ELEVATION ON BOINLING POINT OF COFFE EXTRACT

Abstract

The rise in boiling point of coffee extract was experimentally measured at soluble solids concentrations in the range of 9.2 to 52.4ºBrix and pressures between 5.8 × 10³ and 9.4 × 10(4) Pa (abs.). Different approaches to representing experimental data, including the Dühring's rule, the Antoine equation and empirical models proposed in the literature were tested. In the range of 9.2 to 16.2ºBrix, the rise in boiling point was nearly independent of pressure, varying only with extract concentration. Considerable deviations of this behavior began to occur at concentrations higher than 16.2ºBrix. Experimental data could best be predicted by adjusting an empirical model which consists of a single equation that takes into account the dependence of rise in boiling point on pressure and concentration.

Evaporation; concentration; vapor pressure

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ELEVATION ON BOINLING POINT OF COFFE EXTRACT

J.Telis-Romero^*, R.A.F.Cabral, G.Z.Kronka and V.R.N.Telis

Departamento de Engenharia e Tecnologia de Alimentos, Universidade Estadual Paulista, UNESP,

Phone: (55) (17) 221-2251, Fax: (55) (17) 221-2299, 15054-000, São José do Rio Preto - SP, Brazil

E-mail: [e-mail]

(Received: August 31, 2001 ; Accepted: November 12, 2001)

Abstract - The rise in boiling point of coffee extract was experimentally measured at soluble solids concentrations in the range of 9.2 to 52.4^oBrix and pressures between 5.8 ´ 10³ and 9.4 ´ 10⁴ Pa (abs.). Different approaches to representing experimental data, including the Dühring's rule, the Antoine equation and empirical models proposed in the literature were tested. In the range of 9.2 to 16.2^oBrix, the rise in boiling point was nearly independent of pressure, varying only with extract concentration. Considerable deviations of this behavior began to occur at concentrations higher than 16.2^oBrix. Experimental data could best be predicted by adjusting an empirical model which consists of a single equation that takes into account the dependence of rise in boiling point on pressure and concentration.

Keywords: Evaporation, concentration, vapor pressure.

INTRODUCTION

Coffee is the product with the second high value added worldwide, and Brazil is the major producer and major exporter. In 1998 coffee represented 5.1% of the total value of Brazilian exports, and the coffee industry processed around 12.2 million bags directed to the internal market. The Brazilian coffee production estimate for 2001/02 is of about 33.7 million bags (Embrapa, 2001; Café do Cerrado, 2001). These data indicate the great importance of the coffee industry to the Brazilian economy and justify research aiming to optimize design and operation of processing plants.

Knowledge of the boiling point temperature of coffee extract for a wide range of concentrations is of primary importance to the soluble coffee industry, as it makes wide use of evaporation for extract concentration prior to spray or freeze drying.

Previous work has been conducted on the effect of temperature and concentration on thermophysical properties of coffee extract, including density, specific heat, thermal conductivity, thermal diffusivity, and rheological data (Telis-Romero et al., 2000; Cabral, 2000). Nevertheless, no published data are available on the rise in boiling point at different concentrations of coffee extract, and theoretical equations are of limited use due to the complex composition of the material and lack of knowledge of the components that contribute to the elevation of boiling point.

Experimental data on the rise in the boiling points of fruit juices at different concentrations were presented by Ilagantileke et al. (1991), for Thai tangerine juices, by Crapiste and Lozano (1988) and Moresi and Spinosi (1984) for apple juice, by Moresi and Spinosi (1980) for orange juice, and by Varshney and Barhate (1978) for pineapple, mango and lemon juices.

In this work, the rise in boiling point of coffee extracts at various concentrations and pressures was measured and compared with data on sucrose solutions, which are often used as models to represent the behavior of fluid foods such as fruit juices and coffee extracts.

MATERIALS AND METHODS

Sample Preparation

Roasted and ground Arabica coffee obtained at the local market was used throughout this study. The coffee extract was prepared by manual extraction with boiling distilled water at atmospheric pressure and concentrated under vacuum (about 1.07 ´ 10⁴ Pa, abs.) in a rotary evaporator (Fisatom, São Paulo, Brazil), resulting in extracts with soluble solids contents in the range of 9.2 to 52.4^oBrix. Soluble solids concentration was measured at 25 ^oC with a refractometer.

