:409–421
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These correlations show that the major tocopherol homo-
logue that is acting as an antioxidant in the deep-fried veg-
etable oils seems to depend at least partly on the initial
tocopherol composition in the oil.
Differential Scanning Calorimetry (DSC) of EVOO
and Seed Oils: Fresh Samples and Time Course During
Short-Term Deep-Frying of French Fries
There is increasing need for fast and reliable methods for
monitoring oil quality that produce low levels of chemi-
cal waste. Thermoanalytical techniques are increasing in
importance in analytical chemistry and other interdisci-
plinary areas. These methods have been used in quality
control of oils because they provide data on oil stability
according to its thermal behavior [11] and were compared
with Rancimat® for the assessment of olive oil oxidation
[28]. DSC monitors changes on the physical and chemi-
cal properties of a material as a function of temperature by
monitoring heat transfer during heating. This technique is
used in lipid chemistry to assess the melting and crystal-
lization characteristics of edible oils, heat of fusion, and
polymorphism. Furthermore, DSC has been successfully
applied for the assessment of the identity of vegetable oils
and changes during oxidation [29]. To our knowledge, this
is the first report of DSC heating thermograms of deep-
fried EVOO, and soybean and sunflower oils.
Thermal events occurred below 20 °C, for all samples,
and heating thermograms showed no events from 20 °C up
to 250 °C (data not shown). The heating thermograms of
all the EVOOs exhibited one distinguishable exothermic
event between −40 and −20 °C (Fig. 4). Similar profiles
of DSC thermograms were previously reported for fresh
EVOO [11, 30]. This exothermic event represents crystal-
lization and rearrangement of TAG in EVOOs and was not
distinguishable in seed oils. In thermograms of deep-fried
EVOO this exothermic event was shifted to the right, indi-
cating that thermal oxidation might have changed the crys-
tallization of TAG in EVOO [31]. This shift was possibly
promoted by the accumulation of TAG hydrolysis products,
Fig. 4 Evolution of DSC thermograms in the oil samples after short-term deep-frying French fries at 180 °C for 25, 50, and 75 min
418 J Am Oil Chem Soc (2015) 92:409–421
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such as diacylglycerols, monoacylglycerols, and FFAs
and in the present study was paralleled by changes in acid
value.
Additionally, two endothermal events were observed in
the fresh EVOO and seed oil samples, but the second event
was smaller in the seed oils compared to EVOO. In the
EVOO, these endothermal peaks occurred between −20
and 14 °C. In contrast, in the seed oils, these endothermal
events occurred at a lower temperature interval, from −35
to −18 °C (Fig. 4). These differences are associated with
the samples’ fatty acid composition, because endothermal
bands represent TAG melting [30]. As the more unsaturated
samples (Table 1), the seed oils present endothermal events
at lower temperatures (Fig. 4).
Deep-frying with EVOOs and seed oils led to appreci-
able changes in the endothermal events. The major endother-
mal signal showed peak broadening and decreased height,
and the minor endothermal peak disappeared, or possibly
became embedded into the major one. Similar changes in
endothermal events were observed in heated EVOO [29].
TAG hydrolysis products and secondary lipid oxidation
products in thermo-oxidized EVOOs might have affected the
crystals’ stability, making them less stable than in samples
of fresh oils. Furthermore, these molecules possibly hindered
the rearrangement of TAG polymorphic crystals, changing
the phase transition profile at lower temperatures [2].
Transition enthalpies (Table 2) summarize the changes
observed in Fig. 4 and described above. Overall, the tran-
sition enthalpies of EVOOs, soybean, and sunflower oils
decreased during deep-frying, with differences observable
from the first 25 min. Portuguese EVOO presented the
lowest transition enthalpy among olive oils, and also the
smaller changes during deep-frying (7 % decrease from 0
to 75 min). In contrast, among EVOO, Arbequina and Pic-
ual presented the most pronounced changes (29 and 24 %,
respectively) in transition enthalpy during deep-frying.
These changes might be explained by the accumulation
of lipid hydrolysis and oxidation products, promoting the
formation of mixed and/or less stable polymorphic crystals
than in pure oil, melting at lower temperatures [30] or at a
broadened temperature range, compared to the fresh oils.
These composition changes might have promoted heteroge-
neous structures, which were more easily disrupted in heat-
ing, as previously observed [30, 31].
Deep-Frying in EVOOs (Subsample): Chemical
Compounds Transferred into French Fries and EVOO
Stability During Deep-Frying at 180 °C
During deep-frying there is heat and mass transfer between
the food and the frying medium. Briefly, the heat transferred
from the frying medium (at 180 °C in the present work)
promotes water vaporization and food dehydration. During
dehydration, some of the evaporated water from the fried
food is replaced by oil that is absorbed by food from the
frying medium [3]. Therefore, transferred oil might carry
minor components, such as bioactive and flavor compounds,
from the EVOO that would expectedly enhance nutritional,
functional, and sensorial value of the prepared food. In this
context, we observed that the French fries changed their
chemical composition after deep-frying in Portuguese and
Spanish Arbequina EVOOs (Table 3, Fig. 6).
