Biblid: 1821-4487 (2015) 19; 2; p 67-70
UDK: 547.915
Original Scientific Paper
Originalni naučni rad
COMPARATIVE STUDY OF EDIBLE VEGETABLE OILS
PHYSICAL PROPERTIES
KOMPARATIVNA STUDIJA FIZIČKIH OSOBINA
JESTIVIH BILJNIH ULJA
Vlasta VOZÁROVÁ, Monika BOŽIKOVÁ, Michal VALACH, Ľubomír HÍREŠ,
Ana PETROVIĆ, Ján CSILLAG, Tomáš REGRUT
Slovak University of Agriculture in Nitra, Faculty of Engineering, Department of Physics,
Tr. A. Hlinku 2, SK-949 76 Nitra, Slovakia
E-mail: [email protected]
ABSTRACT
The present work deals with study of physical properties of vegetable oils with focus on rapeseed, sunflower and olive oils.
Physical properties – density, thermal conductivity, thermal diffusivity,dynamic viscosity are measured as a function of temperature.
Energy(calorific) value of vegetable oils expressed by specific heat of combustion is determined as well.
Results show that the temperature is important factorwhich influence thermophysical properties; it is found that in the measured
temperature interval can be used linear regression for temperature dependency vegetable oils density, thermal conductivity and
thermal diffussivity. Examination of the dynamic viscosity shows the exponential decreasing dependency for the each sample in
accordance with Arrhenius equation.
Key words: edible vegetable oils, physical properties:density,thermal conductivity, thermal diffusivity, dynamic viscosity, energy
value.
REZIME
Ovaj rad bavi se proučavanjem fizičkih svojstava biljnih ulja sa fokusom na ulje repice, suncokreta i masline. Fizičke osobine gustina , toplotna provodljivost, toplotna difuzivnost, dinamička viskoznost merene su u funkciji temperature. Energetska (toplotna)
vriednost biljnih ulja izražena preko specifične toplote sagorevanja je određena kod svih ulja.
Rezultati pokazuju da je temperatura važan faktor koji utiče na termofizičke osobine; utvrđeno je da se u izmerenim
temperaturnim intervalima može koristiti linearna regresija za predstvljanje zavisnosti gustine, toplotne provodljivosti i toplotne
difuzivnosti ulja od temperature. Ispitivanje dinamičke viskoznosti pokazuje eksponencijalnu opadajuću zavisnost za svaki uzorak u
skladu sa Arenijusovom jednačinom.
Ključne reči: jestiva biljna ulja, fizička svojstva: gustina, toplotna provodljivost, toplotna difuzivnost, dinamička viskoznost,
energetska vrednost.
INTRODUCTION
Many authors dealing with food physics investigate not only
physical properties and physical processes running in the
materials, but also they are interested in the study of the factors
which influence properties of food or raw materials (Blahovec,
2008; Figura, Texeira, 2007; Sahin, Sumnu, 2006; Mohsenin,
1980 etc.). Temperature is one of the main controlling factors
used in food processing; it is the value which influences nearly
each property of the material.
Knowledge of thermal properties is basic condition for
describing food material’s behaviour during food processing.
Thermal properties are related to heat transfer control and
can be classified as thermodynamic properties (enthalpy and
entropy) and heat transport properties (thermal conductivity and
thermal diffusivity).
According to the authors (Barbarosa-Cánovas et al., 2006)
thermophysical properties not only include thermodynamic and
heat transport properties, but also other physical properties
involved in the transfer of heat, such as freezing and boiling
point, mass, density, porosity and viscosity.
The thermal conductivity k (W/mK) basically characterises
process of heat transfer in the material. It is defined by the
Fourier’s law
q = − k gradT
Journal on Processing and Energy in Agriculture 19 (2015) 2
(1)
where q is the heat flux, T is temperature. The thermal
diffusivity a (m2/s) is connected with the thermal conductivity k
by the formula
k = aρ cp
(2)
where ρ is the density, cpis the specific heat at the constant
pressure.
Chemical composition, structure, moisture content and
binding of the water, temperature and thermal history of the
material are the key factors affecting thermophysical properties
of food materials.
Materials, where internal friction is generated, can be
characterized by viscosity. Dynamic viscosity η (Pas) is defined
as a constant between shear stress τ and shear rate (dv/dy):
τ =η
dv
dy
(3)
Dependency of dynamic viscosity on temperature can be
described by Arrhenius equation
η = η0 e
E 
− A
 RT 
(4)
whereη0is the reference value of dynamic viscosity, EA is
activation energy, R is gas constant and T is temperature.
