./orrnld
ofhod Er1girwring
30 (1YYh)
zyxwvutsrqponmlkjihgfe
I 555162
Copyright
CZ lYY6 Elscvier Scicncc Lmlrcd
Printed in Great Britain. All righta rcscrved
0x0-X774/06
$1.5.00+ (l.O(l
Pll:SO260-8774(96)00017-9
Photopyroelectric (PPE) Measurement of Thermal
Parameters in Food Products
D. Dadarlat,”
J. Gibkes,” D. Bicanic” & A. Pasca,”
” institute of Isotopic and Molecular Technology, POB 700, Cluj-Napoca 5, R34OC)Romania
“Wageningen Agricultural University, Department of Agricultural Engineering and Physics.
Agrotechnion, Bomenweg 4, 6703 HD Wageningen, The Netherlands
(Received
20 April 1995; accepted
I1 January
19%) zyxwvutsrqponmlkjihgfedcba
ABSTR4 CT
The stundard (SPPE) und inverse (IPPE) photopyroelectric configurations were
combined in order to obtuin room temperature values of the thermal pummeters
for vurious types of food products. The SPPE conjiguration (with thermalLy
thick sample and sensor) allows the direct and absolute measurement qf the
thermal diffusivity. The thermal efiusivity was obtained in the inverse
configuration (with thermally thin and optically opaque sensor and thermally
thick sample). The static (volume specific heat) and dynamic (thermal
conductivity, diffusivity und effusiviv) thermal parameters for a variety of liquid
und push’ products such as oils, margarine, butter; creums, musturd. ketchrip
and juices are reported. Copyright 0 1996 Elsel’ier Science Limited
INTRODUCTION
In recent years, the photopyroelectric
(PPE) method has proved to be a useful tool
for optical and thermal characterization
of materials (Bicanic, 1992). On the other
hand, studies of thermophysical
properties of foodstuffs represent the subject of
intensive research effort carried out both in industry and academic institutions.
Once known and their behavior understood, such properties can be used to benefit
industrial manufacturers by improving specific applications, as well as to control the
stability and the shelf-life of the product. Due to their composition and intrinsic
inhomogeneity foods are generally difficult to analyze thermally.
Recently the PPE method, in various configurations,
was proposed as a new
approach to thermal characterization
of fatty acids and triglycerides (Dadarlat et al.,
1995a, 1995b). Essentially, the PPE technique is concerned with the detection (via a
pyroelectric sensor) of the heat developed in a sample exposed to modulated radiation (Mandelis & Zver, 1985; Chirtoc & Mihailescu, 1989). The result of a PPE
155
156
D. Dadarlat et al
measurement, the PPE voltage, is a function of two or in some particular cases one
of the sample’s related thermal parameters (Dadarlat et al., 1995a).
It is well known that the four thermal parameters
(volume specific heat, C,
thermal conductivity, k, diffusivity, c(, and effusivity, e) are connected by two relationships and, consequently,
only two are independent
[see eqn (3)]. For the
purpose of obtaining all four thermal parameters for a given sample, usually one
measures a dynamic thermal parameter and uses literature values for the volume
specific heat (in fact values of the density and mass specific heat). Unfortunately, for
many foods, values for the volume specific heat are not available. For such products,
a method capable of directly measuring two thermal parameters is necessary.
In this paper it is proposed to combine two PPE configurations
in order to
measure the thermal diffusivity and effusivity and to use these values to calculate
the remaining thermal parameters. In such a way, the method is self-consistent,
being independent of other methods and/or literature values. The usability of the
proposed approach is demonstrated
with studies on food specimens for which the
value of the volume specific heat is not known.
THEORY
Standard configuration
In the standard configuration the radiation impinges on the front surface of the
sample; the pyroelectric sensor glued to the sample’s rear side measures its temperature variation.
Marinelli et al. (1992) demonstrated that, in the standard configuration, when the
sample and the sensor are both thermally thick (the geometrical thickness, L, is
much larger than the thermal diffusion length, ~1= (~x/o)]‘~, in the material), the
phase of the complex PPE signal is given by:
(1)
where (I) is the angular chopping frequency of the radiation and the subscript ‘s’
refers to the sample. Equation (1) indicates that, for a calibrated cell (i.e. L, is
precisely known), a frequency scan of the phase of the PPE signal provides the
direct and absolute measurement of the sample’s thermal diffusivity.