As expected, deep-frying promoted a reduction in
moisture content of similar magnitude, 27 % on average,
for all experimental conditions, with only slight differ-
ences between Portuguese and Spanish Arbequina EVOO.
Concurrently, there was a significant increase (on average
40 %; dry basis) in fat content in the French fries deep-
fried in EVOO. These results are consistent with replace-
ment of the water lost by evaporation with the oil used as
frying medium.
Although French fries showed an increase in fat con-
tent, there were beneficial changes to the quality of fat after
deep-frying in EVOO (Table 1). Deep-frying with both
EVOOs significantly, and by several fold, increased the con-
tent of 18:1n-9 in French fries. These changes are consistent
Table 2 Transition enthalpies (J g−1) from DSC thermograms of EVOO and seed oils heated at 180 °C in short-term deep-frying French fries
for 25, 50, and 75 min
Results are expressed as mean ± standard deviation of triplicates
Superscript letters indicate significant differences between samples and between deep-frying time (repeated measures MANOVA): a–d Differ-
ences in the same column (time effect); A–FDifferences in the same row (between samples)
Deep-frying at 180 °C Extra virgin olive oils Seed oils
Brazilian Portuguese Spanish Soybean Sunflower
Arbequina Picual
Control (0 min) 85.5 ± 0.10a,A 54.0 ± 0.20a,B 72.0 ± 0.21a,C 68.6 ± 0.32a,D 51.0 ± 1.22a,E 54.9 ± 0.31a,F
25 min 83.2 ± 0.31b,A 54.9 ± 1.10a,B 72.4 ± 1.13a,C 65.3 ± 2.10a,D 44.8 ± 0.13a,E 51.8 ± 0.25a,F
50 min 80.8 ± 0.11b,c,A 53.9 ± 0.33a,B 64.0 ± 0.17b,C 58.5 ± 0.14b,D 39.5 ± 0.51b,E 48.6 ± 0.12b,F
75 min 75.9 ± 0.11c,A 50.0 ± 1.13b,B 51.2 ± 0.24c,B,C 52.4 ± 0.20c,C 35.9 ± 0.12c,D 42.8 ± 0.21c,E
419J Am Oil Chem Soc (2015) 92:409–421
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with the mass transfer of TAGs from EVOO into the French
fries. The typically high contents of 18:1n-9 in EVOO pro-
moted beneficial changes in the lipid composition in the
French fries, when the health of the consumers is of con-
cern. Similarly, deep-frying in EVOO significantly enriched
French fries with α-tocopherol, the main tocol in Portu-
guese and Spanish Arbequina EVOOs. Alpha-tocopherol
was not detected in non-prepared prefried French fries, but
high contents were detected in the prepared French fries. In
conclusion, French fries deep-fried in EVOO might present
added nutritional value, from absorbed EVOO components.
These changes in chemical composition of French fries
were paralleled by increased oxidative stability of the fries
(Fig. 5a). Oxidative stability of the bulk frying oil during deep-
frying was stable for Portuguese EVOO, but for the Spanish
Arbequina showed a significant decrease in the first 25 min
and stabilized thereafter (Fig. 5b). Possibly this was due to
a lower induction period (Fig. 5b) and to a higher peroxide
value (Table 3) in the fresh Portuguese EVOO. In both EVOO,
the induction period stabilized at approximately 20 h and was
not significantly different from 50 min of deep-frying.
Considering flavor compounds, we also observed that
some volatile compounds of EVOOs were transferred
to the French fries when deep-fried in these oils (Fig. 6).
Volatile compounds produced enzymatically by the lipoxy-
genase pathway in olives, such as hexanal, hexanol, hexyl
acetate, (Z)-3-hexenal, (E)-2-hexenal, (E)-2-hexenol, (Z)-
3-hexenol, and (Z)-3-hexenyl acetate confer green notes to
the flavor of EVOO [15]. Volatile compounds in the frying
oil might be transferred to French fries, changing its fla-
vor [32]. In the present work, we investigated absorption
of volatile compounds from EVOO into the French fries
(Fig. 6). The major volatile compounds observed in both
EVOOs were (Z)-3-hexenal, (E)-2-hexenal, (E)-2-hexenol,
and (Z)-3-hexenyl acetate. Four flavor compounds were
transferred from the EVOOs to the French fries (Fig. 6)
and were therefore quantified (Fig. 6a). Almost all of
these compounds were more concentrated in the French
fries than in the fresh EVOO (Fig. 6b), with the excep-
tion of hexane. Except for 2-heptenal, absorption of vola-
tiles in the French fries was not different between the two
EVOO tested (Fig. 6b). Unlike other vegetable oils, EVOO
