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INFORMATION PROCESSING IN AGRICULTURE 2 (2015) 142–148

journal homepage: www.elsevier.com/locate/inpa

Capacitive sensor probe to assess frying oil

degradation

Alfadhl Yahya Khaled a, Samsuzana Abd Aziz a,*, Fakhrul Zaman Rokhani b
a
Department of Biological and Agricultural Engineering, Faculty of Engineering, Universiti Putra Malaysia, Serdang, Selangor,
Darul Ehsan 43400, Malaysia
b
Department of Computer and Communication Systems Engineering, Faculty of Engineering, Universiti Putra Malaysia, Serdang, Selangor,
Darul Ehsan 43400, Malaysia

A R T I C L E I N F O A B S T R A C T

Article history: The repeated usage of frying oil has been proven hazardous due to the degradation process
Received 19 November 2014 by chemical reactions that lead to changes in the quality of the oil. Currently, the degree of
Received in revised form frying oil degradation is indicated by the percentage of its total polar compounds (TPC). In
6 July 2015 this study, a capacitive sensor was designed to assess frying oil degradation at several heat-
Accepted 21 July 2015 ing time intervals by measuring changes on its electrical capacitance. The sensor was
Available online 7 August 2015 designed using interdigitated electrode structure. A total of 30 samples of 130 ml palm
oil were heated at 180 °C up to 30 h. For each one hour interval, one sample was moved
Keywords: out from the laboratory oven. The electrical capacitance, total polar compound (TPC) and
Capacitance sensor viscosity of the samples were measured for analysis. Preliminary results demonstrated
Frying oil quality significant correlation between oil electrical capacitance with TPC and viscosity with R2
Heating ranged from 0.83 to 0.90. The designed sensor has good potential for simple and inexpensive
way of determining frying oil quality.
Ó 2015 China Agricultural University. Production and hosting by Elsevier B.V. All rights
reserved.

1. Introduction occur, such as hydrolysis, polymerization and thermal

oxidation, thereby producing a considerable number of harm-
Frying is one of the earliest and most popular ways of ful compounds such as polymer and ketones, which signifi-
cooking. It is the process that food is submerged into the cantly change the quality of the frying oil [7,13,24,35,37]. In
hot oil or fat at elevated temperatures (150–190 °C). It gives addition, improper methods to determine the time to discard
tasty flavor and charming appearance in food which is very the oil will result in over using it which poses a public health
simple and fast in operation as it requires a limited amount risk [16,22].
of time in food preparation [5]. During the frying process, in In most cases frying oil degradation is evaluated based on
the presence of air and moisture many chemical reactions visual inspection wherein for instance the chefs experience to
decide when to discard the oil for example based on excessive
foaming, odor, smoking, color changes, and by tasting the
* Corresponding author. Tel.: +60 3 8946 4455; fax: +60 3 8946 food products. However, these are not reliable methods due
6425. to their subjective nature and these parameters may manifest
E-mail address: samsuzana@upm.edu.my (S.A. Aziz). only when the oil has already become unsafe to be reused.
Peer review under the responsibility of China Agricultural
Studies on both physical and chemical characteristics of the
University.
http://dx.doi.org/10.1016/j.inpa.2015.07.002
2214-3173 Ó 2015 China Agricultural University. Production and hosting by Elsevier B.V. All rights reserved.
 Information Processing in Agriculture 2 ( 2 0 1 5 ) 1 4 2 –1 4 8 143

