Emissions of volatile aldehydes from heated cooking oils

Abstract

Emissions of volatile organic compounds, including aldehydes, formed during heating of cooking oils: coconut, safflower, canola, and extra virgin olive oils were studied at different temperatures: 180, 210, 240, and 240°C after 6h. Fumes were collected in Tedlar® bags and later analysed by GC–MS. The emissions of volatiles were constant with time and increased with the oil temperature. When the temperature of the oil was above its smoke point, the emission of volatiles drastically increased, implying that oils with low smoke point, such as coconut, are not useful for deep-frying operations. Canola was the oil generating the lowest amount of potentially toxic volatile chemicals. Acrolein formation was found even at low temperatures, indicating that home cooking has to be considered as an indoor pollution problem.

Full text

Emissions of volatile aldehydes from heated cooking oils
Harinageswara Rao Katragadda
a
, Andrés Fullana
b
, Sukh Sidhu
a
, Ángel A. Carbonell-Barrachina
c,*
a
University of Dayton, Environmental Sciences and Engineering Group, 300 College Park, Dayton, OH 45469-0132, USA
b
Universidad de Alicante, Departamento de Ingeniería Química, Apartado de correos 99, 03080 Alicante, Spain
c
Universidad Miguel Hernández, Departamento Tecnología Agroalimentaria, Grupo Calidad y Seguridad Alimentaria, Carretera de Beniel, km 3.2, 03312 Orihuela, Alicante, Spain
article info
Article history:
Received 18 May 2009
Received in revised form 27 July 2009
Accepted 21 September 2009
Keywords:
Acrolein
Canola oil
Coconut oil
Deep-frying
Indoor emissions
Olive oil
Safflower oil
abstract
Emissions of volatile organic compounds, including aldehydes, formed during heating of cooking oils:
coconut, safflower, canola, and extra virgin olive oils were studied at different temperatures: 180, 210,
240, and 240 °C after 6 h. Fumes were collected in Tedlar
Ò
bags and later analysed by GC–MS. The emis-
sions of volatiles were constant with time and increased with the oil temperature. When the temperature
of the oil was above its smoke point, the emission of volatiles drastically increased, implying that oils
with low smoke point, such as coconut, are not useful for deep-frying operations. Canola was the oil gen-
erating the lowest amount of potentially toxic volatile chemicals. Acrolein formation was found even at
low temperatures, indicating that home cooking has to be considered as an indoor pollution problem.
Ó2009 Elsevier Ltd. All rights reserved.
1. Introduction
The operation known as deep-fat frying is widely used both
domestically and commercially. Frying may be considered as a ra-
pid combination of drying and cooking (Moyano & Pedreschi,
2006). During deep-frying, fats and oils are repeatedly used at ele-
vated temperatures (between 160 and 240 °C, with an optimal va-
lue of 180 °C) in the presence of atmospheric oxygen and receive
maximum oxidative and thermal abuse (Andreu-Sevilla et al.,
2008; Frankel, 1998; Moreira, Castell-Perez, & Barrufet, 1999).
Different oils (both of animal and vegetable origin) are used for
deep-frying in different cultures. However, recent studies have
shown that unsaturated oils like olive and canola oils are healthier
to use than hydrogenated oils or animal based fats such as butter
and lard. (Frankel, 1998; Medina-Navarro, Mercado-Pichardo, Her-
nández-Pérez, & Hicks, 1999; Moreira et al., 1999). For instance,
the beneficial properties of olive oil have been attributed to its
fatty acid composition characterised by a high monounsaturated-
to-polyunsaturated fatty acid ratio and the presence of minor com-
pounds with powerful antioxidant activity, among which polyphe-
nols stand out (Aparicio & Harwood, 2003; Velasco & Dobarganes,
2002).
Aldehydes, ketones, alcohols, dienes and acids, commonly
formed during edible oil degradation, create unpleasant flavour, re-
duce the shelf-life of edible oils, and may further cause health
problems (Fullana, Carbonell-Barrachina, & Sidhu, 2004a, 2004b).
Recent reports indicate that oil fumes resulting from heating edible
oils, such as rapeseed oil, soybean oil, peanut oil and lard, to high
temperature exhibit mutagenicity and genetic toxicity (Chiang
et al., 1997; Moreira et al., 1999; Qu et al., 1992). Since aldehydes
are major products of this degradation and due to their capacity to
induce toxicological effects (e.g., their reactivity with amino groups
of proteins), aldehydes like acrolein are considered to have high
relevance (Umano & Shibamoto, 1987; Yen & Wu, 2003; Zhu,
Wang, Zhu, & Koga, 2001). Within this chemical family, unsatu-
rated aldehydes, such as alkenals and alkadienals, show more se-
vere toxicity than alkanals (Meacher & Menzel, 1999; Tovar &
Kaneda, 1977).