Inverse configuration
In the inverse configuration,
the modulated radiation is absorbed by the front
surface of the pyroelectric sensor, the sample (in good thermal contact with the
sensor’s rear side) acting as a heat sink.
Dadarlat (Dadarlat & Frandas, 1993; Dadarlat et al., 1995) showed that, in this
configuration, when the sensor is thermally thin (Lp <pp; .p3 referring to the pyroelectric sensor) and optically opaque, and the sample is thermally thick (L, $ pJ, the
amplitude of the PPE signal is given by:
Photopyroelectric measurement of thermul parumeters
v=!x
157
(2)
es
where V,, is a calibration factor depending on the radiation intensity, on the characteristics of the pyroelectric
sensor and the electrical circuit used for signal
processing (Dadarlat et al., 1995a). Equation (2) indicates that, provided an appropriate calibration was completed, the direct measurement
of the sample’s thermal
effusivity is possible. The remaining two thermal parameters can be derived using
the well known relationships:
k = ,(,)‘I2
and C = e(x)
“’
(3)
EXPERIMENT
The experimental set-up and the PPE cells, both for standard and inverse configurations, were presented elsewhere (Dadarlat et zyxwvutsrqponmlkjihgfedcbaZYXWVUTSR
al., 1995a, 1995~). Only some details
are given here. The pyroelectric sensors consisted of a 300-Lirn thick LiTa03 single
crystal in the standard configuration
and a 9-pm thick polyvinylidene
difluoride
(PVDF) foil in the inverse geometry. The signal from the detectors was processed
with a SR850 model Stanford Research lock-in amplifier. The radiation source was
a 31) mW Melles Griot diode laser, the radiation of which was electronically chopped
by the internal oscillator of the lock-in.
For the standard configuration a calibrated cell was designed (Fig. 1). The thickness of the cell was measured by two methods: (i) using a microscope; and (ii) by
performing a frequency scan with water as reference sample (Fig. 2). Using the
literature value of the thermal diffusivity of water (x = 1.42 x lo-’ m’is) in eqn (1)
one obtains the thickness of the cell. Both methods lead to a thickness of 570
( +5) itm, considered constant for all the investigated samples.
Radiation
GlilSS
rings
Fig. 1. Calibrated
cell used in the standard
configuration.
D. Dadarlat et al.
158
The frequency range for the scan was 0.05-l Hz. Figure 2 indicates that (for
water), starting at about 0.5 Hz, the conditions for a thermally thick regime for the
sample and sensor are fulfilled. Due to the fact that the composition of the samples
under investigation is based mainly on water or fats, we expect a similar frequency
range for them.
In the inverse configuration
the thickness of the sample exceeded 3 mm. A
chopping frequency of 0.1 Hz assures the validity of eqn (2) with an accuracy of
about 2% (Dadarlat et al., 1995). The measurement was calibrated with water again
(f&MU = 1600 W.s”2/m2.K).
After each measurement the sensor was cleaned with water and chloroform; the
procedure was repeated until the signal obtained with the empty sensor was always
the same within 0.2%. In both configurations the typical signal to noise ratio was
better than 1000.
The investigated samples included: different types of oils (soya, sunflower, olive,
pumpkin, motor oils), margarine, butter, peanut butter, apple juice, mustard, sour
cream and ketchup. No data about the volume specific heat of these products were
found in the literature. The following data about the origins and composition of the
investigated samples are available:
Soya zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA
oil: produced by Albert Heijn, The Netherlands;
name of product: Slaolie;
typical composition: saturated fatty acids (C16:0, Cl&O) 14%, unsaturated (Cl&l)
21%, (C18:2) 56%, (C18:3) 8%.
3.0
zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDC
2.5
2.0
I3
-2.5
-3.0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
sqrt (f) zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPON
(Hz’9
Fig. 2. Frequency
scan performed
(with water as sample)
sample.
to measure
the thickness
of the
Photopyroelectric
measurement
qf’ thermal parameter? ;
1%
oil: produced
by Carbonell, Spain; composition: (Cl60 and Cl8:O) 43%,
and C18:l) 2176, (C18:2 and C18:3) 6396, higher fatty acids trace level.
Sunflower
oil: produced
by VNR (Reform), The Netherlands;
main fatty acid
composition: (C16:l and C18:l) 21%, (C18:2 and C18:3) h3%, saturated fatty acids
12%.