used to observe frying oil at all stages of its used and deter-
Table 1 – Physical and chemical Indicators of frying oil
quality. mine its viscosity [11]. Other than that, instruments such as
Testo 270 (InstruMartlnc, Germany), and Ebro FOM 310
Physical indicators Chemical indicators (ebroÒElectronic GmbH, Germany) were developed by elec-
Smoke point Free fatty acids (FFA) tronic companies to measure the quality of frying oil by
Color Peroxide value (PV) testing the total polar materials (TPM) based on changes
Viscosity Iodine values (IV) in the dielectric constant of the oil. In addition, kits such
Taste Total polar compounds (TPC) as Fritest (E. Merck, Germany) and Oleh TestTM (Panreca,
Odor Polymeric triglycerides
Spain) were developed to measure the quality of frying oil
Foam persistence Anisidine value (AV)
by testing the FFA and TPC, respectively based on the color
Polymerized and oxidized material
reaction of the oil [9,37]. However, there are some limita-
tions with the current devices such as complex calibration
frying oil are important to determine the deterioration level of requirement, suitability for different type of oil as well as
the oil [6]. distinct temperature dependencies [27].
Several physical indicators (Table 1) are used to evaluate Numerous studies have attempted to explain the molecu-
the quality of frying oil: smoke point, color, viscosity, taste, lar polarizability and the orientational effects of polar media
odor, and foam persistence [24,31,35]. These tests are by observing changes in their electrical properties. Morgan
extensively used; however they are not decisive in them- et al. [25] reported that the dipole moment of the biological
selves. Color, for example, basically depends on the sort particles is induced when subjected to an AC field. Where
of food fried as well as the oil; taste and odor depend the polarized particles gain a force that can cause them to
on the food type used for frying. Smoking amount of fry- replace to electric field, relying on the particle polarizability
ing oil is related to the temperature as well as to the as comparing to the suspending medium. The molecular
amount of low molecular weight breakdown in the oil. polarizability is effecting the magnitude of the dipole
Viscosity measurements can be used as an indicator to moment, and this in turn is controlled by the dielectric prop-
detect the quality deterioration of frying oil, but it is not erties of the medium and molecular. Bagchi et al. [4] stated
conclusive in itself. that the values of the polarizability of the molecule can expe-
While the chemical indicators listed in Table 1, can be a rience substantial changes from their values when a molecule
more reliable way to assess the deterioration of frying oil is excited to higher electronic state. According to Hughes [15]
[22]. Innawong et al. [16] stated that the volatile compounds a molecule experience negative or positive dielectrophoresis
produced from chemical reactions through frying process is relying on its polarizability relative to its nearby medium.
contribute to raise the peroxide value (PV) in the oil. As Hughes noted that the variances in the quantity of induced
Tsuzuki et al. [34] argued that PV increases with time as the charge at the interface between medium and particle lead to
oil is heated at 180 °C. In addition, iodine value (IV) is used oriented dipole counter to applied field where the polarizabil-
for the assessment of the suitability of the oils [23]. Garba ity of medium is less than that of molecule, and in the same
et al. [12] reported that oil with high IV exhibited poor perfor- direction as the applied field where it is less. The relative
mance due to the oxidation reactions of lipids and the polarizability depends on applied field frequency, it has a
hydroperoxide formation between the unsaturated fatty acids strong frequency dependence, beside the conductivity and
and oxygen. Also, free fatty acid (FFA), polymeric triglycerides, permittivity, because it is a complex function. According to
anisidine value (AV), and polymerized and oxidized material Darma [8], the movement of dipole throughout polarization
(POM) are broadly used as the pointers of the frying oil quality, resulting in displacement current, and this contributes to
but are not conclusive in themselves [21]. At present, mea- total current and improves the conductivity.
surement of the total polar compounds (TPC) is considered Generally, studies on changes of electrical properties have
to be the most commonly used method to evaluate the qual- been introduced and some sorts of instruments were
ity of oil because it determines overall chemical degradation proposed to be used in the agricultural field [17]. The parallel
taking place in the oil [10]. planar electrodes are one of the generally used probes to
To date various methods have been developed and sense the moisture content in peanut oil [18], analysis of egg-
introduced to measure the different chemical and physical plant pulp and effects of drying and freezing–thawing treat-
parameters of frying oil. For example, chemosensory system ments on its capacitance characteristics [38]. Presently,
for controlling the quality of oil in food industries [36], interdigitated electrodes (IDEs) are applied in many sensing
Fourier transforms infrared (FTIR) to differentiate between devices including surface acoustic wave, chemical sensors,
good and inadmissible oils [17], chromatography to measure and MEMS biosensors [33]. Furthermore, IDEs have been
dielectric constant, smoke point and viscosity [28] and image studied in cancer cell detection as well as other biological
analysis to determine the TPC rate in frying oil [14]. However, associated applications [3,19]. Therefore, IDE could be used
these methods are complicated, time consuming, and expen- in solving complex calibration requirements and improving
sive. Thus, developing a simple sensing system to help in the accuracy of sensory sensitivity. Also, IDE shape has some
appraising the quality of frying oil is required. advantages such as no moving parts, ease of fabrication, flex-
More recently, there are many instruments and kits that ible in design, and cost effective [30]. In this study, a capacitive
can be used to determine oil degradation. For example, vis- sensor was designed using IDE platform to assess frying oil
cosity meters and electronic-based physical tests such as degradation due to heating at different frequencies. The aims
Vibro Viscometer (A&D Company Limited, Japan) can be of this study were to develop and evaluate a new sensor for
144 Information Processing in Agriculture 2 ( 2 0 1 5 ) 1 4 2 –1 4 8

