J Sci Food Agric 1998, 78, 182È186

Photopyroelectric Observation of Melting inValeric (C5 : 0), Linoleic (C18 : 2), Linolenic(C18 : 3) and Tricosanoic (C23 : 0) FreeFatty AcidsDorin Dadarlat,1* Dane Bicanic,2 Jurgen Gibkes,2 Vasile Surducan1and Aurel Pasca11 Institute of Isotopic and Molecular Technology, POB 700, Cluj-Napoca 5, R-3400, Romania2 Wageningen Agricultural University, Department of Agricultural Engineering and Physics, Agrotechnion,Bomenweg 4, 6703 HD, Wageningen, The Netherlands(Received 19 August 1997 ; revised version received 20 November 1997 ; accepted 5 February 1998)

Abstract : Photopyroelectric (PPE) study of some saturated (C5 : 0, and C23 : 0)and unsaturated (C18 : 2 and C18 : 3) free fatty acids was performed in a tem-perature range that included their melting points. The standard PPE conÐgu-ration was used consistently during the experiment ; the special PPE caserequests thermally thick sample and sensor and optically opaque sample. Forthose fatty acids for which literature data on volume-speciÐc heat were available(C18 : 2, C18 : 3), the behaviour around the melting point of all static anddynamic thermal parameters (speciÐc heat, thermal conductivity, di†usivity ande†usivity) was obtained. Since no data for volume-speciÐc heat of C5 : 0 andC23 : 0 were found in the literature, only the temperature behaviour of thethermal di†usivity was obtained for these specimens. 1998 Society of Chemical(Industry

J Sci Food Agric 78, 182È186 (1998)

Key words : photothermal phenomena ; photopyroelectric technique ; fatty acids ;phase transitions

INTRODUCTION

The thermal and optical characterisation of foodstu†sare important for their industrial manufacture, develop-ment and improvement, prediction of stability, qualityassessment and taste, consumer protection and otheraspects (Mohsenin 1980). Among the data of interest arethose associated with phase transitions, which, in thecase of food products, are usually melting processes.

Recently, the photopyroelectric (PPE) method wasproposed as a new technique for accurate calorimetricinvestigations of food products (Dadarlat et al 1995a,b).In the PPE method, the amount of heat developed in asample, due to the absorption of a modulated light, ismeasured with a pyroelectric sensor (Mandelis and Zver1985 ; Chirtoc and Mihailescu 1989). Usually, the lightis sinusoidally modulated in intensity using electro-mechanical or acousto-optical choppers. Various PPE

* To whom correspondence should be addressed.

conÐgurations were proposed in order to study meltingprocesses in fatty acids and triglycerides (Dadarlat et al1995a,b). Among them, the standard geometry withthermally thick sensor and sample and optically opaquesample has proved to be very suitable, because, from asingle measurement, one can obtain the temperaturebehaviour of all static and dynamic thermal parameters.A comparison between PPE and di†erential scanningcalorimetry (DSC) was recently made (Dadarlat et al1996) : it was shown that the two methods are in a waycomplementary, one of them providing informationabout the melting temperature and the temperaturebehaviour of the thermal parameters in the meltingregion, and the other one providing information aboutthe melting temperature and the quantity and sign oflatent heat involved in the process of phase transition.Due to the fact that the pyroelectric sensors detect onlythe temperature variation, and not the temperatureitself, the latent heat is a DC heat and, therefore, not

1821998 Society of Chemical Industry. J Sci Food Agric 0022È5142/98/$17.50. Printed in Great Britain(

Melting in free fatty acids 183

sensed by the sensor. As a last comparison betweenPPE and DSC, it is well known that, when investigatingphase transitions, the accuracy of the results is higher ifthe temperature variation rate is lower. With PFE onecan use temperature variation rates as low as fewmK min~1 (Marinelli et al 1994). With DSC this is notalways possible, because the sensitivity of the methodincreases with increasing the temperature variation rate.

This paper reports on new results concerning thetemperature behaviour of the thermal parameters in atemperature range that included the melting point forsome saturated and unsaturated fatty acids. The investi-gated fatty acids were C5 : 0 (valeric), C18 : 2 (linoleic),C18 : 3 (linolenic) and C23 : 0 (tricosanoic). The methodused for investigation was the standard PPE conÐgu-ration described above.

THEORY

In the standard PPE conÐguration, the radiationimpinges on the front surface of the sample, and thepyroelectric sensor, situated in good thermal contactwith the sampleÏs rear side, measures its temperaturevariation. A given medium (sensor or sample) is said tobe thermally thick if its geometrical thickness (L ) islarger than the thermal di†usion length (u) in thematerial.

