II Congresso Brasileiro de Plantas Oleaginosas, Óleos, Gorduras e Biodiesel
Realização: Universidade Federal de Lavras e Prefeitura Municipal de Varginha
SPECIFIC HEAT OF SOME VEGETABLE OILS BY DIFFERENTIAL
SCANNING CALORIMETRY AND MICROWAVE OVEN
José Carlos Oliveira Santos 1
José Pires Dantas 2
Antônio Gouveia de Souza 3
Marta Maria da Conceição 4
ABSTRACT
The feasibility of the microwave oven method to determine the specific heat of edible
vegetable oils was investigated, by comparison with the differential scanning calorimeter
method. The results obtained using differential scanning calorimetry (at 40-190oC) and
microwave oven (at 76-111oC) are presented in this work. It was observed that the specific
heat of the edible oils increase as a function of the saturation of the fatty acids.
KEYWORDS: Specific heat. DSC. Microwave.
1 INTRODUCTION
The knowledge of the specific heat of the oils and fats is quite useful to determine
their behavior during different technological processes. Although these specific heat are
similar to the ones of the original triglycerides, they present the tendency to increase as a
result of the unsaturation of the fatty acids, both in the liquid and in the solid states. All these
facts are directly related to the mobility of the molecules of oils and fats in the different
physical states (MORAD et al., 2000).
In the last years, new analytical methods have been required by researchers to evaluate
their processing and storage conditions. The use of thermal analysis methods in the study and
characterization of oils and fats is receiving increasing interest. Therefore, recently the use of
thermal analytical methods, Thermogravimetry (TG), Differential Thermal Analysis (DTA)
_____________________
1
Professor Titular DQ/CCT/UEPB, zecarlosuepb@yahoo.com.br
2
Professor Titular DQ/CCT/UEPB, gpcnpq@terra.com.br
3
Professor Adjunto DQ/CCEN/UFPB, gouveia@quimica.ufpb.br
4
Bolsista DCR do DQ/UFRN, martamaria8@yahoo.com
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Realização: Universidade Federal de Lavras e Prefeitura Municipal de Varginha
and Differential Scanning Calorimetry (DSC) for oil and fat characterization has gained a
great interested for the food industries (SANTOS et al., 2002).
In the present work, the specific heats of some edible vegetable oils were determined
by microwave oven and compared with the results obtained by DSC to evaluate the proposed
method.
2 MATERIALS AND METHODS
Six samples of edible vegetable oils: soybean, olive, rapeseed, rice, corn, sunflower
and two oil blends: olive (3%) / soybean (97%) and olive (30%) / sunflower (70%) were
investigated in this study. The chemical characterization of the edible vegetable oil samples is
displayed in Table 1.
Table 1. Composition of some edible vegetable oil samples.
Edible
Oils
Olive
Rapeseed
Sunflower
Corn
Soybean
Rice
Soybean/olive
Sunflower/olive
Monounsaturated
71.3
65.0
23.0
33.5
24.3
40.8
29.2
41.3
Fatty Acids (%)
Polyunsaturated
12.7
29.0
65.0
51.0
60.0
40.1
57.0
46.7
Saturated
16.0
5.0
12.0
15.5
15.7
19.1
13.8
12.0
Artificial
antioxidants
⎯
⎯
⎯
Citric acid/TBHQ
Citric acid/TBHQ
⎯
⎯
Citric acid
A differential scanning calorimeter Shimadzu, model DSC-50, was used. The edible
vegetable oil samples were placed in aluminum crucibles and heated from room temperature
up to 200oC at a heating rate of 10oC/min, under a dynamic atmosphere of nitrogen (50
mL/min), according to the procedure described in Table 2.
Table 2. Experimental conditions to determine the specific heat of edible vegetable oils.
Heating hate (ºCmin –1)
5.0
10.0
Temperature (ºC)
30.0
200.0
Time (min)
5.0
5.0
Three measurements were carried out for each sample: 1) Measurements with an
empty crucible; 2) Measurements with a reference material (alumina), with a known specific
heat capacity; 3) Measurement of specific heats of the edible oil samples, with a fixed mass of
22.0 ± 0.5 mg.
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II Congresso Brasileiro de Plantas Oleaginosas, Óleos, Gorduras e Biodiesel
Realização: Universidade Federal de Lavras e Prefeitura Municipal de Varginha
A BRASTEMP, model 38MWO, commercial microwave furnace without any
adaptation, was used for obtaining the specific heat capacity values by the microwave oven
method. The experiments consisted in monitoring the temperatures of water (used as standard)
and of the edible oil samples, under determined conditions of power (900 W) and time (30,
60, 90 s).
3 RESULTS AND DISCUSSION
In the DSC method, as the specific heat of the reference material (co) is known, the
specific heat capacity of the edible oil samples (c) can be calculated by:
c=
m0 c 0 S 3 − S1
⋅
m S 2 − S1
(1)
as S1, S2 and S3 are continuous functions relating to the temperature, the specific heats can be
determined continuously. Where S1, S2, S3 are the thermal displacements of the DSC
respectively related to the blank, the reference and the sample.
For all the investigated edible oils, the specific heats do not vary substantially and
their values can be used for engineering design purposes. The specific heat values obtained
from DSC of the investigated edible oils in the 40 – 180oC range are listed in Table 3.