Nowadays consumers are demanding healthier oils; this de-
mand is based on a proper fatty acid composition (high content
of monounsaturated fatty acids, such as oleic acid), and also on a
reduced emission of potentially toxic compounds, such as alde-
hydes. The correct choice of an oil for frying is important for sev-
eral reasons. Firstly, the oil is used as the heat transfer medium
and must be able to withstand high temperatures and have high
enough stability to allow its reuse. Secondly, products being fried
will take up some of the oil, and therefore the oil needs to maintain
a high oxidative stability during the life of the product, and be both
palatable and nutritious (Andreu-Sevilla, Hartmann, Burló, Poquet,
& Carbonell-Barrachina, 2009; Andreu-Sevilla et al., 2008; Talbot &
Zand, 2006). Finally, the oil should be as stable as possible during
0308-8146/$ - see front matter Ó2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.foodchem.2009.09.070
*Corresponding author. Fax: +34 966749754.
E-mail address: angel.carbonell@umh.es (Á.A. Carbonell-Barrachina).
Food Chemistry 120 (2010) 59–65
Contents lists available at ScienceDirect
Food Chemistry
journal homepage: www.elsevier.com/locate/foodchem

its heating and originated a low emission of potentially toxic vola-
tile organic compounds.
The emission of indoor air pollutants is related to the smoke
point of the oils being heated. The smoke point is the temperature
at which oil begins to smoke continuously. In the present experi-
ment, different oils covering a wide range of smoke points were
used (Table 1): coconut (175 °C), extra virgin olive oil (195 °C), saf-
flower (212 °C), and canola (238 °C). To our knowledge, no studies
have been conducted that compare the impact of heating oils with
different smoke points on the emission of volatile organic com-
pounds. In this paper, gas chromatography–mass spectrometry
(GC–MS) was used to isolate, identify, and quantify the volatile
compounds from four types of cooking oils: coconut, extra virgin
olive, safflower, and canola oils at four different temperatures:
180, 210, 240 and 270 °C, for 6 h. In previous studies (Fullana
et al., 2004a, 2004b), this research group studied the emissions of
aldehydes from two different types of olive oils, (a) extra virgin ol-
ive (edible oil prepared without any chemical treatment) and (b)
olive (oil physically refined, eliminating off-flavours but also natu-
ral antioxidant compounds); canola oil was used as a control
material.
2. Materials and methods
2.1. Materials
Canola (liquid at room temperature; 58% and 38% of monoun-
saturated and polyunsaturated fatty acids, respectively), extra vir-
gin olive (liquid at room temperature; 72% of monounsaturated
fatty acids), safflower (liquid at room temperature; 75% of polyun-
saturated fatty acids), and coconut (solid at room temperature;
91% of saturated fatty acids) oils were purchased from a local
supermarket in Dayton (OH, USA).
2.2. Experimental system and sampling
The experimental system used to conduct this study is shown in
Fig. 1. This system consists of a hermetic Pyrex reactor heated by a
hemispherical heating mantle and the heating is controlled by a
thermocouple that was placed inside the oil. For this study, cooking
oils were heated to 180 °C (regular deep-frying temperature), 210,
240, and 270 °C (temperature above the maximum smoke point of
the cooking oils studied) for 6 h and samples were taken every
Table 1
Fatty acid composition and smoke points of coconut, safflower, canola, and extra
virgin olive oils.
Oil MUFA
a
(%) PUFA (%) SFA (%) Smoke point (°C)
Coconut 6 ± 1 3 ± 1 91 ± 5 175 ± 4
Safflower 15 ± 1 75 ± 4 8 ± 1 212 ± 4
Canola 58 ± 3 32 ± 2 10 ± 1 238 ± 5
Extra virgin olive 72 ± 4 14 ± 1 14 ± 1 195 ± 4
a
MUFA = monounsaturated fatty acids; PUFA = polyunsaturated fatty acids;
SFA = saturated fatty acids.
OFF
ON
Air
Tedlar bag
Cooking oil
Food
Tª controller
Heating system
Thermo couple
180ºC
Air
Fig. 1. Schematic representation of experimental system used in frying simulation.
Table 2
Emission rate (mg h1loil) of volatile aldehydes from safflower oils heated at 210 °C during different times (1–6 h).
