Sorption Thermodynamics of Soursop Powders Obtained by Different Dehydration Technologies

Termodinámica de Sorción de Polvos de Guanabana Obtenidos por Diferentes Tecnologías de Deshidratación

 

Adela M. Ceballos Peñaloza¹; Gloria Inés Giraldo Gómez²; Carlos Eduardo Orrego Alzate³ and Javier Telis-Romero⁴

 

¹ Titular Professor. Universidad de Caldas - Facultad de Ingenierías - Departamento de Ingeniería. Calle 65 No 26-12. Manizales, Colombia. <adela.ceballos@ucaldas.edu.co>
² Associate Professor. Universidad Nacional de Colombia - Sede Manizales- Facultad de Ciencias Exactas Naturales. - Departamento de Física y Química. Manizales, Colombia. <gigiraldogo@unal.edu.co>
³ Titular Professor. Universidad Nacional de Colombia - Sede Manizales- Facultad de Ciencias Exactas Naturales. - Departamento de Física y Química. Manizales, Colombia. <corregoa@unal.edu.co>
⁴ Titular Professor. Universidad Estadual Paulista - Departamento de Ingeniería. San Jose de Rio Preto. Brazil. <javier@ibilce.unesp.br>

 

Received: January 29, 2013; accepted: August 04, 2013.

 

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Abstract. The soursop (Annona muricata L.) is a perishable tropical fruit with high nutritional value. The aim of this work was to evaluate the sorption thermodynamics of soursop powders with maltodextrin (MD), obtained by different drying technologies, thus allowing understanding the mechanisms that govern this process. Adsorption isotherms of freeze dried (FD), spray drying (SD) and vacuum drying (VD) powders were determined at 20, 30, 40 and 50 °C by using the static gravimetric method. The results showed that the better fits the adsorption experimental data is the GAB model (R²>0.9893). The influence of the drying system in the model parameters was also shown. The monolayer moisture values between 5.28 and 9.30% (kg water/kg d.s) were within the reported range for dehydrated food.

Key words: Sorption isotherms, thermodynamic properties, isosteric heat of sorption, compensation theory.

Resumen: La guanábana (Annona muricata L.) es una fruta tropical perecedera y de gran valor nutricional. El objetivo de éste trabajo fue evaluar la termodinámica de sorción de polvos guanábana con maltodextrina (MD) obtenidos por diferentes tecnologías de secado, lo que permite la comprensión de los mecanismos que regulan este proceso. Se determinaron las isotermas de adsorción por método gravimétrico estático a 20, 30, 40 y 50 °C, para polvos secados por liofilización (FD), aspersión (SD) y vacío (VD). Los resultados mostraron que el modelo de GAB se ajustó a los datos experimentales de adsorción (R²>0,9893). Se mostró la influencia del sistema de secado en los parámetros del modelo. Los valores de humedad de la monocapa, entre 5,28 y 9,30% (kg de agua/kg s.s), se encuentran dentro de los rangos reportados para alimentos deshidratados.

Palabras clave: Isotermas de sorción, propiedades termodinámicas, calor isostérico de sorción, teoría de compensación.

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Annona muricata (L.) (family, Annonaceae) is a well-known tropical fruit tree, named "soursop", "guanábana" in spanish speaking countries and "graviola" in Brazil. It is classified as sour fruit and a source of fiber, calcium, phosphorus and vitamin C (Chaparro et al., 2004). It is considered as a nutraceutical and has a good functionality due to its polyphenol content and higher antioxidant capacity (Márquez, 2009). The soursop fruit is usually transformed into juice, pulp and concentrate. The largest producers are in the Caribbean, Central and South America (Colombia and Brazil), southeast China, Vietnam, Australia, New Zealand and Africa (IICA-PRODAR, 2006). Problems of crop seasonality and its high perishability make think that dehydration is a good alternative to add value and diversify soursop market.