Apparatus and Procedure

A schematic diagram of the apparatus used for experimental measurement, which is similar to that described by Moresi and Spinosi (1980), is shown in Figure 1. It was made of glass and consisted of a flat bottom flask (F) with three openings. Samples were introduced into the flask by means of tube A and heated by an electric heater provided with a magnetic stirrer (Fisatom, São Paulo, Brazil). When the extract reached boiling temperature, a recirculation flow was established between tubes B and C. The liquid-vapor mixture freed from the liquid surface flowed up through tube B, thus heating the thermocouple installed in the well, which was connected to a temperature transmitter (model TT302, SMAR, Sertãozinho, SP, Brazil). Entrained liquid particles were trapped in compartment D and returned to flask F, allowing vapor to enter reflux condenser R. Condensed vapor also returned to flask F through tube C with valve V controlling the recirculation flowrate to keep the extract concentration constant.

The condenser was connected to a vacuum pump, allowing pressure to vary in the range of 5.8 ´ 10³ to 9.4 ´ 10⁴ Pa (abs.). Differential pressure transmitters (model LD-301, SMAR, Sertãozinho, SP, Brazil) were used to measure static pressure at two different positions in the vacuum tube. An HP model 75.000-B data logger, an HP-IB interface and a PC running a data acquisition program written in IBASIC monitored temperature with an accuracy of 0.6^oC and pressure with an accuracy of 4.3 mPa.

In each experiment, a 180 mL sample of concentrated extract was introduced into the boiling vessel. The cooling water flow was initiated in the reflux condenser, the vacuum pump was turned on with a valve regulated to provide a pressure of about 6 ´ 10³ Pa (abs.), and the fluid was mixed and heated slowly. Temperature and pressure were then continuously recorded, and final values for solution boiling point and associated pressure were registered after readings had been constant for at least 5 minutes.

The procedure was repeated almost up to atmospheric pressure, allowing measurement of boiling points at different pressures with the same extract concentration. In order to check extract concentration, heating was periodically interrupted, the vessel was cooled down to room temperature, and a sample of fluid was removed for measurement of ^oBrix. When necessary, extract was substituted and the run was repeated.

For extracts of different concentrations, all of the above procedure was repeated.

RESULTS AND DISCUSSION

Performance of the apparatus was checked using aqueous solutions of LiCl and NaOH, whose boiling points at various concentrations and pressures are known. Table 1 permits comparison of experimental data obtained in this work and data presented by Perry and Chilton (1986), while Figure 2 shows a typical histogram of data distribution. The agreement between experimental and reported data was good as was the reproducibility of results, since the histogram shows a normal distribution of measurements around the average value. Values of the standard deviation (sd) and standard error (se) included in Table 1 were calculated by equations (1) and (2) respectively

where x_n is the value of each measurement, the average value, and n the number of measurements.

The typical manner of presenting boiling point data on fluid foods consists of relating them with the boiling temperature of water at the same pressure. Dühring's rule (Foust et al., 1960) states that the temperature at which one liquid exerts a given vapor pressure is a linear function of the temperature at which a reference liquid exerts the identical vapor pressure (Heldman and Singh, 1981). Therefore, at a constant concentration we have

where T_A and T_A0 are respectively the boiling temperatures of coffee extract and water at the same pressure. Parameters m₀ and m₁ were determined by linear regression for each coffee extract concentration and are shown in Table 2 together with the correlation coefficient, r².

Defining the rise in boiling point, DT_B, as

and substituting it into equation (3), it is observed that for m₁ = 1, DT_B = m₀, indicating that at this condition the rise in boiling point varies only with extract concentration and is independent of pressure. Table 2 and Figure 3, which shows DT_B versus the boiling point of pure water, show that the slope of equation (3), m₁, was practically equal to one for lower concentrations, but considerable deviations began to occur at higher concentrations. This behavior led to the conclusion that the pressure influence should be taken into account in any proposed method for predicting boiling points of coffee extract.

