^{1}Doctor en tecnología avanzada, fgordillo@uniquindio.edu.co,
Universidad del Quindío, Armenia, Colombia.

^{2}Ingeniero electrónico, dfvalenciag@uqvirtual.edu.co, 0000-0002-5555-9210
Universidad del Quindío, Armenia, Colombia

^{3}Ingeniero electrónico, jagarciag_1@uqvirtual.edu.co, , Universidad del Quindío, Armenia, Colombia.

**How to cite:**

: F. Gordillo-Delgado, D. Valencia-Grisales and J. García-Giraldo, “Specific heat measurement of organic and conventional coffee samples by thermal relaxation”. Respuestas, vol. 24, no. 3, pp. 65-71, 2019.

Received on February 15, 2018 - Approved on June 25, 2018

The specific heat at constant pressure (Cp) is a necessary thermal parameter in the description of the heat transport in a material, related to the adaptation to changes of temperature, which is very important in the valuation and inspection of inputs for the construction, adhesives, insulators thermal and electronic devices. In this work, a thermal relaxation system with infrared thermometry was used to determine the Cp of green coffee beans, measuring the temperature of the material placed inside a vacuum chamber that reaches a pressure of 10-2 Torr. The sample was heated by radiation with laser light and the data was obtained by means of an acquisition card. The calibration of the system was made comparing the values obtained of Cp of Zinc, Tungsten, Titanium and Steel sheets with those reported and similarity was found. This method was used to measure the Cp of samples of organic and conventional coffee; these values were subjected to an analysis of variance and significant differences were found with a confidence level of 95 %. This technique could be used for the discrimination of organic coffee in the certification process

**Keywords:**Coffee, Specific heat, Differentiation, Instrumentation.

El calor específico a presión constante (Cp) es un parámetro térmico necesario en la descripción del transporte de calor en un material, relacionado con la adaptación a cambios de temperatura, lo que es importante en la valoración e inspección de insumos para la construcción, pegantes, aislantes térmicos y dispositivos electrónicos. En este trabajo se utilizó un sistema de relajación térmica con termometría infrarroja para determinar el Cpde granos de café verde, midiendo la temperatura del material colocado dentro de una cámara de vacío que alcanza una presión de 10-2 Torr. La muestra se calentó por radiación con luz láser y los datos fueron obtenidos mediante una tarjeta de adquisición. La calibración del sistema se hizo comparando los valores obtenidos de Cp de láminas de Zinc, Wolframio, Titanio y Acero con los reportados y se encontró similitud. Este método se usó para medir el Cp de muestras de café orgánico y convencional; estos valores se sometieron a un análisis de varianza y se hallaron diferencias significativas con un nivel de confianza del 95%, lo que permite afirmar que la técnica podría ser utilizada para la discriminación de café orgánico en el proceso de certificación.

**Keywords:**Café, Calor específico, Diferenciación, Instrumentación.

The fundamentals of heat transfer were established with Fourier’s first works [1]. The thermal characteristics of solids determine their behaviour to a temperature difference [2], which is important in industry and material sciences. In particular, specific heat expresses the amount of heat dissipated in a unit volume sample when its temperature varies by one degree [3].

The development of simple systems for measuring this parameter is a matter of interest, since commercial equipment is high cost and analytical methods and calculations are restricted to initial conditions and different temperature ranges; however, the thermal relaxation technique is one of the simplest and most functional [4, 5]. This method is based on the disturbance of the state of thermodynamic equilibrium, by means of constant illumination, of a sample suspended adiabatically inside a vacuum chamber and on the measurement of the variation of its absolute temperature as a function of time [6]. Infrared radiometry is often used to measure the temperature of the sample, taking into account the influence of convection and radiation heat losses. The analytical solution of the heat diffusion equation allows to obtain the Cp of the material through the analysis of the temperature evolution between two parallel surfaces of the solid sample [7]. In recent years this method has been used for the thermal characterization of semiconductor materials, food, wood, zeolites, clays and polymers [3] and to develop discrimination criteria [8], since the specific heat depends on the internal structure of the material and is characteristic of each substance.

On the other hand, in organic coffee cultivation, environmentally friendly practices bring this activity closer to being sustainable, because they give added value to the product [9]. However, the commercialization of this product requires the intervention of certifying bodies that inspect and guarantee its denomination, through a costly and lengthy process that is based on on-site inspection and testing on each farm. For this reason, the use of a quantitative method, such as the one that can be visualized from the results of this work, could support the discrimination of organic coffee, since it is made from a precise measurement that does not require sample preparation and a quick and lowcost analysis. In this work, the thermal relaxation method was used to measure the Cp of organic and conventional coffee samples for differentiation purposes. The single factor analysis of variance (ANOVA) led to a significant difference between the data for the two types of coffee. In the implementation of the measurement system it was guaranteed that the sensed temperature was acquired and saved as a data in function of time, using programming and synchronization of the algorithms in the interfaces.

The front surface of the samples (where the laser beam strikes) was dyed with a thin layer of carbon paint to ensure uniform heating and heat transfer. The light absorbed by the solid is converted, in whole or in part, to heat by non-radiative de-excitation processes. Assuming a known heat source, called Q, which radiates a sample of thickness L and mass m, the temperature variation is found.

ΔT. Assuming that it is adiabatically suspended at atmospheric pressure, it has to be:

Deriving (8) with respect to time and replacing in (3) has to be:

With the initial condition ΔT(0)=0, the solution to equation (9) is:

Table I shows the dimensions and mass of the samples in sheet form, obtained by grain cuts. It can be noted that the two types of coffee are in the same size and weight range.

The values of Cp and C of the 10 samples of coffee of each type were analyzed. Table IV shows the average values obtained with their respective standard deviation and coefficient of variation. The coefficient of variation for the two grain classes is very low, which is related to the precision of the measuring instrument and the homogeneity of the data. The average Cp value of the conventional and organic coffee samples was 1,320±0,003 and 1,157±0,003 J g-1 K-1, respectively.

The Cp value of zinc sheets, tungsten, titanium and steel, measured with the thermal relaxation system was close to that reported by other authors. This served to establish the reliability of the method.

The ANOVA applied to the Cp data, obtained through the thermal relaxation technique, allowed establishing a difference between samples of organic and conventional coffee. This single factor analysis, in which the response variable was the value of this parameter, led to define that this magnitude ranged between 1.3165 and 1.3233 J g-1 K-1 and between 1.1538 and 1.1596 J g-1 K-1, for conventional and organic coffee beans, respectively. On the other hand, information was obtained on the precision of the measurements, which had a coefficient of variation of less than 1%.

Possibly the Cp of conventional coffee is higher due to the different content of lipids, proteins and acids [15].

The electronic instrumentation implemented was of low cost and easy commercial acquisition, which gives added value to the development of the Cp measurement system and makes it a viable option for the purpose of supporting the organic coffee certification process.

The authors thank the Universidad del Quindío for funding this work, through project 924 and the support of the Instituto Interdisciplinario de las Ciencias de la Facultad de Ciencias Básicas y Tecnologías.

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