The study of the thermodynamic sorption of water, allows knowing the behavior of the water inside and on the surface, its structure, properties, and the calculation of both energy and mass transfer requirements. For a biological system in equilibrium with its surroundings, it provides information related to the affinity sorbent-water, spontaneity of sorption processes and the concept of order and disorder into water, sorbent and the whole system (McMinn et al., 2005). Likewise, it is important to understand the relationship between the thermodynamic states and how they are reached through time. The thermodynamic properties derived from the analysis of the adsorption isotherms, are interesting for a large number of applications in food science and technology, as shelf life prediction of dehydrated products, storage conditions selection and packaging design to ensure good conditions for conservation (Chuzel, 1992). The sorption isotherms show the functional relationship between water activity and moisture content in equilibrium at a given temperature; changes with temperature and generally, for the same water activity, an increase in temperature causes a decrease in the water content of the product and thus a decrease in its hygroscopicity (Chuzel, 1992). Literature reports a large number of mathematical expressions for modeling the sorption isotherms, most of them reviewed by (Iglesias et al. (1976). The model of GAB (Guggenheim-Anderson-de Boer) is a semi-theoric three parameter model, considered as one of the best fits to the experimental data of food in a wide range of values (Giraldo et al., 2011; McMinn and Magee, 2003; Rahman, 1995). The objective of this work was to evaluate the sorption thermodynamic soursop powders added with maltodextrin (MD), obtained by freeze dried (FD), spray drying (SD) and vacuum drying (VD), as well as the adjustment to the GAB model of the isotherms at four different temperatures. The determination of thermodynamic properties to understand the mechanisms governing the adsorption process of water in the soursop powder with MD was also studied.

 

MATERIALS AND METHODS

Experimental procedure. Ripe soursop was purchased in a local market at Manizales-Colombia. Pulp was extracted and mixed with maltodextrin (MD) MOR-REX® 1920 with 20 dextrose equivalent (DE) until reached relation of 55% MD in dry basis (db). The operating conditions of the dryers were: Freeze drying (FD) Virtis (Genesis25, USA), an average freezing speed of 2.04 °C min^-1, chamber pressure of 1.3 ± 0.3 mmHg, plate temperature between -10 °C and 80 °C, maximum product temperature 60 ± 5 °C. Spray drying (SD) Buchi -290 equiped with a nozzle was operated in co-current, with an inlet air temperature of 140 °C and a feed flow rate between 1 and 1.5 L h^-1). Vacuum drying (VD) Marconi mark dryer (MA-030) at 26 mm Hg and 70 °C for 24 h. FD and VD samples were homogenized by grounding them into a mortar.

The adsorption isotherms were obtained using the static gravimetric method at 20, 30, 40, and 50 °C (Chuzel, 1992; Gabas et al., 2007; Simal et al., 2007) with saturated salt solutions of NaOH, KC₂H₃O₂, MgCl₂, K₂CO₃, Mg(NO₃)₂, NaNO₂, NaCl, KCl and BaCl₂, corresponding to a range of water activities of 0.059 to 0.9 (Fernández y Madriñán, 1992). Each solution was transferred into separated jars. Triplicate samples of around 1 g of soursop powder previously dried in an oven at 40 °C were weighed into small plastic receptacles and placed on tripods in the jars, which were then tightly closed and placed in a temperature-controlled chamber. The required equilibration time was 4 to 5 weeks, based on the change in samples weight expressed on a dry basis, which did not exceeded 0.1% (0.001 g/g dry solids). The equilibrium moisture content was determined in a vacuum oven at 60 °C for 48 h (AOAC, 2001). To evaluate the influence of the drying method on the parameters of GAB, an ANOVA with 95% confidence level was made.

Adsorption isotherms adjustment. The experimental data of the adsorption isotherms for soursop powders with MD at four temperatures were adjusted to the GAB model, which has been extensively tested to describe isotherms of foods (Rahman, 1995) and fruit powders dried with MD as described by pineapple and loquat (Telis et al., 2000; Gabas et al., 2007), soursop (Ceballos et al., 2009) and pineapple (Gabas et al., 2009). The GAB model is shown in equation (1), (Rahman, 1995):

Where, X is moisture in equilibrium of the isotherms expressed as kg of water/kg d.s; a_w is water activity; C, K, are dimensionless constants of the GAB model; X_m represents the monolayer moisture content. K indicates the difference in free enthalpy (standard chemical potential) between the sorbate molecules in the multilayer above the first layer and the pure liquid (Brunauers et al., 1938; Telis-Romero et al., 2005). C measures the difference between the chemical potentials of the sorbate in the upper layers of sorption and the monolayer (Al-Muhtaseb et al., 2004). In order to adjust the GAB model, a nonlinear regression analysis was performed using ORIGIN software. The regression coefficient, the standard error associated with parameters and the mean relative deviation (% E), which evaluates the data adjusting degree to the model were estimated. Equation (2):

Where, mi is the moisture content at the observation i; mpi is predicted moisture content at that observation and N is number of experimental data. For values %E<10 , the fit is considered acceptable (McMinn and Magee, 2003; Al-Muhtaseb et al., 2004; Simal et al., 2007).

Determination of thermodynamic properties. The changes in the adsorption of water have effect on the Gibbs free energy. The calculation of DG is defined by the Gibbs equation (3) as follows:

Where a_w is water activity; T is temperature the isotherm in K; R is the universal gas constant.