Fig. 1 – (a) A top-view illustration of capacitance sensor probe; (b) A photograph of a prototype device next to a Malaysian coin.

determining the degradation of frying oil by measuring the A ¼ L  ðw  NÞ ð2Þ

changes of its electrical properties. where L is the length of electrode measured by (mm) and w is
electrode width measured by (lm).
2. Materials and methods The materials used to construct the sensor were 100 ang-
stroms titanium/2000 angstroms of gold, 1000 angstroms of
2.1. Capacitive sensor design silicon dioxide (SiO2), and polished alumina (AI2O3). Gold
was applied as the layer of metal for the interdigitated finger
The capacitive sensor was designed based on the interdigi- electrodes because it has very low resistance. While titanium
tated electrodes (IDE) as shown in Fig.1(a). The sensor scheme was used as an adhesion layer because sputtered gold will not
was drawn using a CAD software before the photomask of the adhere to alumina substrate by itself as it will peel off. It is
probe was formed. The variables of the sensor are the number sputtered first so that the gold will adhere to the substrate.
of the electrodes N, width of the electrode w, electrode space SiO2 was used as insulating layer between interdigitated fin-
s, and the length of the electrode L, with dimension of 41, ger electrodes and gold bonding pads and AI2O3 was used as
100 lm, 60 lm, and 8 mm, respectively. Every other electrode rigid substrate as illustrated in Fig.1(b).
finger is connected electrically together through a common The mechanism of this sensor is that once the oil oxida-
electrode arm. In addition, the length and width of the sub- tively and thermally breaks down during the frying, there will
strate were 11.50
18.50 mm with bonding pad size of be a rise in the number of polar molecules, which directly
2
2 mm and insulation layer of 11.20
4.95 mm. These increases the dielectric constant. While an electric field is
dimensions have been experimented based on the maximum enforced across the faces of IDEs, the dipole and molecular
fabrication capability, to carry through the requirements for charges in the testing of the frying oil are forced out from
detecting the changes in the frying oil quality. The equation their equilibrium locations, and those dipole charges lay up
of the electrical capacitance is given by: through the electrodes of the detector. Consequently, as long
eo er A
C¼ ð1Þ
sN1
where eo is the permittivity of free space which is equal to
8.854 pF/m, er is the relative dielectric constant of oil which
is equal to 2–4, s is the electrode space and measured by
(lm), and N is the number of electrodes. Also A is the area
of the sensor which is calculated by:

Fig. 2 – Capacitance sensor connected to LCR meter. Fig. 3 – Measurement of TPC using Testo 270.
 Information Processing in Agriculture 2 ( 2 0 1 5 ) 1 4 2 –1 4 8 145

2.2. Sample preparation

Fresh oil was bought from a local market in Seri Kembangan,

Selangor, Malaysia. Then the oil was divided into 30 samples
where each sample contained 130 ml. All samples were
heated in a laboratory oven at the temperature of 180 °C.
The heated time of the samples was ranging from 1 to 30 h,
where one sample was taken out from the oven at every hour.
After heating, all samples were kept in amber glass bottles at
40 °C for further analyses.

2.3. Electrical capacitance, TPC, and viscosity

measurement

The distinction among each heated oil sample was analyzed

Fig. 4 – Measurement of the viscosity using Vibro
by measuring its electrical capacitance, TPC and viscosity.
Viscometer.
The electrical capacitance was measured using the custom
built IDE sensor immersed into the oil sample. The sensor
was connected to a LCR meter (4263B, Agilent, Japan) with
as the frying oil yields polar molecules through different Kelvin clip leads (16089E, Agilent, Japan) as depicted in Fig.2.
chemical reactions during frying the more charges will lay The LCR meter has a frequency range from 100 Hz and to
up in the electrodes. 100 kHz. Before starting the measurements using the LCR
meter, calibration was performed following the standard pro-
cedure of the instrument operation manual [2]. Then the TPC
of each heated sample was measured using a frying oil tester