The thermal di†usion length depends on the angularmodulation frequency (w) and the thermal di†usivity,(D) through the relationship

u \A2D

wB1@2

(1)

The thermal di†usivity is related to the remaining threethermal parameters, volume-speciÐc heat (C) thermalconductivity (k) and e†usivity (e) by

k \ CD e\ (Ck)1@2 (2)

In the standard conÐguration with thermally thicksample and sensor and optically opaque sample, theamplitude V and the phase F of the complex PPEsignal are given by the following equations (Marinelli etal 1994) :

V \ H0 prepL pCp[1] (wtE)2]1@2

]exp[[L s(w/2Ds)1@2]

es(em/es ] 1)(ep/es] 1)(3)

and

F\ [L s(w/2Ds)1@2 (4)

In eqns (3) and (4) is the intensity of the incidentH0radiation, r and represent the electrical resistance andtEelectrical time constant of the equivalent sensor lock-incircuit (from electrical point of view, the pyroelectricsensor and the lockin ampliÐer are in parallel), and p is

the pyroelectric coefficient of the sensor. The subscriptsp, s and m refer to the pyroelectric sensor, the sampleand the medium in front of the sample.

Equation (4) indicates that the phase of the PPEsignal depends only on the sampleÏs di†usivity, allowingits direct measurement (providing and w are known).L sSubstituting from eqn (4) in eqn (3), one obtains aDssecond thermal parameter (in our case the thermal e†u-sivity, The values of remaining thermal parameterses).can be derived from eqn (2).

EXPERIMENTAL

The experimental set-up used for PPE calorimetricinvestigations was discussed elsewhere (Dadarlat et al1995a) (see Fig 1), only some details are presented here.The PPE cell is a cold-Ðnger-based system, allowing theoperation both below and above the ambient (Dadarlatet al 1995a). The typical temperature variation rate was0É35 K min~1 with acquisitions at each 0É05 K. Thepyroelectric sensor was a 0É3 mm thick singleLiTaO3crystal. The sample accommodated the all available

Fig 1. (a) The experimental set-up used in the experiment. (b)The PPE cell geometry used to accommodate the sample.

184 D Dadarlat et al

Fig 2. The phase of the PPE signal as a function of sqrt (f) forthe investigated fatty acids. The linear dependence limits thethermally thick regime for the sample and sensor. The slope is

used to calculate the thermal di†usivity from eqn (4).

space between the sensor and an opaque calibrated cell(0É57 mm thick) (Fig 1b) (Bicanic et al 1995 ; Dadarlat etal 1995a). The sample was introduced in the cell alwaysin a liquid form and Ðlled the space between the sensorand an opaque metallic layer due to capillarity. Thesignal from the detector was processed with an EG andG 5110 Princeton Applied Research lock-in ampliÐer.The radiation source was a 30 mW Melles Griot diodelaser electronically chopped by the internal oscillator ofthe lock-in. A personal computer was used for dataacquisition. The purity of the samples was minimum92%.

Prior to phase transition measurements, a frequencyscan was performed for each sample in order to Ðnd theappropriate chopping frequency that satisÐes therequirements imposed by the special PPE case(thermally thick regime for the sample and sensor) andto obtain an absolute calibration value for the thermaldi†usivity. The results of the frequency scans for thephase of the PPE signal are presented in Fig. 2.

As stated above (see eqn 4), in the thermally thickregime for both sample and sensor, the room tem-perature value of the thermal di†usivity can be obtainedfrom the slope of the curve (providing is known) :L s

slope \ [L s(n/Ds)1@2 (5)

For all samples investigated here, a chopping frequencyof 0É8 Hz was considered suitable for phase transitionsinvestigations. When processing measured data, thetemperature dependence of the PPE signal for theunloaded sensor was used for normalisation (Dadarlatet al, 1995a,b) ; in such a way, the temperature depen-dence of the pyroelectric coefficient and electrical capac-itance of the sensor are eliminated.

RESULTS

The room temperature value of the thermal di†usivityof the investigated samples ranges between 0É0015 and

0É0025 cm2 s~1, in agreement with isolated valuesreported in the literature for some oils or fats.

A typical result obtained for the amplitude and phaseof the PPE signal for one of the investigated fatty acids(C18 : 2) is presented in Figs 3 and 4.

Large anomalies around the melting point are presentboth in the amplitude and phase of the PPE voltage.They are practically associated with the critical anom-alies of the thermal parameters at a Ðrst-order phasetransition. If the melting points are taken as the tem-peratures at which the amplitude of the PPE signalexhibits a minimum, when heating, then the relevantvalues are 241 K (C5 : 0), 268 K (C18 : 2), 262 K(C18 : 3) and 346 K (C23 : 0). These melting points arenot far from the data reported in the literature (FattyAcid Data Book 1992). When cooling down, the criticaltemperatures are always lower, the observed hysteresis,associated with nucleation phenomena, being typical formelting processes occurring in fatty acids.

For quantitative results, a calibration of the measure-

Fig 3. Temperature dependence of the amplitude of the PPEsignal for C18 : 2, in a temperature range that includes the

melting point.

Fig 4. Temperature dependence of the phase of the PPEsignal for C18 : 2, in a temperature range that includes the

melting point.