Net isosteric heat of adsorption or differential enthalpy DH_diff in J mol^-1, is defined as the difference between total isosteric heat of water sorption H_v(T) and the heat of vaporization of pure water H_v⁰(T). Equation (4):

The differential enthalpy changes in the water-solid interface, at different stages of the adsorption process in food and agricultural products may be calculated from Clausius-Clapeyron equation (5) (Telis-Romero et al., 2005):

Where [DH_diff]T is a molar amount which depends on the isotherm temperature and represents the amount of energy over the heat of vaporization of water required for the water molecules to bind by a sorption process at a specific moisture level. In order to apply the isosteric method, it is assumed that the heat of vaporization of pure water and the heat of sorption do not vary with temperature, hence it is recommended to measure isotherms at more than two temperatures (McMin et al., 2004; Telis-Romero et al., 2005; Lee and Lee, 2008).

With values obtained for enthalpy changes, the variation in the molar differential entropy [DS_diff]T may be estimated using equation (6), (Gabas et al., 2000; Fasina, 2006):

Where, S_L is differential molar entropy of water adsorbed on the solid; S_L is molar entropy of pure water in equilibrium with vapor.

Enthalpy-entropy compensation theory. Enthalpy-entropy compensation theory or the isokinetic theory explains the mechanisms that control the sorption of water vapor in the food and its spontaneity (Thys et al., 2010). It establishes a linear relationship between differential entropy and enthalpy for a particular reaction generated by changes in the solute-solvent interactions. If the compensation theory is assumed to be valid for sorption, the isokinetic T_b temperature must be constant for all points, its value corresponds to the slope of the graph, (Rooney, 1998) and represents the temperature at which all the reactions temperature in the series proceed at the same rate (McMinn et al., 2005). The result line is given by equation (7):

Where T_b is isokinetic temperature in K; DG_b is Gibbs free energy in J mol^-1 at T_b. A negative sign indicates that the process of water adsorption is spontaneous and a positive sign a non-spontaneous process at T_b (Simal et al., 2007).

RESULTS AND DISCUSSION

Adsorption isotherms. Figure 1 (A-C) shows the equilibrium moisture content X dry basis (db) obtained experimentally for soursop powders with 55% MD (db), dried by FD, SD and VD at 4 temperatures, 20, 30, 40 and 50 ºC, and the isotherms adjusted to GAB model. It can be noticed that for a same a_w at the 4 temperatures, X values for SD powder are larger than those of FD and VD; this suggests that the powders obtained by spray drying are more porous and thus more hygroscopic than the vacuum dried powders. The adsorption isotherms of soursop powder with MD, were affected by the type of dryer used (Ceballos et al., 2009), this result was also observed by (Lee and Lee, 2007) showing differences in the pores formed in dried yeast powder using different dehydration technologies.

All the drying systems presented inversion points. For a_w values smaller than the inversion point, the behavior is similar to the described in literature (the higher the temperature, the lower moisture values) after the inversion point is reached, this ratio is changed (the higher the temperature, the higher moisture content). This result has been widely demonstrated in works with high-sugar foods such as peaches, raisins, currants, prunes and biscuits (Rahman, 1995). The inversion points of the isotherms between 20 and 50 °C are in a_w values between 0.53 to 0.50 for FD; 0.5 to 0.70 for SD and 0.3 to 0.35 for VD, respectively. This point is explained by an increase in sugar solubility caused by an increase in temperature. Inversion point values between 0.55 and 0.70 of a_w have been reported for products with high sugar content such as dried fruit (Tsami et al., 1990; Telis-Romero et al., 2005).

Table 1 shows the estimated parameters for GAB model adjusted to the experimental data for soursop powders with MD at the experimental temperatures and statistical parameters. C value decreases with an increase of temperature for the 3 drying systems, indicating that at higher temperature, the binding energy of the first adsorbed layer decreases. For a temperature increase from 20 to 50 °C, there is a decrease in the C value for all the samples. Strong adsorbent-adsorbate interactions are exothermic, favored by lower temperatures, and are the cause of an increase in the C parameter with decrease in temperature (Diosady et al., 1996; Gabas et al., 2009). As well as, dependence on the drying type was also showed. Neither the K value nor X_m showed a clear dependence with temperature. X_m values were higher for SD powders (range from 0.0811 to 0.0972), followed by FD and VD (0.618 to 0.085 and 0.0528 to 0.0595), respectively. A similar result was reported by (Giraldo et al., 2011) in blackberry powder with MD dried under the same conditions used in this work.