According to this equation, viscosity exponentially decreases
with temperature.
67
Vlasta Vozárová et. al./Comparative Study of Edible Vegetable Oils Physical Properties
0,9400
0,9200
MATERIAL AND METHOD
RESULTS AND DISCUSSION
Measurement results of oils density are presented in the
graph on the Figure1; it is shown that density linearly decreases
with temperature of oil, even with the same slope. Olive oil and
sunflower oil have nearly the same values of density (925.5
kg/m3 or 925.6 kg/m3 at the temperature 0 °C and 864.4 kg/m3 or
864.6 kg/m3 at 90 °C); rapeseed oil has higher values of density
(930.5 kg/m3 at the temperature 0 °C and 870.1 kg/m3 at 90 °C).
Obtained temperature dependencies of thermal conductivity and
thermal diffusivity with regression equations are in the graphs
(Figure 2 and Figure 3).
68
Sunflower Oil
y = -0,000x + 0,925
R² = 0,999
0,9100
0,9000
Rapeseed Oil
y = -0,0007x + 0,9253
R² = 1
0,8900
0,8800
0,8700
0,8600
0
20
40
60
Temperature (°C)
80
100
Fig.1. Temperature dependencies of oil density
1,2
Thermal conductivity
(W.m-1.K-1)
1
0,8
Olive oil
y = 0,019x + 0,218
R² = 0,999
0,6
Rapeseed oil
0,4
y = 0,023x + 0,011
R² = 0,964
0,2
Sunflower oil
y = 0,015x + 0,353
R² = 0,998
0
0
5
10
15
20
25
30
35
40
45
Temperature ( C)
Fig. 2. Thermal conductivity of oils as a function of temperature
3,5
3
Thermal diffusivity (m2.s-1)
Sunflower (Helianthus annuus L.) or oilseed rape (Brassica
napus L.) are produced for several purposes. Their growing
conditions and properties are investigated from the processing
and/or utilization point of view (Babićetal, 2012; Vujakovićetal,
2014); one of several possibilities is vegetable oil production.
Vegetable oils are components of food materials and they are
also used as biodiesel (about 16 % of vegetable oils production).
Edible oils and fats are composed primarily of the following
components: triacylglycerols (94.4 – 99.1 or 91.8 – 99.0 %),
phospholipids (up to 4.0 %), free fatty acids (0.3 – 1.8 %) and
other components:unsaponifiables, tocopherols, chlorophylls,
sulphur etc. (Gunstone, 2002). Vegetable oils comprise mixtures
of fatty acids in proportions that depend on the source materials.
These fatty acids vary with respect to carbon chain length and
degree of saturation. Fatty acid composition has a major impact
on the properties of the oil and on food (or fuel) quality.
Knowledge of this impact should facilitate the process of
selecting oil sources for the purpose of producing food (or fuel)
with optimal characteristics (Hansen et al., 2011).
This study is focused on comparison of rapeseed, sunflower
and olive oilsphysical properties; density,thermal conductivity,
thermal diffusivity,dynamic viscosity are measured as a function
of temperature. Energy (calorific) value expressed byspecific
heat of combustion is determined as well.
Density of oils is measured by digital density meter DM40
LiquiPhysics™ Excellence (Mettler Toledo) in the temperarture
interval from 0 °C to 90 °C.
Unsteady-state method with heat source (hot-wire method)
and apparatus ISOMET 2104 (Applied Precision) is used for
measurement
of
vegetable
oils
thermophysical
properties:temperature dependancy of thermal conductivity and
thermal difussivity is measured in the temperature interval from
10 °C to 40 °C.
Measurements of vegetable oils dynamic viscosityisrealised
byrotational viscometer DV2TLV(Brookfield). Measurements of
the dynamic viscosity in the temperature interval from room
temperature (22 °C or 23 °C) to 100 °C are provided.
Measurement of energy values expressed by specific heat of
combustion is done by calorimeter IKA C5000.
Olive Oil
y = -0,000x + 0,930
R² = 0,999
0,9300
Density (g.cm -3)
Thermophysical and rheological properties of different
biological (food) materials were measured by many authors
(Božiková, Hlaváč, 2010, 2013; Hlaváč, Božiková, 2011, 2012;
2014; Hlaváč et al., 2013). Severa et al., (2010) examined
influences of storing and temperature on viscosity of egg fluids.