is not refined and keeps most of its original components,
such as volatiles, thus conserving its flavor [33]. Although
hexanal is considered an off-flavor in refined oils, it is an
important flavor component in EVOOs [15, 34], to which
Table 3 Chemical composition and peroxide value of prefried French fries, extra virgin olive oil (EVOO) used for preparing the fries, and of
French fries prepared in the EVOO, after consecutive deep-frying (25, 50, and 75 min) at 180 °C
Results are expressed as mean ± standard deviation of triplicates. One-way ANOVA with repeated measures with Tukey’s post-test was used for
comparisons between means
DB result expressed in a dry basis, ND not detected
* Significantly different (P < 0.05) from control (prefried French fries, non-prepared)
a Contents in the oil absorbed by French fries
Chemi-
cal quality
parameters
Before frying French fries prepared in EVOO
Prefried
French fries
Portuguese
EVOO
Spanish
Arbequina
EVOO
Fried in Portuguese EVOO Fried in Spanish Arbequina EVOO
25 min 50 min 75 min 25 min 50 min 75 min
Moisture
(g/100 g)
70.2 ± 0.35 – – 50.7 ± 0.59*50.6 ± 0.74*50.8 ± 0.19*52.9 ± 0.08*51.0 ± 1.24*51.4 ± 0.78*
Lipids
(g/100 g DB)
41.0 ± 5.83 – – 62.1 ± 4.70*58.6 ± 4.13 52.3 ± 3.67 58.1 ± 0.47 50.6 ± 6.06 56.4 ± 1.71
Peroxide
value
(mEq
O2/kg)a
44.5 ± 3.54 9.58 ± 0.11 1.58 ± 0.04 16.5 ± 0.42*17.1 ± 1.20*22.5 ± 2.02*4.23 ± 0.33*5.62 ± 1.06*9.58 ± 0.07*
Fatty acids
(g/100 g)a
18:1n-9 13.4 ± 5.52 64.3 ± 4.09 70.2 ± 1.26 49.4 ± 1.22*48.2 ± 1.69*48.4 ± 2.00*52.5 ± 3.10*50.7 ± 6.21*60.9 ± 2.53*
18:2n-6 26.5 ± 10.8 5.16 ± 0.34 5.16 ± 0.11 7.67 ± 0.13 7.72 ± 0.19 8.47 ± 0.21 8.40 ± 0.44 8.33 ± 0.89 10.8 ± 0.48
18:3n-3 ND 0.68 ± 0.28 0.54 ± 0.02 0.58 ± 0.05*0.60 ± 0.08*0.55 ± 0.02*0.49 ± 0.00*0.47 ± 0.05*0.51 ± 0.04*
Tocopherols (mg/100 g)a
αND 14.6 ± 1.71 10.8 ± 0.08 4.73 ± 1.08 5.65 ± 1.05 3.39 ± 0.06 6.19 ± 0.22 4.75 ± 1.08 3.79 ± 0.12
β5.22 ± 0.89 8.48 ± 1.29 2.83 ± 0.82 4.14 ± 0.40*4.38 ± 0.68*5.43 ± 1.01*6.33 ± 0.65*4.02 ± 1.20*3.14 ± 0.29*
γ0.45 ± 0.30 0.37 ± 0.06 0.62 ± 0.04 0.55 ± 0.04*0.58 ± 0.12*0.55 ± 0.00*0.83 ± 0.04*0.65 ± 0.13*0.58 ± 0.13*
420 J Am Oil Chem Soc (2015) 92:409–421
1 3
it is attributed a “green” odor. The green odor in EVOO is
attributed to low contents of hexanal, whereas a fatty odor
is observed in high contents. In oils, contents higher than
3-fold the odor threshold of hexanal (0.9 µg/g) presented a
fatty odor [35]. In the present study, we observed that hexa-
nal contents in the French fries were below this threshold:
0.61 ± 0.1 µg/g and 0.32 ± 0.01 µg/g of French fries, when
deep-fried in Portuguese and Spanish Arbequina EVOO,
respectively. Therefore, the absorption of hexanal by fries
to levels below the off-flavor threshold could positively
impact in the flavor of French fries deep-fried in EVOO.
Conclusion
EVOO is a highly attractive medium for short-term deep-fry-
ing of French fries. EVOO used as a frying medium showed
high oxidative stability, preservation of unsaturated fatty
acids, and low formation of FFAs and carbonyl compounds.
Additionally, deep-frying in EVOO added value to the French
fries. Transfer of compounds from the EVOO frying medium
into the French fries added nutritional value to the fries, and
increased oxidative stability. Transfer of flavor compounds
possibly improved the flavor of the French fries. Additionally,
differential scanning calorimetry seemed useful to investigate
the quality of EVOO during deep-frying. Future investiga-
tions concerning associations between sensorial attributes and
absorption of minor compounds from EVOO used as the fry-
ing medium for French fries are of interest.
Acknowledgments We thank Alma Brasilis® (Rio de Janeiro, Bra-
zil), which kindly provided the three commercial samples of Euro-
pean EVOOs. The financial support of FAPERJ, CAPES, and CNPq
(Brazil) is greatly acknowledged. E. Akil was a recipient of an MSc
scholarship, A.M.M. Costa was a recipient of a DSc scholarship, and
V. Calado was a recipient of a research fellowship, from CNPq (Bra-
zil). V.N.Castelo-Branco was a recipient of a DSc scholarship from
CAPES (Brazil).
Conflicts of interest The authors declare that there are no conflicts
of interest to disclose.
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