The C values obtained for soursop powders with MD dried by SD, VD and FD at 20 °C were 38.6739, 21.2418 and 19.9598, correspondingly. Similar tendency was observed in all the tested temperatures. These values are within the limits reported for acai powder dried with air (11.82) and by lyophilization (7.13) (Pavan et al., 2012). Comparatively, C values for soursop powder with MD obtained by FD were lower than those reported for pineapple powder with MD and dried under the same conditions used in this work (22.852 and 12.798); and close to those reported for SD (18.097 and 6.030) and VD (25.384 and 11.093) at 20 and 50 °C, respectively (Gabas et al., 2007), in blackberry powder with MD. In all of the cases, C value was much higher than 2. This constant is related to the net sorption heat and is used to classify the isotherms according to the Brunauer's classification (Brunauers et al., 1938; Rahman, 1995); for C values higher than 2, isotherms are classified as type II (Martínez et al., 1998). It can be assumed that the monolayer adsorption heat is greater than the condensation heat, and it is constant for the monolayer occupation (Rahman, 1995). From isotherms analysis and the C values it can be inferred that soursop powder with MD isotherms are type II, this result agrees with the ones that were reported for fruit powders with MD as camu-camu (Silva et al., 2006) and noni (Fabra et al., 2011).

K values between 0.8829 and 0.9529 were obtained, which are within the range of values reported in the literature for many foods (Rahman, 1995). Its variation did not present a clear dependence on temperature, but an ANOVA at 95% confidence, showed a significant difference among the drying systems. By comparing the results of this work (0.8829, 0.9444, 0.8971 at 20 °C and 0.9096, 0.9301 and 0.9298 at 50 °C for FD, SD and DV), with those of pineapple powder (0.901, 0.875 and 0.911 for FD, SD and VD at 20 °C, and 0.936, 0.956 and 0.920 at 50 °C, respectively; Gabas et al., 2007), lyophilized acai juice (0.96 and hot-air drying at 25 °C of 0.92; Pavan et al., 2012) and naranjilla powder (0.894 to 0.951; Gabas et al., 2009), all are in the same range.

The X_m values (kg of water/kg d.s.) indicate the saturation moisture of the adsorption sites or the monolayer water content, and are dependent on the type of drying process. For the soursop powder with MD, values in the monolayer water were between 0.0528 and 0.0930 for the SD, 0.0637 and 0.0850 for FD, and 0.0528 and 0.0595 for VD. Although they do not show any definite trend with the temperature, the results were similar to those reported in different studies with fruit added with MD as pineapple pulp between 0.060 and 0.068 for FD; 0.093 and 0.115 for SD; 0.063 and 0.065 for VD (Gabas et al., 2007), freeze-dried strawberry pulp (0.075) (Mosquera et al., 2012), pineapple powder 0.049 and 0.074 (Gabas et al., 2009). For noni free of MD is reported a value of 0.082 and at higher contents of MD the value went down to 0.034, indicating that the MD content decreases the equilibrium moisture (Fabra et al., 2011); the work of (Lee and Lee, 2008) indicates that X_m is affected by the drying system.

The R² values for the adjustment of the curves were very close to unity and the %E = 10% , both confirm the good fit of the experimental data with the GAB model for the temperature range tested. This result agrees with the concept of numerous researches in food, which recommend the use of GAB model because of its good adjustment and the constants have a physical meaning.

Thermodynamic properties. In the Figure 2, changes in the Gibbs free energy are shown as a function of moisture content on dry basis (%), during the adsorption of water by the soursop powders with MD obtained by different drying systems at 30 °C. For a 1% of moisture, powders obtained by VD had a DG value of -13948 J mol^-1, -12857 J mol^-1 for SD and -12134 J mol^-1 for FD, indicating that were more hygroscopic at low moisture. However, the spray dried powders at humidities higher than 3.5% had lower DG values, were the most hygroscopic over a wide range of humidities. The higher negative values of DG are achieved for the same moisture, the more water molecules are easily trapped due to the greater forces of adsorption among the active points of the surface and the water molecules in the environment.

Figure 3 shows the relationship between the differential enthalpy DH_diff or net heat of sorption and the moisture content of the soursop powder, and its dependence on the three drying systems.