Table 2 – The mean of capacitance, TPC and viscosity at different heating time.
Heating time (h) TPC (%) Viscosity (mPas) Capacitance
100 Hz 1000 Hz 10 kHz 20 kHz 100 kHz

0 6.50 50.80 0.0035 3.47 3.47 3.48 3.48

1 8.00 60.30 0.0037 3.63 3.65 3.66 3.71
2 8.50 57.75 0.0037 3.69 3.71 3.71 3.76
3 9.00 61.35 0.0035 3.71 3.72 3.71 3.77
4 9.25 61.05 0.0038 3.77 3.78 3.79 3.85
5 9.75 62.40 0.0037 3.77 3.79 3.80 3.85
6 10.50 61.05 0.0037 3.78 3.80 3.80 3.86
7 10.25 60.40 0.0037 3.78 3.80 3.81 3.87
8 10.50 59.00 0.0037 3.76 3.78 3.78 3.84
9 10.75 65.15 0.0037 3.70 3.72 3.73 3.78
10 11.00 61.90 0.0037 3.72 3.74 3.74 3.80
11 11.75 63.85 0.0037 3.75 3.76 3.77 3.82
12 11.25 61.55 0.0039 3.84 3.85 3.85 3.88
13 15.75 68.00 0.0039 3.93 3.94 3.95 3.97
14 15.00 70.00 0.0040 3.93 3.94 3.95 3.97
15 16.75 68.25 0.0040 3.99 3.99 3.99 4.02
16 13.75 66.30 0.0039 3.91 3.92 3.93 3.96
17 18.00 72.05 0.0041 4.11 4.12 4.12 4.15
18 20.50 79.75 0.0042 4.14 4.15 4.16 4.18
19 17.50 71.85 0.0041 4.08 4.09 4.10 4.12
20 18.75 76.20 0.0040 4.05 4.06 4.06 4.09
21 23.25 77.55 0.0041 4.05 4.07 4.07 4.10
22 19.00 80.10 0.0041 4.08 4.10 4.11 4.14
23 19.50 74.55 0.0041 4.05 4.07 4.08 4.11
24 23.00 81.20 0.0041 4.07 4.07 4.09 4.12
25 22.75 78.40 0.0041 4.04 4.05 4.06 4.09
26 23.25 80.85 0.0042 4.20 4.22 4.22 4.25
27 22.00 77.30 0.0040 4.01 4.03 4.04 4.06
28 22.50 80.10 0.0041 4.06 4.08 4.09 4.11
29 25.25 90.60 0.0041 4.14 4.15 4.16 4.19
30 21.00 78.75 0.0039 3.97 3.99 4.00 4.03
146 Information Processing in Agriculture 2 ( 2 0 1 5 ) 1 4 2 –1 4 8

Fig. 5 – The capacitance and TPC measurements at 100 kHz Fig. 7 – The capacitance and viscosity measurements at
at different heating time. 100 kHz at different heating time.

Fig. 6 – Capacitance measurement at 100 kHz of heated

Fig. 8 – Capacitance measurement at 100 kHz of heated
frying oil regressed on their TPC values.
frying oil regressed on their viscosity values.

(Testo 270, InstruMartlnc, Germany) (Fig.3). Before TPC mea-

to Japan Calibration Service System (JCSS) [32]. All measure-
surement, the oil tester was calibrated using the reference
ment was repeated three times.
oil supplied with the device which has a TPC value of 6.5
± 0.5%. The reference oil was heated to approximately 50 °C
2.4. Statistical analysis
for 10 min. Then, the oil tester was immersed in the reference
oil and adjusted to get the reference value. After each testing,
Regression analysis was performed to evaluate the relation-
the IDE sensor and the oil tester probe was cleaned by soft tis-
ships between degradation indices of TPC and viscosity with
sues. Later, the viscosity of the oil sample was measured
the capacitance of the heated oil samples. The regression
using a viscometer (SV-10 Vibro Viscometer, A&D Company
equations were evaluated by the coefficient of determination
Limited, Japan) as illustrated in Fig.4. Before using the vis-
(R2) and the root mean square error (RMSE) calculated by:
cometer, calibration was done using purified water according

Table 3 – The correlation coefficient and the RMSE of the regression equation used to predict TPC using capacitance
measurements at different frequencies.