Table 3. Specific heat (Cp) for edible oils at the 40–180oC range obtained by DSC.
Temp.
(oC)
Olive
Soybean
Corn
Cp (J g-1K-1)
Rice Sunflower Rapeseed
40
60
80
100
120
140
160
180
2.721
2.890
3.052
3.092
3.293
3.483
3.701
3.868
2.269
2.470
2.531
2.547
2.646
2.703
2.756
2.812
2.039
2.250
2.319
2.346
2.462
2.527
2.605
2.673
1.860
2.032
2.089
2.109
2.201
2.238
2.287
2.342
1.833
2.030
2.076
2.070
2.152
2.181
2.219
2.252
1.833
2.008
2.059
2.077
2.165
2.217
2.271
2.319
Olive /
Sunflower
2.741
2.933
3.057
3.113
3.300
3.483
3.694
3.828
Olive /
Soybean
2.731
2.907
3.034
3.052
3.286
3.457
3.615
3.770
In the considered range, the specific heat values for the olive oil samples and for the
mixtures containing olive oil were higher than those of the other oils. This increase can be
attributed to a higher concentration of monounsaturated fatty acids present in the olive oil.
Generally, the results demonstrate that the specific heat capacity is probably related to the
unsaturation degree of the analyzed oils. Comparing the results of the edible oil samples
presented in Table 3 with those obtained by other authors (KASPRZYCKA-GUTTMANN
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Realização: Universidade Federal de Lavras e Prefeitura Municipal de Varginha
and ODZENIAK, 1991), a small difference in the values of specific heats was observed,
which might probably be attributed to a difference in the methods used.
In the Microwave Oven method, the specific heat values of these samples were also
calculated taking into account the power effectively absorbed by the samples during the
heating in a microwave oven, as shown by the Eq. 2:
P=
k .C p .m.ΔT
(2)
t
where P is the absorbed power (W), k is the unit conversion factor from cal/s to watts (4.184 J
cal-1), Cp is the specific heat of sample (J g-1 K-1), m is the sample mass (g) that undergoes the
temperature variation, ΔT (K), during the exposition time, t (s) (PERCORARO et al., 1997).
The specific heat values obtained by the Microwave Oven method (Table 4) show that
there are no meaningful differences in the values of the edible vegetable oil samples analyzed,
indicating the low sensitivity of the method.
Table 4. Specific heats (Cp) of the edible vegetable oils.
Edible
Oils
Olive
Soybean
Corn
Rice
Sunflower
Rapeseed
Olive + Sunflower
Olive + Soybean
Temperature (oC)
79
97
110
Cp (J g-1K-1)
Microwave Oven
2.575
2.845
2.882
DSC
3.019
3.009
3.197
76
97
108
77
99
109
78
97
110
77
97
110
80
99
110
78
101
111
80
97
109
2.697
2.892
2.993
2.565
2.821
2.957
2.615
2.899
2.915
2.615
2.850
2.915
2.481
2.813
2.915
2.615
2.704
2.849
2.454
2.845
2.930
2.515
2.496
2.611
2.313
2.333
2.420
2.084
2.061
2.180
2.072
2.036
2.123
2.059
2.060
2.147
3.047
3.118
3.223
3.034
3.004
3.177
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II Congresso Brasileiro de Plantas Oleaginosas, Óleos, Gorduras e Biodiesel
Realização: Universidade Federal de Lavras e Prefeitura Municipal de Varginha
Comparing these results with those obtained by DSC, it was observed that the specific
heat values are higher for all samples, except for these containing olive oil. These differences
can be due to the amount of the oil sample used in the microwave method, which is much
higher than that employed in the DSC, turning homogenization more difficult. Also no
correlation was found between the specific heats obtained by DSC and obtained by the
microwave oven method.
4 CONCLUSIONS
It was observed that the specific heat capacities of the edible oils present the tendency
to increase with the saturation of t he fatty acids. Consequently, the specific heats of the
edible vegetable oils are dependent of the fatty acid composition.
A new technique, for the determination of the specific heat capacity of edible
vegetable oils, based on the heating in a microwave oven was also investigated. Although
quick results were obtained, their accuracy ts was not as good as the one obtained upon the
use of the DSC technique. These two methods are able to rapidly supply thermodynamic data
that are important for design purposes.
5 REFERENCES
KASPRZYCKA-GUTTMANN, T., ODZENIAK, D. Specific Heats of Some Oils and Fats.
Thermochimica Acta, v.191, p. 41-46, 1991.
MORAD, N. A., KAMAL, M. A. A., PANAU, F., YEW, T. W. Liquid Specific heat
Capacities of Oils and Tryglicerides. Journal of American Oil Chemists Society, v. 77, p.
1001-1012, 2000.
PERCORARO, E., DAVOLOS, M. R., JAFELLICI JUNIOR, M. Microondas e suas
Aplicações. Química Nova, v. 20, p. 89-96, 1997.
SANTOS, J. C. O., SOUZA, A. G., SANTOS, I. M. G., SANTOS, A. V., PRASAD, S.
Thermal Stability and Kinetic Sutdy on Thermal Decomposition of Commercial Edible Oils
by Thermogravimetry. Journal of Food Science, v. 67, n. 4, p. 1393-1399, 2002.
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