Severa and Los (2008) were looking for temperatureinfluence on
dynamic viscosity of dark beer during storage. Measurement of
energy values and rheological properties of bioethanol and biolubricants were published by Kardjilova et al. (2012, 2013).
2,5
Olive oil
2
y = 0,066x + 0,260
R² = 0,998
1,5
Rapeseed oil
y = 0,033x + 0,461
R² = 0,997
1
Sunflower oil
y = 0,038x + 1,042
R² = 0,973
0,5
0
0
10
20
30
40
50
Temperature (°C)
Fig. 3. Thermal diffusivity of oils as a function of temperature
Theory and high values of determination coefficients show
that in the measured temperature interval (close to room
temperatures) can be used linear regression for temperature
dependency thermal conductivity and thermal diffusivity for
each sample of investigated vegetable oil. We can see that slope
of individual regression is different: sunflower oil thermal
conductivity increase mildest with temperature and olive oils
thermal diffusivity increase the most sharply.
Temperature dependencies of vegetable oils viscosity and
regression equations with value of determination coefficient R2
for each sample are in the Figures 4 – 6.
Journal on Processing and Energy in Agriculture 19 (2015) 2
Vlasta Vozárová et. al./Comparative Study of Edible Vegetable Oils Physical Properties
As for energy (orcalorific) value of vegetable oils;
comparative measurement of vegetable oilsspecific heat of
combustion is presented in the Table 1. The highest energy value
has sunflower oil; olive oil and rapeseed oil have close energy
values.
Table 1. Energy value of oils
Sample
Energy value (J/g)
Olive oil
40 086
Rapeseed oil
39 433
Sunflower oil
49 364
80
Viscosity (mPa.s)
70
60
50
y = 134,0e-0,02x
R² = 0,994
40
30
20
10
0
0
20
40
60
80
100
120
Temperature (°C)
Fig. 4. Temperature dependency of olive oil dynamic viscosity
Progress of obtained dependencies can be described by
decreasing exponential function:
η = Ae
 t 
− B  
 t0 
Viscosity (mPa.s)
(5)
wheret is temperature, t0 = 1 °C; A and B are constants
dependent on kind of material, and on other factors (e.g. ways of
processing or storing).
80
70
60
50
40
30
20
10
0
Measured temperature dependencies of physical properties
(density,thermal conductivity, thermal diffusivityand dynamic
viscosity)show
good
accordance
with
published
results.Differences are caused by several factors (chemical
composition, way of processing, condition of storing, thermal
history etc.), which affect nearly each physical property of
biological materials.
Presented linear dependencies of thermal conductivity and
thermal diffusivity on the temperature,althoughin the small
temperature interval, indicate significant impactof the
temperature on oils thermal properties. Obtained regression
equations for temperature dependency of dynamic viscosityare
Arrhenius type, with different values of coefficients A, B for
each sample depending on the kind of oil.
ACKNOWLEDGEMENTS: This work was supported by the
project of Scientific Grant Agency of Slovak Republic under No.:
VEGA 1/0854/14 and it was co-funded by European Community
under project No 26220220180: Building Research Centre
„AgroBioTech“.
y = 112,1e-0,02x
R² = 0,990
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0
20
40
60
80
100
120
Temperature (°C)
Fig.5.Temperature dependency of rapeseed oil dynamic viscosity
The exponential dependency for the each sample was
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Experimental values of dynamic viscosity at room
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of dynamic viscosity are measured for rapeseed oil. Measured
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mPa s at 100 °C, sunflower oil: 78.60 mPa s at the temperature
22 °C to 8.42 mPas at 100 °C, and rapeseed oil: 68.80 mPa s at
the temperature 22 °C to 8.28 mPas at 100 °C.
Dynamic viscosity at the temperature 100 °C is very similar
for each sample.
90
80
Viscosity (mPa.s)
CONCLUSION
70
60
y = 132,42e-0,029x
R² = 0,9904
50
40
30
20
10
0
0
20
40
60
80
100
Temperature ( C)
Fig. 6. Temperature dependency of sunflower oil
dynamic viscosity
Journal on Processing and Energy in Agriculture 19 (2015) 2
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70
Accepted: 30.03.2015.
Journal on Processing and Energy in Agriculture 19 (2015) 2