Melting in free fatty acids 185

Fig 5. Temperature behaviour of the thermal di†usivity of theinvestigated fatty acids, in a temperature range that includes

the melting point.

Fig 6. Critical behaviour around the melting point of thevolume-speciÐc heat, thermal conductivity and e†usivity for

C18 : 2.

Fig 7. Same as Fig 5, but for C18 : 3.

ments is necessary in order to obtain the value of theÐrst factor in eqn (3).

The value of the thermal di†usivity obtained, via afrequency scan, at a certain temperature (usually roomtemperature) allows one to obtain the temperaturebehaviour of this thermal parameter in the temperaturerange of interest. The results obtained for the investi-gated fatty acids are presented in Fig. 5.

In order to obtain the temperature behaviour of theremaining thermal parameters, one needs a value of asecond thermal parameter, at the same temperature atwhich the frequency scan was performed. For somefatty acids this second thermal parameter (available inthe literature) necessary to calibrate eqn (3) is thevolume-speciÐc heat (the product of the mass-speciÐcheat and density) (Fatty Acid Data Book 1992). Unfor-tunately, for the fatty acids investigated in this paper, tothe best of the authorsÏ knowledge, no data about thethermal parameters are reported. However, assumingthat the mass-speciÐc heat of C18 : 2 and C18 : 3 is thesame as that of C18 : 1, the authors derived all thethermal parameters and their temperature behaviour forthese two fatty acids too (Figs 6 and 7).

186 D Dadarlat et al

CONCLUSIONS

The PPE method, in standard conÐguration with ther-mally thick sample and sensor and optically opaquesample was applied to investigate melting processes insome saturated and unsaturated fatty acids. The tem-perature behaviour of the thermal di†usivity in a tem-perature range that included the melting point, forC5 : 0, C18 : 1, C18 : 2 and C23 : 0, was obtained for theÐrst time.

When isolated values for the mass-speciÐc heat wereavailable in the literature, the temperature behaviour ofthe remaining static and dynamic thermal parameterswas derived.

Large anomalies around the melting point for thethermal parameters and hysteresis to heatingÈcoolingcycles were observed for all investigated samples.

Unfortunately, due to the lack of data in the liter-ature about the thermal parameters of the fatty acids,no comparison with the present results is possible.However, the order of magnitude of the values of thethermal parameters obtained here agrees with isolatedvalues reported for some oils or fats.

Finally, there are some important aspects concerningthe method. The formulas used in order to obtain thetemperature behaviour of the thermal parameters arevalid for homogeneous samples. During a meltingprocess a coexistence of phases can appear and, conse-quently, the validity of these formulas is doubtful.However, due to the small quantity of sample (about0É7 ml) requested in such a PPE experiment, it isassumed that, in the Ðrst approximation, the wholesample melts at the same time, and the formulas arevalid in the melting region too. The melting processes infatty acids, triglycerides and in food products generallyare usually strong Ðrst-order phase transitions. For thistype of transition, the enthalpy has a step at the meltingpoint. As a consequence, the speciÐc heat must have adivergency at this point. Experimentally, due to a Ðnitetemperature variation rate and due to impurities, oneobtains peaks. These heat-capacity peaks are sometimescalled “anomalousÏ and they cannot be eliminatedexperimentally. More than that, due to the fact that thevalues of the speciÐc heat are used to derive otherthermal parameters, this “anomalousÏ behaviour propa-gates to them too. In conclusion, one can never claim todo “critical phenomenaÏ when investigating melting pro-cesses in food products ; what the experimentalist per-forms is in fact a PPE calorimetry in a temperaturerange that includes the melting point. The values of thethermal parameters in the liquid and solid phase andthe melting temperature are absolutely correct. Themagnitude and the width of the critical anomalies aremore or less dependent on the experiment, and this iswhy a clear description of the experimental conditions(temperature variation rate, rate of data acquisition,samples purity, etc) is always necessary.

Work is in progress in combining this standard PPEconÐguration with an inverse one. The inverse (front)conÐguration permits one to obtain directly thesampleÏs thermal e†usivity (necessary for calibration). Insuch a way, the PPE method will become independenton other methods or data available in the literature.

Concerning the general utility of the PPE method indetecting phase transitions, it is well known, since 1994,that it is one of the most sensitive techniques forsecond-order phase transition investigations. It wasused in measuring the critical exponents of the thermalparameters and in identifying the universality classes.This paper (together with a few others mentionedabove) tries to demonstrate that (with some reser-vations, presented above) it can be a powerful methodfor studying phase transitions in fats.

ACKNOWLEDGEMENTS

One of the authors (DD) acknowledges the receipt of avisiting scientist fellowship from the Dutch Organiz-ation for ScientiÐc Research (NWO), The Hague, TheNetherlands, which greatly stimulated this work. Usefulconversations with Prof J Thoen are also acknow-ledged.

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