Frequency (Hz) Equation R2 RMSE

2
100 y = 1E + 07x  48020x + 57.105 0.83 3.88
1000 y = 111.51ln(x)  136.2 0.85 4.75
10,000 y = 0.0003x7.9507 0.90 4.51
20,000 y = 111.45ln(x)  136.75 0.85 5.04
100,000 y = 0.0002x8.2087 0.90 4.85
 Information Processing in Agriculture 2 ( 2 0 1 5 ) 1 4 2 –1 4 8 147

Table 4 – The correlation coefficient and the RMSE of the regression equation used to predict viscosity using capacitance
measurements at different frequencies.

Frequency (Hz) Equation R2 RMSE

598.06x
100 y = 6.7699e 0.79 3.64
1000 y = 22.54x2  127.73x + 223.9 0.84 3.83
10,000 y = 23.85x2  138.26x + 244.2 0.85 4.04
20,000 y = 25.85x2  154.44x + 276.54 0.85 4.07
100,000 y = 32.967x2  209.43x + 380.88 0.87 4.99

vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi This is where the trend of capacitance is measured by the IDE

u
u1 X N
RMSE ¼ t ðY t  Y e Þ2 ð3Þ sensor following the same trend of viscosity measured by the
Ns n1 Viscometer. The viscosity results can be explained by the
increasing saturation of oil constituents and also the
where Ns is the number of samples in the dataset, Yt is the polymerization of the oil during the heating process [29].
predicted value calculated using the regression equation Fig. 8 illustrates the changes of capacitance value of the
and Ye is the measurement obtained through experimental frying oil as a function of viscosity across heating times at
procedures. 100 kHz. Based on the graph, it can be seen that the capaci-
tance measurements have significant positive correlation
3. Result and discussion with viscosity values with R2 of 0.87. Besides that, when the
regression equations were validated using the validation data
3.1. Relationship between capacitance measurements and set, the RMSEs found were between 3.64% and 4.99% (Table 4).
TPC measurements
4. Conclusion
The capacitance measurements data was analyzed to evalu-
ate the deterioration of frying oil quality. Overall, the capaci- The present study was designed to develop a custom built
tance of frying oil increased as the heating time increased sensor to evaluate frying oil degradation. Electrical capacitive
(Table 2). For example, at 100 kHz, the capacitance rose from based on spectroscopy technique was adapted as an alterna-
approximately 3.48 pF to 4.24 pF with the rising of heating tive method for measuring the oil degradation. The variations
time from 0 to 30 h, and increment of TPC values from of capacitance measurements had significant correlation
6.50% to 25.50% (Fig.5). In this result, the trend of capacitance with the changes of TPC and viscosity during the heating pro-
measured by the IDE sensor followed the same trend of TPC cess in frying oil. The capacitance measured by sensor has
measured using Testo 270. This finding is in agreement with shown significant correlation with TPC measured by commer-
the findings of [1,11,20] which showed that as the quantity cially tester (Testo 270) where the coefficient correlation R2 is
of the polar materials in the oil increased, the dipole density from 0.83 to 0.90, which also showed substantial correlation
and electric susceptibility also increased. For this reason, the with viscosity where the highest R2 is 0.87. The capacitive
capacitance of the frying oil samples increased. However, the sensing technique has good potential in developing a simple
fluctuation in the capacitance results might due to changes in and inexpensive way of monitoring frying oil degradation.
temperature of the oven during the experiments. The drawback of this type of technique are interferences by
Fig. 6 shows the capacitance measurements of the frying moisture since only a few pF were observed as sensor
oil regressed on the TPC values at different heating times. response, therefore a more extensive research on this area
Based on the graph, it can be seen that the electrical capaci- is recommended. A gravimetric measurements approach
tance measurements have significant positive correlation used to determine the degradation process of engine oil [26]
with TPC. The highest correlation was at 100 kHz with R2 of might be of interest; however, no known literature on its
0.90. When the regression equations were validated using application in edible oil were yet reported.
the validation data set, the lowest RMSE of 3.88% was found
at 100 Hz (Table 3). Acknowledgement

3.2. Relationship between capacitance measurements and This study is funded by the Prototype Research Grant Scheme,
viscosity measurements Ministry of Higher Education; project number PGRS/1/12/
TK02/UPM/02/2.
The viscosity values of the frying oil samples generally went
up from 50.80 mPas to 90.60 mPas as the heating time
increased (Table 2). On the other hand, the capacitance mea- R E F E R E N C E S
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