A second manner of presenting boiling point data on aqueous solutions is based on extending the use of expressions suitable for describing the temperature dependence of pure water vapor pressure, as in the case of the Antoine equation (Perry and Chilton, 1986), which is written as

In equation (5), P is the pressure in Pa, T_A is the boiling temperature of coffee extract in K, and A, B, and C are empirical constants dependent on concentration. Table 3 shows values of these constants for the coffee extract, obtained by a nonlinear regression procedure, while Figure 4 presents a comparison of experimental vapor pressure data and predictions of equation (5). Although the correlation coefficient (r²), also included in Table 3, indicates a good correlation between the model and experimental data, it was not possible to establish an explicit dependence of these constants as a function of soluble solids content of coffee extract. On the other hand, using a similar model to describe vapor pressure dependence on temperature of concentrated apple juices, Moresi and Spinosi (1984) were able to express the empirical constants as functions of an "equivalent sucrose weight fraction."

Crapiste and Lozano (1988) proposed an alternative approach to representing the elevation of boiling point of aqueous solutions, which consisted of adopting an empirical model that was simultaneously dependent on pressure and soluble solids concentration, given by the following equation:

where DT_B = T_A - T_A0 is the rise in boiling point in ^oC, W represents the mass concentration of soluble solids in ^oBrix, and the parameters a, b, g, and d could be evaluated by nonlinear regression.

In the case of coffee extract, fitting equation (6) to experimental data resulted in a very good agreement, with a high correlation coefficient (r² = 0.997) and a favorable distribution of residuals (Figure 5). The adequacy of equation (6) for predicting the elevation of boiling point of coffee extract can also be checked using an observed (experimental) versus predicted data plot, as in Figure 6. The great majority of data are contained within a ± 10% range of error, with larger deviations occurring only for dilute solutions, where values of rise in boiling point are nearly zero.

Numerical values for parameters a, b, g and d are shown in Table 4. In order to permit comparison, values of the same constants for apple juice and sugar solutions obtained by Crapiste and Lozano (1988) were included. A similar order of magnitude was observed for coffee extract and apple juice. Nevertheless, these values were considerably different from those corresponding to sucrose solution, leading to the conclusion that important errors could arise from the use of data on sucrose solutions to estimate the rise in boiling point of coffee extract, which is a common practical procedure.

It is important to stress that this work was conducted with coffee extract produced at atmospheric pressure. Industrial extraction process involves an additional step, the hydrolysis, which is carried out at high pressures and consists of the breakdown of large molecules of water insoluble carbohydrates into smaller, water-soluble compounds. These are mostly reducing sugars, but there are also large sugar molecules, as well as proteins that also undergo hydrolysis under process conditions (Sivetz and Desrosier, 1979). Hydrolysis products could affect boiling point of industrial coffee extracts in a different manner from that have been shown in this work. On the other hand, the experimental data presented in this paper consists of a considerable advance against the use of data on sucrose solutions.

CONCLUSION

The rise in boiling point of coffee extract at soluble solids concentrations in the range of 9.2 to 52.4 ^oBrix has been measured at pressures between 5.8 ´ 10³ and 9.4 ´ 10⁴ Pa (abs.). In the range of 9.2 to 16.2^oBrix, the rise in boiling point was nearly independent of pressure, varying only with extract concentration. Considerable deviations from this behavior began to occur at concentrations higher than 16.2^oBrix. Experimental data could be adequately predicted by adjusting the empirical model proposed by Crapiste and Lozano (1988), which consists of a single equation that takes into account the dependence of the rise in boiling point on pressure and concentration.

NOMENCLATURE

a, b, g, d Constants in equation (6) DT_B

Elevation of boiling temperature of coffee extract

A, B, C Constants in equation (5) m₀, m₁ Constants in equation (3) P Pressure T_A Boiling temperature of coffee extract T_A0 Boiling temperature of pure water W Soluble solids concentration (^oBrix)

ACKNOWLEDGMENTS

The authors wish to express their thanks to FAPESP for its financial support (Proc. 98/08738-7) and to Prof. Dr. Fernando Ferrari (DCCE/UNESP - São José do Rio Preto) for his helpful suggestions in the statistical analysis.

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To whom correspondence should be addressed

Publication Dates

• Publication in this collection
  13 May 2002
• Date of issue
  Mar 2002

History

• Accepted
  12 Nov 2001
• Received
  31 Aug 2001