Chemical compound ANOVA test Time of heating (h)
0 1 234 5 6
Alkanals
Acetaldehyde
*A
5.17 ± 0.26
B
c
C
8.81 ± 0.44b 9.17 ± 0.46b 9.76 ± 0.49b 10.4 ± 0.52ab 11.2 ± 0.56a 9.09 ± 0.45b
Propanal N.S. 2.99 ± 1.15 5.66 ± 1.08 5.39 ± 0.99 5.51 ± 0.88 5.69 ± 1.28 5.75 ± 1.03 5.17 ± 0.85
Butanal N.S. 20.3 ± 2.0 22.8 ± 3.1 10.8 ± 7.5b 19.3 ± 1.0 22.4 ± 1.1 17.9 ± 1.9 18.9 ± 1.7
Pentanal
*
152 ± 8b 190 ± 10a 195 ± 10a 182 ± 9a 204 ± 10a 190 ± 10a 186 ± 9a
Heptanal N.S. 79.9 ± 4.0 109 ± 16 59.8 ± 3.0 80.2 ± 4.0 86.9 ± 4.3 78.9 ± 3.9 82.3 ± 4.1
Octanal N.S. 16.7 ± 0.9 17.4 ± 0.9 17.5 ± 0.9 18.2 ± 0.9 19.0 ± 0.95 19.7 ± 1.0 18.1 ± 0.9
Nonanal
*
40.1 ± 2.0b 54.1 ± 2.7a 51.2 ± 2.6a 50.2 ± 2.5a 52.1 ± 2.61a 51.2 ± 2.6a 49.8 ± 2.5a
Alkenals
Acrolein
*
87.2 ± 4.4c 129 ± 7a 106 ± 5.3b 89.6 ± 4.5c 87.1 ± 4.4c 74.1 ± 3.7d 95.5 ± 4.8c
2-Hexenal N.S. 69.6 ± 3.5 84.4 ± 10.2 70.2 ± 3.5 59.2 ± 7.5 69.3 ± 3.5 65.1 ± 3.3 69.7 ± 3.5
2-Heptenal
**
436 ± 22b 643 ± 32a 234 ± 12d 255 ± 13d 209 ± 10d 432 ± 22b 368 ± 18c
2-Octenal
*
46.0 ± 2.3c 68.4 ± 3.4a 61.1 ± 3.1a 62.7 ± 3.1a 58.5 ± 2.9b 40.8 ± 2.0c 56.2 ± 2.8b
2-Decenal N.S. 14.0 ± 4.7 22.1 ± 3.1 22.3 ± 2.5 22.8 ± 3.1 23.1 ± 2.3 23.4 ± 3.2 21.3 ± 2.5
Alkadienals
2,4-Heptadienal N.S. 13.4 ± 5.7 23.3 ± 2.2 19.1 ± 1.0 20.3 ± 1.9 20.1 ± 1.6 19.0 ± 1.3 19.2 ± 1.1
2,4-Decadienal N.S. 15.3 ± 6.8 24.8 ± 1.2 24.6 ± 1.2 25.1 ± 1.3 24.9 ± 1.2 24.3 ± 1.2 23.2 ± 1.2
Total
Alkanals
*
317 ± 16c 408 ± 20a 349 ± 18b 365 ± 18b 400 ± 20ab 375 ± 19b 369 ± 19b
Alkenals
**
653 ± 33b 947 ± 47a 494 ± 25c 489 ± 25c 447 ± 22c 635 ± 32b 611 ± 31b
Alkadienals
*
28.7 ± 4.4b 48.1 ± 2.4a 43.7 ± 2.2a 45.4 ± 2.3a 45.0 ± 2.3a 43.3 ± 2.2a 42.4 ± 2.1a
ALDEHYDES
*
999 ± 50bc 1403 ± 70a 886 ± 44d 900 ± 45c 892 ± 45c 1053 ± 53b 1022 ± 51b
*
,
**
, and
***
, significant at p< 0.05, 0.01, and 0.001, respectively.
A
N.S. = not significant Fratio (p< 0.05).
B
Treatment means of the ANOVA test (mean of three replications) ± standard error.
C
Values followed by the same letter, within the same raw, are not significant different (p< 0.05), Tukey’s multiple-range test.
60 H.R. Katragadda et al. / Food Chemistry 120 (2010) 59–65

hour. Air at flow rate of 1 l min
1
was supplied to the reactor. This
air flow ensured total removal of fumes generated from heating the
oil. The inlet air line was designed such that it was far enough from
oil surface to minimise changes in the diffusion layer but at the
same time the air flow was sufficient enough to prevent stagnant
flow zones and was able to transport all the compounds evolved
during heating of cooking oils. The reactor exhaust gases were col-
lected in 500 ml clear layered Tedlar bags (Supelco, Bellefonte, PA,
USA; Supelco catalogue number 24655). The collected gas samples
were then analysed by GC–MS. The product stability was analysed
according to the specifications in US. EPA sampling method 40 dur-
ing the time between sampling and analysis. Experiments were run
in six replicates.
Authors checked that no artefacts were introduced by the Pyrex
reactor or the Tedlar bags. Different set ups for trapping the vola-
tile compounds were previously used in different studies using ol-
ive oils: (a) Tedlar bags (Fullana et al., 2004a) and (b) tenax traps
(Fullana et al., 2004b); results were quite similar with the only
exception that tenax traps were unable to trap small and reactive
compounds such as acrolein or acetaldehyde. These previous re-
sults advise that Tedlar bags were the best way of collecting fumes
from frying simulations.