Pumpkin
oil: produced
by Oljarna FRAM, Slovenia; name of product: GIMA,
bucino olje; composition: not specified.
Motor oil: produced
by Adam Opel AG, Ruesselsheim,
Germany; name of
product: Motor Oil SAE 15 W-40, API-SGICD CCMC G4iPD2; composition: not
specified on the package.
Murgurine: produced by Albert Heijn, The Netherlands;
name of product: Esona:
composition: mainly C18:2 and C18:3 (45%).
Butter: produced
by van der Pol en Zonen BV, Wijk en Aalburg, The Netherlands; name of product: Echte Boter; composition: mostly saturated fatty acids.
some amount of C18:l.
Peunut huttcr: produced
by Calve, The Netherlands;
name of product: Calve
Pindakaas; composition: peanuts, oil and fat of plant origin.
Mustard: produced
by Kuhne Netherlands:
name of product: Franse Mustard:
composition: water, mustard flour, vinegar, salt, sugar, seasonings.
Ketchup:
produced by Heinz, Elst, The Netherlands;
name of product: IHeinz
Super Hot Ketchup; composition: tomato puree, vinegar, sugar, glucose syrup, salt.
seasonings.
SOLIYcream: produced by Bubi, Dortmund, Germany; name of product: Saurc
Sahne; composition: 10% fat; no other data available on the package.
Orunge juice: produced by Riedel, Dranken Industrie, The Netherlands;
composition: 100% orange juice.
Olive
(Cl&l
RESULTS
Typical results of the frequency scan measurements
performed
in the standard
configuration for soya, olive and sunflower oils are displayed in Fig. 3. The slopes
obtained in the standard confieuration and the PPE signals obtained in the inverse
configuration are presented in Table 1. According to gqn (2), the sample’s thermal
effusivity can be calculated from:
e, = ewatcl.VLI’<IICI./V\
All four thermal parameters
are presented
(3)
in Table 2.
DISCUSSION
The PPE technique in two configurations (SPPE and IPPE) was proposed in order
. .
to obtain room temperature
values of static and dynamic thermal parameters tor
some (semi-)liquid foodstuffs. The method is able to directly measure two dynamic
thermal parameters (thermal diffusivity and effusivity); the remaining two thermal
parameters can be easily derived.
The method is self-consistent, being independent
of results obtained by other
calorimetric
techniques. The only literature data used for calibration were the
160
zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA
D. Dadarlat et al.
3.0
1
2.5 2.0 A.
u
0
L
V
\
1.5-
4
%
1.0 -
5
'I
E
0
_c
a.5 -
a
0.0 -
l.'*.
w
n_ -0.5 -
CL
-1.0 -1.5 -
-2.0
/,
0.0
0.2
0.4
1.2
sqrt(f)0~6(Hz@
Fig. 3. Frequency
zyxwvutsrqponmlkjihgfe
I,0
scan for: (a) soya; (b) olive; and (c) sunflower oils.
thermal parameters of water (literature scatter for thermal parameters of water is
low).
Due to the very good signal to noise ratio, the typical accuracy of such a measurement is about 0.1%. Unfortunately,
the final accuracy of the results is much lower.
TABLE 1
The Slope in the Direct Configuration
and the PPE Signal Amplitude
Inverse Configuration for the Investigated Samples
Sample
Soya oil
Olive oil
Sunflower oil
Pumpkin oil
Motor oil
Margarine
Butter
Peanut butter
Mustard
Sour cream
Apple juice
Ketchup
Water
IPPE amplitude (m V)
Slope
3.662
3.632
3.382
3.638
4.154
3.111
3.213
3.332
1.477
1.532
1.463
1.548
1.410
- 2.695
- 2.654
- 2.665
- 2.627
- 2.506
- 2.959
- 3.253
-3.216
- 3.236
- 2.983
- 2.754
- 3.474
“Water was used for calibration. The precision
residuals about mean explained (PRME).
