Photopyroelectric Calorimeter for Phase Transitions Monitoring: Application to Chocolate
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Photopyroelectric Calorimeter for Phase Transitions Monitoring: Application to Chocolate E. I. Martínez-Ordoñez, E. Marín1, J. A. I. Díaz-Góngora and A. Calderón Centro de Investigación en Ciencia Aplicada y Tecnología Avanzada, Instituto Politécnico Nacional, Legaria 694, Col. Irrigación, C.P. 11500, México D.F., México 1 Corresponding author: [email protected] ABSTRACT In this work we report about the design and construction of a simple and cheap calorimeter for phase transitions monitoring using Peltier elements and based in the well known inverse (front) photopyroelectric method for thermophysical characterization of materials. We describe its application for the detection of phase transitions in chocolate samples, as an alternative, for example, to the most widely used and more expensive Differential Scanning Calorimetry technique. The manufacture of chocolate requires an understanding of the chemistry and the physical properties of the product. Thus the involved problems during the confection process are those of the so-called materials science. Among them, those related with tempering are of particular importance. Because the fats in cocoa butter experience the so-called polymorphous crystallization, the primary purpose of tempering is to assure that only the best form is present in the final product. One way to characterize this is by measurement of the temperature dependence of the thermal properties of the chocolate and the monitoring of the temperature at which phase transitions take place. We show that the photopyroelectric method, aided with Peltier cells temperature control, can be a useful choice for this purpose. INTRODUCTION In the last years the photopyroelectric (PPE) technique [1] in its several experimental configurations has become great attention due its possibilities to perform the thermal characterization of materials. In the most used configurations, the analyzed sample is placed in intimate thermal contact with one of the metal coated surfaces of the sensor, while a periodical intensity modulated light beam impinges on its opposite metalized side or on the sample’s surface, which act as a light absorber. Following the absorption of light energy, the pyroelectric (PE) temperature fluctuates periodically at the modulation frequency of the incident beam (these temperature oscillations are the so-called thermal waves) thereby generating a voltage, whose amplitude at a given frequency can be measured using a Lock-in amplifier. Due to the good thermal contact that can be achieved between liquids and detector, the majority of the published works concern the characterization of these kind of materials. When the light beam impinges on the metal contact of the sensor, the experimental variant is called the front or inverse PPE technique, which has been found mainly suitable for thermal effusivity measurements [2]. In the other useful configuration light is absorbed by a sample, and the thermal diffusivity can be determined [3]. Applications in the fields of foods characterization [4], study of liquid m
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