2.3. Analytical procedure
A Hewlett-Packard (HP) model 5890 gas chromatograph com-
bined with an HP-5970 series mass selective detector equipped
with a 30.0 m 0.25 mm i.d. DB 625 was used to perform the anal-
Temperature (ºC)
170 180 190 200 210 220 230 240 250 260 270 280
Total Volatiles (mg h-1 L-1)
0
2000
4000
6000
8000
10000
12000
14000
Coconut
Safflower
Canola
Extra Virgin Olive
Fig. 2. Emission rates of total volatiles (mg h
1
l
1
oil
) being released from coconut,
safflower, canola, and extra virgin olive oils heated at four different temperatures
(180–270 °C).
Temperature (ºC)
160 180 200 220 240 260 280
160 180 200 220 240 260 280
Concentration (mg h-1 L-1)
0
1000
2000
3000
4000
5000
Aldehydes
Alkanals
Alkenals
Alkadienals
Temperature (ºC)
Concentration (mg h-1 L-1)
0
2000
4000
6000
8000
10000
Hydrocarbons
Alcohols
Ketones
Fig. 3. Emission rates of aldehydes, hydrocarbons, alcohols, and ketones
(mg h
1
l
1
oil
) being released from coconut oil heated at four different temperatures
(180–270 °C).
Temperature (°C)
180 210 240 270
2-Heptenal (mg h-1L-1)
0
200
400
600
800
1000
1200
1400
1600
Coconut
Safflower
Canola
Extra Virgin Olive
Temperature (°C)
180 210 240 270
2-Hexanone (mg h-1L-1)
0
500
1000
1500
2000
Coconut
Safflower
Canola
Extra Virgin Olive
Fig. 4. Emission rates of 2-heptenal and 2-hexanone (mg h
1
l
1
oil
) being released
from coconut, safflower, canola, and extra virgin olive oils heated at four different
temperatures (180–270 °C).
H.R. Katragadda et al. / Food Chemistry 120 (2010) 59–65 61

yses of collected gas samples. For this analysis, 1.0 ml of gas was
directly injected in the gas chromatograph. The limit of detection
observed (1 mg m
3
) was enough for the purposes of this study,
and therefore no concentration or derivatisation methods were
used. The oven temperature was held at 35 °C for 5 min and then
programmed to 230 °Cat5°C min
1
and held for another
12 min. Products were identified by comparing product mass spec-
tra with mass spectral database (NIST).
Standards of acrolein, pentanal, hexanal, heptanal, 2-octenal,
and 2,4-heptadienal in the available purest form were bought from
Aldrich Chemicals (Milwaukee, WI). These chemicals were diluted
in methanol and injected into GC–MS to obtain response factors
from the standard calibration curves. The average value of all the
response factors from these standards was calculated and used to
quantify experimental results.
2.4. Statistical analyses
Data from volatile aldehydes emissions were examined by anal-
ysis of variance (ANOVA) using STATGRAPHICS Plus 5.0 software
(Manugistics Inc., Rockville, MD). Wherever Fvalues were signifi-
cant, Tukey’s multiple-range test was used to separate the mean
effects (different temperatures for each oil type). Significance
was defined at p60.05. Graphics were done using Sigma Plot 9.0
(SPSS Science, Chicago, IL, USA).
3. Results and discussion
3.1. General
A total of 30 compounds were tentatively identified, based on
matches with library mass spectral data, including aldehydes, ke-
tones (2-hexanone, 2-heptanone, and ethylcyclopentanone),
hydrocarbons (pentane, 1-heptene, heptane, cyclopropane, oc-
tane, nonane, decane, butylbenzene, undecane, and 6-dodecene),
and alcohols (pentanol, hexanol, and 1-octen-3-ol). From these
compounds, the most interesting are aldehydes due to their po-
tential toxicity for those working in restaurants or home kitchens.
It has been reported that large amounts of aldehydes are present
in the headspace of cooking oil containers, and they are consid-
ered as possible contributors to carcinogenicity (Yen & Wu,
2003; Zhu et al., 2001). Sixteen aldehydes were tentatively iden-
tified in this study and can be grouped as follows: (a) alkanals:
acetaldehyde, propanal, butanal, pentanal, heptanal, octanal, and
nonanal; (b) alkenals: acrolein, 2-hexenal, 2-heptenal, 2-octenal,
and 2-decenal, and (c) alkadienals: 2,4-heptadienal and 2,4-
decadienal.
3.2. Effect of time
In this experiment oils were heated at 210 °C up to 6 h and sam-
ples were taken every hour. The generation rate of aldehydes was
almost constant with respect to time for all four oils under study.