of the fit is also presented
Obtained
in the
PRM E
100
100
100
100
99
99
99
100
100
100
99
100
by the percent
of
Photopyroelectric
measurement
161 zyxwvuts
of thermul purumefers
TABLE 2
The Static and Dynamic Thermal
Parameters
(W .,sI::,m’.K)
1.40
I .45
143
1.62
I .48
I.11
0.06
WY9
0.97
I.14
I .?i
0~85
616
621
667
620
543
725
702
677
1527
1462
1542
1457
Olive oil
Sunflower oil
Pumpkin oil
Motor oil
Margarine
Butter
Peanut butter
Mustard
Sour cream
Apple juice
Ketchup
Samples
c’ x lo- (’
(J/m-‘ .K) zyxwvutsrqp
2 x 10’
(m’ /s)
Sample
Soya oil
of the Investigated
1.64
I .hS
I .7s
I54
I .42
2.16
2.2’)
2.12
J.Y4
4.30
3.22
4.04
0.23
0.24
0.25
0.25
0.21
0.24
0.22
0.2 1
0.48
0.4’)
057
0.42
In the IPPE measurement,
eqn (2) is valid only within 2% (Dadarlat
et al., 1955).
The reproducibility
of the slope value (for the same material) in the standard
configuration is about 2% (see Fig. 4 for margarine), giving about 4% accuracy for
Y. This discrepancy is not due to the method, but probably sample inhomogeneity.
We want to stress the fact that this inhomogeneity of real food samples is the main
limitation on the final accuracy of the results. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQP
7.
‘A
‘.;..
*:.
*.*.
*.*.
0.‘.
*.
*I)
*.
*.
l
.
*’
0;.
. *
-.
** .
*.
‘*
-1.0
b
slopes:
zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA
a -3.032
\
a
!Ib -2.959
-1.5 1
-2.00(
0.2
0.4
sqrt(f)
0.6
1.0
1.2
(Hz’%:
Fig. 4. Frequency scans performed
for margarine
(two samples from the same
Between the two successive runs the cell was dismounted and cleaned.
pack).
162 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA
D. Dadarlat et al.
Some of the values obtained are typical for fats (oils, margarine, butters) and the
others are not far from the values of the thermal parameters of water (for mustard,
sour cream, apple juice, ketchup), denoting the high water content of these samples.
Various oils exhibit similar values for their thermal parameters. Some difference was
found in the thermal effusivity of butters and margarine, thus explaining the different taste sensation in the mouth for these two types of products.
As a final conclusion, the method proposed in this paper is simple, fast and
sensitive. Its main merit is the possibility of obtaining independently
of other
methods all four thermal parameters. In this paper the method was used with liquid
and semi-liquid samples, but its use can be extended to solid food samples; in this
case a coupling fluid must be used between the sample and the sensor. The method
should be a good tool for reducing the present lack of thermal parameter data for
many food products.
ACKNOWLEDGEMENT
The authors acknowledge the financial
Scientific Research (NWO).
support
from the Dutch Organization
for
REFERENCES
Bicanic, D. (ed.) (1992). Photoacoustic and
Optical Sciences, Vol 69. Springer, Berlin.
Photothermal
Phenomena
III,
Springer Series in
Chirtoc, M. & Mihailescu, G. (1989). Theory of the photopyroelectric
method for investigation of optical and thermal materials properties. Phys. Rev., B40, 906.
Dadarlat, D., Bicanic, D., Visser, H., Mercuri, F. & Frandas A. (1995a). Photopyroelectric
method for determination
of thermophysical parameters and detection of phase transitions
in fatty acids and triglycerides. Part I: Principles, theory and instrumentational
concepts. 1.
Amer. Oil Chem. Sot., 72, 273.
Dadarlat, D., Bicanic, D., Visser, H., Mercuri, F. & Frandas A. (199%). Photopyroelectric
method for determination
of thermophysical parameters and detection of phase transitions
in fatty acids and triglycerides. Part II: Temperature dependence of thermophysical parameters. J. Amer. Oil Chem. Sot., 72, 281.
Dadarlat, D. & Frandas, A. (1993). Inverse photopyroelectric
detection method. Appf. Phys.,
A56, 235.
Dadarlat, D., Visser, H. & Bicanic D. (199%). Improved inverse photopyroelectric
cell for
thermal effusivity measurements of food products. M eas. Sci. Technol., 6, 1215.
Mandelis, A. & Zver, M. M. (1985). Theory of photopyroelectric
spectroscopy of solids. J.
Appl. Phys., 57, 442 1.
Marinelli, M., Zammit, U., Mercuri, F. & Pizzoferrato R. (1992). High-resolution
simultaneous photothermal
measurements
of thermal parameters at a phase transition with the
pyroelectric technique. J. Appl. Phys., 72, 1906.