This property provides information about the energy changes that occur in food at a specific moisture level, produced by changes in the equilibrium moisture of the product. In the soursop powders with MD, differential enthalpy values decreases exponentially with increase in the humidity, tending to zero; this means that the heat of adsorption is equal to the heat of vaporization of water, and that this water begins to be more available for product degradation processes (Chuzel, 1992). The lowest values were presented for powders dried by VD and the highest for powders obtained by FD; these are the ones that require more energy to remove water from the monolayer. Net isosteric heat is zero when the heat of sorption value is the same as the heat of vaporization of water. The DH_diff results showed that VD powders with moistures of 1% require 20 kJ/kmol, 32 kJ kmol^-1 for SD powders and 50 kJ kmol^-1 for FD powders, suggesting that all are affected by the changes in relative humidity, FD powders are the most sensitive to this change (Viganó et al., 2012). These results are within the ranges reported for carrot 23 kg kmol^-1 and tomato 52 kJ kmol^-1 (Nunes and Rotstein, 1991), and higher than the pineapple powders with MD obtained by VD 17.5 kJ kmol^-1, FD 16.5 kJ kmol^-1 and SD 13.0 kJ kmol^-1 (Viganó et al., 2012).

The point at which DH_diff =0 is identified with the start of the solubilization of the low molecular weight solutes. The three drying systems presented the solubility point at humidity of 14.7 g water/100 g d.s for VD, 12.8 for FD and 11.9 for SD. Despite having the same composition, each drying system produces powders with different structures, which has an influence in the solubility (Ceballos et al., 2009).

Figure 4 represents the ratio of total sorption heat / heat of vaporization of water for the three drying systems at different humidities. This value expresses the increase of energy needed to evaporate water from the product in relation to the evaporation of the same amount of water in the free state (Viganó et al., 2012). The heat of evaporation of water was taken at the average temperature of experimental temperature range (35 °C) (McMinn and Magee, 2003).

It can be concluded from Figure 4 that drying soursop powder with MD until a final moisture content of 0.05 g of water/g d.s., requires approximately 215% more energy than if the same amount of free water were evaporated at the same temperature. Reaching the same moisture with spray and vacuum dryers requires approximately 600% more energy. Until a equilibrium moisture of 0.1 kg water/kg d.s, the lyophilizer requires less energy than other dryers to reach the same moisture. These results allow predicting the energy costs for the different technological systems.

Differential entropy is calculated for each equilibrium moisture and each drying system with equation (6), results at 30 °C are shown in Figure 5. Experimental results indicate moisture dependence and negative value in the range of 0.5% to 10.0% of moisture content for SD powders; 0.5% to 8.5% for DV; 0.01% to 10.0% for FD, and at values greater than 10% of moisture, it depends only on moisture. The differential entropy is proportional to the number of available adsorption sites at a specific energy level, decreases for low moisture contents and increases rapidly when water is all over the surface (Al-Muhtaseb et al., 2004). Entropic changes are associated with attraction or repulsion forces presented by the interface of water and sorbent; their quantification can contribute to the interpretation of dissolution and crystallization processes.

From fitting data of [DH_diff ]T versus [DSdiff ]T by linear regression, according to equation (7), the isokinetic temperature was calculated. Values of R² greater than 0.9988 in all cases were obtained.

Table 2 shows the T_b and DGT_b values dependence of the drying system. The highest value of free energy to the isokinetic temperature was presented at the spray dried powder, followed by lyophilized and vacuum drying powders; in every DGTb case the values were positive, suggesting that the adsorption process at this temperature is non-spontaneous for soursop powder with MD at T_b. The DGT_b values and the sign are indicatives of the affinity of the sorbent for water (McMinn et al., 2005). This may explain the low solubility of the powders in water.

The results are consistent with the ranges reported for isokinetic temperatures of various materials such as potato and potato starches 376.6 K and 466.8 K, respectively, and a DG between -1.525 and +0.107 kJ mol^-1 (McMinn et al., 2005); mango 463 K (Telis-Romero et al., 2005); loquat pulp between 458 and 475 K (Telis et al., 2000); pineapple 342 K and 1.67 kJ mol^-1 of free energy (Simal et al., 2007).

 

CONCLUSIONS

The isotherms for soursop powder with MD, dehydrated using three different technologies, presented an influence of the drying system and a good fit with the GAB model (R²>0.9893). All isotherms showed a sigmoid shape type II and an inversion point for all drying systems. The negative DG values suggest that adsorption process is spontaneous for the soursop powder with MD. The thermodynamic properties values of the soursop powders were influenced by the drying system, suggesting that differences in the structures caused by the drying technology used alter its porosity, and therefore influence their solubility, hygroscopicity and spontaneity of the adsorption processes.

 

ACKNOWLEDGMENTS

This work was supported and funded by the Food Fruit Research Group at the Universidad Nacional de Colombia at Manizales and Food Research Group at UNESP-Sao Paulo.

 

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