As an example, data for the generation of aldehyde during heating
of safflower oil are compiled in Table 2. The concentrations of total
aldehydes were 999, 1403, 887, 900, 892, 1053, and
1002 mg h
1
l
1
oil
for 0, 1, 2, 3, 4, 5, and 6 h of heating at 210 °C
(mean of 1019 ± 75 mg h
1
l
1
oil
). Similar patterns were found for
(a) coconut oil: 894, 1215, 1893, 1143, 1064, 1200, 1139
mg h
1
l
1
oil
, for 0, 1, 2, 3, 4, 5, and 6 h, respectively (mean of
1221 ± 119 mg h
1
l
1
oil
); (b) canola oil: 1302, 993, 1167, 1033,
1053, 1056, and 1076 mg h
1
l
1
oil
, respectively (mean of
1097 ± 43 mg h
1
l
1
oil
); and (c) extra virgin olive oil: 1908, 1049,
2338, 2185, 2440, 1731, and 2001 mg h
1
l
1
oil
, respectively (mean
of 1950 ± 193 mg h
1
l
1
oil
).
Consequently, the effect of time was omitted in the next exper-
iments (averages of all concentrations at different time intervals
will be used).
Temperature (°C)
180 210 240 270
2-Decenal (mg h-1L-1)
0
200
400
600
800
Coconut
Safflower
Canola
Extra Virgin Olive
Temperature (°C)
180 210 240 270
Nonanal (mg h-1L-1)
0
200
400
600
800
Coconut
Safflower
Canola
Extra Virgin Olive
Fig. 5. Emission rates of 2-decenal and nonanal (mg h
1
l
1
oil
) being released from
coconut, safflower, canola, and extra virgin olive oils heated at four different
temperatures (180–270 °C).
Temperature (°C)
180 210 240 270
Acrolein (mg h-1L-1)
0
100
200
300
400
Coconut
Safflower
Canola
Extra Virgin Olive
Fig. 6. Emission rates of acrolein (mg h
1
l
1
oil
) being released from coconut,
safflower, canola, and extra virgin olive oils heated at four different temperatures
(180–270 °C).
62 H.R. Katragadda et al. / Food Chemistry 120 (2010) 59–65

3.3. Effect of temperature
The total amounts of volatile compounds present in the fumes
of cooking oils were positively correlated with the frying tempera-
ture. Fig. 2 represents the total volatile generation rate for coconut,
safflower, canola and extra virgin olive oils as affected by the heat-
ing temperature. Total concentrations of aldehydes, hydrocarbons,
alcohols and ketones (Fig. 3) increased with increasing frying tem-
peratures. The dependence of volatile emissions with temperature
was more evident for alkanals and hydrocarbons. Therefore, the
first recommendation is to use the lowest possible temperature
for reaching the desired cooking and technological effects during
deep-frying operations.
3.4. Effect of the smoke point
The smoke point is the temperature at which oil begins to
smoke continuously and can be seen as bluish smoke. This smoke
is an indication of chemical breakdown of the fat to glycerol and
free-fatty acids (FFA). The glycerol is then further broken down
to acrolein (2-propenal), which is one of the main components of
the bluish smoke. This point is greatly dependent on the content
of free-fatty acids and to a lesser degree on partial glycerides.
The influence of degree of unsaturation is minimal but chain length
has an important effect; oils containing short chain fatty acids (e.g.,
lauric acid) have lower smoke appoint than oils with predomi-
nantly longer chain fatty acids. A general rule is that the higher
the smoke point, the better suited a fat is for frying; fats with
smoke point below 200 °C are not suitable for deep-fat frying.
Volatile aldehyde emission from all four oils increased gradually
with temperature until they reached their smoke point (Tables 3
and 4); from this point, emission increased more sharply (Fig. 2).
For instance, in safflower oil (smoke point = 212 °C) the emission
rate of total volatile compounds increased 879 mg h
1
l
1
oil
when
the temperature was raised from 180 to 210 °C, while the incre-
ment was significantly higher, 1971 mg h
1
l
1
oil
, when the temper-
ature was raised from 210 to 240 °C (above the smoke point of this
oil). The same behaviour was found for the other oils.
This experimental finding is very important and leads us to the
following conclusion: the temperature of any oil used for deep-fry-
ing operations should be established below its smoke point, because
otherwise the emission of potentially toxic compounds will increase
significantly. This first conclusion leads us to a second one: coconut
oil should not be used as a deep-frying matrix because its smoke
point (175 °C) is below the optimal temperature for frying opera-
tions (180 °C). However, even emissions below the smoke point of
oils might be harmful, especially those of acrolein and acetaldehyde
in closed rooms/kitchens with limited ventilation.
Table 3
Emission rate (mg h1loil) of volatile aldehydes from coconut and safflower oils heated 6 h at different temperatures (180, 210, 240 and 270 °C).
Chemical compound ANOVAtest Heating temperature (°C)
180 210 240 270
Coconut oil
Alkanals
Acetaldehyde
***A
13.1 ± 2.0
B
d
C
44.7 ± 3.9c 99.2 ± 5.8b 327 ± 11a
Propanal
***
10.8 ± 2.0d 44.7 ± 4.9c 86.3 ± 3.4b 217 ± 6a
Butanal
***
36.5 ± 7.6d 155 ± 18c 297 ± 13b 683 ± 23a
Pentanal
***
104 ± 18d 423 ± 65c 786 ± 27b 1685 ± 45a
Heptanal
***
94.2 ± 4.2c 151 ± 11b 290 ± 22a 302 ± 27a
Octanal
***
51.4 ± 2.2b 66.9 ± 4.3b 135 ± 14a 131 ± 13a
Nonanal
*
60.1 ± 2.6c 66.0 ± 5.3c 160 ± 20a 84.6 ± 4.3b
Alkenals
Acrolein
***
10.4 ± 2.2d 18.6 ± 2.3c 44.4 ± 1.7b 232 ± 10a
2-Hexenal
**
11.7 ± 0.9d 41.9 ± 2.7a 30.0 ± 2.3b 20.7 ± 0.5c
2-Heptenal
**
119 ± 10b 138 ± 14b 268 ± 20a 124 ± 8b
2-Octenal
*
27.7 ± 1.8b 26.8 ± 2.8b 54.9 ± 5.0a 26.7 ± 1.5b
2-Decenal
**
29.9 ± 2.4b 30.1 ± 2.6b 95.7 ± 3.9a 27.3 ± 2.8b
Alkadienals
2,4-Heptadienal
**
10.9 ± 1.7b 8.73 ± 1.81c 20.9 ± 1.6a 12.5 ± 1.3b
2,4-Decadienal
*
4.71 ± 0.34b 5.15 ± 1.26b 13.7 ± 2.8a 5.88 ± 0.94b
Safflower oil
Alkanals
Acetaldehyde
**
9.41 ± 0.84c 9.09 ± 0.80c 1072 ± 34b 62.7 ± 1.3a
Propanal
**
6.68 ± 0.58c 5.17 ± 0.41c 12.8 ± 0.3b 43.3 ± 1.2a
Butanal
***
16.2 ± 1.4c 18.9 ± 1.7c 47.8 ± 1.2b 127 ± 4a
Pentanal
***
133 ± 14d 186 ± 7c 281 ± 11b 600 ± 14a
Heptanal
**
115 ± 16c 82.3 ± 6.0d 168 ± 7b 350 ± 25a
Octanal
**
35.4 ± 3.9b 18.1 ± 0.4d 29.0 ± 1.1c 65.4 ± 4.0a
Nonanal
*
72.4 ± 9.0b 49.8 ± 1.9c 47.9 ± 2.2c 123 ± 11a
Alkenals
Acrolein
***
57.3 ± 6.7d 95.5 ± 7.3c 122 ± 5b 343 ± 27a
2-Hexenal
**
63.6 ± 6.2c 69.7 ± 3.1c 82.1 ± 1.1b 161 ± 16a
2-Heptenal
**
586 ± 74b 368 ± 63c 691 ± 12b 1297 ± 135a
2-Octenal
**
68.0 ± 10.0b 56.2 ± 4.0c 64.0 ± 2.4b 113 ± 9a
2-Decenal
*
24.1 ± 3.3a 21.3 ± 1.4b 10.1 ± 0.8c 28.4 ± 2.6a
Alkadienals
2,4- Heptadienal
**
30.0 ± 5.5b 19.2 ± 1.2c 13.0 ± 4.5d 51.0 ± 4.8a
2,4- Decadienal
**
17.5 ± 2.8c 23.2 ± 1.5b 12.2 ± 0.3d 29.7 ± 4.3a
*
,
**
, and
***
, significant at p< 0.05, 0.01, and 0.001, respectively.
A
N.S. = not significant Fratio (p< 0.05).
B
Treatment means of the ANOVA test (mean of six replications) ± standard error.
C
Values followed by the same letter, within the same raw, are not significant different (p< 0.05), Tukey’s multiple-range test.
H.R. Katragadda et al. / Food Chemistry 120 (2010) 59–65 63

3.5. Effect of fatty acid concentration
Different studies have shown that the following compounds de-
rived from linoleic acid (C18:2): acetaldehyde, pentanal, hexanal,
2-heptenal, 2-octenal, 2-nonenal, 2,4-decadienal, pentane, and 1-
pentanol (Feron et al., 1991; Qu et al., 1992; Ruiz-Méndez, 2003;
Subramanian & Nakajima, 1997; Wan, Pakarinen, & Miscella,
1996). In this way, the emission rates of, for instance, 2-heptenal
should be higher in those oils having higher contents of linoleic
acid: safflower > canola > extra virgin olive > coconut. Fig. 4 shows
the emission rates of 2-heptenal and 2-hexanone as examples of
the experimental situation found and, in general, it was clear that
the emissions of compounds such as 2-heptenal and 2-hexanone
were significantly higher in safflower oil compared to the other
three types of oil under study (Tables 3 and 4). The differences
among canola, extra virgin olive and coconut were only visible at
the highest temperature (270 °C).
In most cases, coconut oil emissions are out of their supposed
position in this pattern due to the fact that all the assayed temper-
atures were above its smoke point.
From the literature, heptane, heptanol, octane, octanal, nonanal,
decanal, 2-decenal, 1-undecene, and 2-undecenal are emitted after
degradation of oleic acid (C18:1), and more precisely from the hae-
molytic fission of the R–O bond of this fatty acid (Feron et al.,
1991; Qu et al., 1992; Ruiz-Méndez, 2003; Subramanian & Nakajima,
1997; Wan et al., 1996). In this way, the emission rates of, for in-
stance, 2-decenal should be higher in those oils having higher con-
tents of oleic acid: extra virgin olive Pcanola > safflower P
coconut. Fig. 5 shows the emission rates of 2-decenal and nonanal
as examples of emission of volatile compounds originating in oleic
acid (Tables 3 and 4). Again the effect of the fatty acid concentrations
on the emission rates was clearer at the highest temperature as-
sayed, 270 °C: the emission of compounds such as 2-decenal and
nonanal were significantly higher in extra virgin olive oil, followed
by canola, and then safflower and coconut.
3.6. Activation energy
Temperature is the most important factor to be considered in
evaluating the oxidative stability of fats, especially unsaturated,
because the mechanism of oxidation changes with temperature
and different hydroperoxides of linoleate, acting as precursors of
volatile flavours, decompose at different temperatures. Because
the rate of oxidation is exponentially related to temperature, the
shelf-life of a food lipid decreases logarithmically with increasing
temperature (Frankel, 1998).
The activation energy (additional energy needed by reactants to
form products) of lipid oxidation is much higher in the presence of
antioxidants than in their absence because antioxidants lower the
rates of oxidation by increasing the overall energy of activation
Table 4
Emission rate (mg h1loil) of volatile aldehydes from coconut and safflower oils heated 6 h at different temperatures (180, 210, 240 and 270 °C).
Chemical compound ANOVA test Heating temperature (°C)
180 210 240 270
Canola oil
Alkanals
Acetaldehyde – n.d. n.d. n.d. n.d.
Propanal
**A
21.4 ± 1.8
B
c
C
36.4 ± 2.9b 10.9 ± 0.6d 93.5 ± 19.0a
Butanal
**
9.77 ± 0.56c 23.7 ± 1.9b 5.57 ± 0.38d 86.6 ± 3.7a
Pentanal
***
33.2 ± 1.4d 50.7 ± 4.1c 216 ± 7b 564 ± 39a
Heptanal
***
64.0 ± 4.0c 85.4 ± 4.1b 46.8 ± 1.9d 415 ± 58a
Octanal
***
54.7 ± 5.1b 51.2 ± 3.3b 8.80 ± 0.34c 167 ± 15a
Nonanal
**
120 ± 8b 88.0 ± 6.8c 51.0 ± 2.5d 365 ± 31a
Alkenals
Acrolein
***
53.5 ± 3.9d 116 ± 8c 234 ± 14a 135 ± 7b
2-Hexenal
**
18.7 ± 0.9b 18.8 ± 1.9b 62.0 ± 2.7a 76.1 ± 12.0a
2-Heptenal
***
108 ± 5d 156 ± 4c 463 ± 15b 841 ± 73a
2-Octenal
***
60.1 ± 5.7c 242 ± 6b 13.6 ± 0.9d 359 ± 35a
2-Decenal
***
63.7 ± 2.4c 74.8 ± 3.8b 33.5 ± 1.4d 138 ± 21a
Alkadienals
2,4-Heptadienal
***
95.8 ± 11.3d 141 ± 3c 163 ± 17b 260 ± 15a
2,4-Decadienal
***
14.9 ± 3.8c 17.2 ± 4.7c 32.3 ± 2.0b 159 ± 12a
Extra virgin olive oil
Alkanals
Acetaldehyde – n.d. n.d. n.d. n.d.
Propanal
**
21.1 ± 6.0b 18.4 ± 2.6b 40.6 ± 3.7a 44.5 ± 0.9a
Butanal
**
18.7 ± 4.9c 30.1 ± 3.4b 82.2 ± 9.5a 90.6 ± 6.0a
Pentanal
*
115 ± 30a 80.1 ± 3.3b 139 ± 14a 151 ± 8a
Heptanal
**
121 ± 15c 215 ± 13b 300 ± 20a 319 ± 11a
Octanal
**
71.3 ± 6.6c 150 ± 11b 182 ± 28ab 193 ± 29a
Nonanal
***
134 ± 17d 362 ± 30c 545 ± 15b 594 ± 13a
Alkenals
Acrolein
***
12.1 ± 2.5d 39.7 ± 3.7c 106 ± 14b 120 ± 4a
2-Hexenal
***
12.2 ± 1.8c 36.4 ± 6.0b 63.5 ± 14.0a 69.7 ± 14.3a
2-Heptenal
***
161 ± 39c 213 ± 17b 413 ± 44a 460 ± 7a
2-Octenal
***
92.6 ± 16.2b 344 ± 37a 369 ± 29a 352 ± 27a
2-Decenal
***
32.9 ± 7.5d 306 ± 73c 611 ± 31b 698 ± 37a
Alkadienals
2,4-Heptadienal
**
17.6 ± 3.0b 60.5 ± 8.2a 50.2 ± 4.3a 52.0 ± 4.4a
2,4-Decadienal
***
9.7 ± 0.7c 102 ± 21b 587 ± 90a 684 ± 6a
*
,
**
, and
***
, significant at p< 0.05, 0.01, and 0.001, respectively.
A
N.S. = not significant Fratio (p< 0.05).
B
Treatment means of the ANOVA test (mean of six replications) ± standard error.
C
Values followed by the same letter, within the same raw, are not significant different (p< 0.05), Tukey’s multiple-range test.
64 H.R. Katragadda et al. / Food Chemistry 120 (2010) 59–65

(Frankel, 1998). Arrhenius plots of the logarithmic of emission
rates of the total volatile compounds versus the reciprocal of the
temperatures (K) were prepared. The adjustments of the experi-
mental data to the Arrhenius equation were good and high coeffi-
cients of determinations (R
2
) were obtained: 0.9885, 0.8070,
0.9571, and 0.9423 for coconut, safflower, canola, and extra virgin
olive oils, respectively. The calculated values of the ratio activation
energy (E/R) were: 5919, 4675, 2875, 2140 for coconut, extra virgin
olive, canola, and safflower oils, respectively. The experimentally
calculated activation energies are quite logical and fully agree with
the nature and fatty acid composition of the oils under study. The
highest activation energy was found for coconut oil because con-
tains the highest concentration of saturated fatty acids, which re-
quire more energy to decompose than unsaturated fatty acids.
The next activation energy was that of extra virgin olive oil due
to its high content of monounsaturated fatty acids and likely high
concentration of natural antioxidants. Safflower oil was had the
lowest activation energy due to its high content of polyunsaturated
fatty acids.
3.7. Acrolein
Acrolein (2-propenal) is a compound of special interest among
the emitted volatile compounds because of its carcinogenicity.
Unfortunately, there is no consensus on its origin. For instance,
acrolein can form via thermolysis of glycerol at temperatures high-
er than 230 °C(Fujisaki, Endo, & Fujimoto, 2002; Zhu et al., 2001);
however, in the present study acrolein was formed even at the
lowest studied temperature, 180 °C. This means that even at low
temperatures, pathways for its formation also exists. Umano and
Shibamoto (1987) based on their observation of acrolein formation
under a nitrogen stream, proposed that its formation was likely
due to a free-radical mechanism involving homolytic fission of
R–O bond (Schauer, Kleeman, Cass, & Simoneit, 2002). Probably
all three formation pathways previously described in the literature
contributed to the acrolein emissions observed in this study; how-
ever, based on the data collected in this study, it is difficult to iden-
tify the dominant formation pathway.
Acrolein emission from safflower oil was high compared to the
other three oils, especially extra virgin olive oil; this might be due
to the high content of polyunsaturated fatty acids present in saf-
flower (Fig. 6). Emission rates were also high for canola oil at tem-
peratures of 240 °C and lower. The high increase observed in
coconut oil at 270 °C might be due, among other factors, to the fact
that this temperature is quite close to the flash point of this oil
275–280 °C; the flash point is the temperature at which the vola-
tile products are evolved at such a rate that they are capable of
being ignited but not capable of supporting combustion.
4. Conclusions
From an industrial point of view, one of the most important
conclusions to be drawn from this study is that a proper control
of oil temperature will increase the shelf-life of oils being used
for deep-frying purposes and at the same time it will reduce the in-
door emissions of aldehydes. Formation of aldehydes and volatile
compounds during deep-frying operations depends mainly on oil
temperature and formation rate is constant with time. The ten-
dency of formation of volatiles also depends on the fatty acid com-
position of heated oils and it increases when it reaches its smoke
point and will enhance drastically when oil approaches its fire
point. Total volatiles and aldehydes emitted from all four oils in-
creased almost linearly with temperature. Even at temperatures
as low as 180 °C (deep-frying optimum), acrolein formation was
found in all four cooking oils. Therefore, formation of acrolein in
home cooking has to be considered as an indoor pollution problem.
However, it is important to mention that during cooking opera-
tions, acrolein would be distributed into an open ambient area,
so the actual concentration to which persons in kitchen and food
industry workers would be exposed will be lower. Proper ventila-
tion in kitchen will help in reducing the impact of these potentially
toxic compounds. Reheating of the oil is not recommended as
used-oil will contain a higher free-fatty acid content and conse-
quently drastically decrease its original smoke point, which will re-
sult in higher emissions of volatiles at lower temperatures.
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