International Journal of Agricultural and Food Science. ISSN Original Article Moisture sorption isotherms of mango slices

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1 Available online at International Journal of Agricultural and Food Science Universal Research Publications. All rights reserved ISSN Original Article Moisture sorption isotherms of mango slices 1, *Elamin O.M. Akoy, 2 Dieter von Hörsten 1 Faculty of Environmental Sciences and Natural Resources, University of Al-Fashir, Sudan 2 Section of Agricultural Engineering, Faculty of Agriculture, University of Göttingen, Germany. *Corresponding author. Tel.: ; Fax: address: aminoz71@googl .com (Elamin Akoy) Received 02 November 2013; accepted 16 November 2013 Abstract Moisture sorption isotherms (desorption and adsorption) of mango fruit, var. Kent, were investigated using the standard static gravimetric method. The sorption isotherms were obtained at the temperatures of 20, 30 and 40 C and in the water activity (a w ) range from to The sorption isotherms of mango fruit followed type III (J-shaped) isotherms, characteristic of high sugar products. The reverse temperature effect occurred at a w >0.75, resulting in a crossing over of the sorption curves for the adsorption isotherms only. The application of the GAB model to the experimental data, using direct nonlinear regression analysis, provided agreement between the experimental and predicted values. The GAB model parameters monolayer moisture content (Mo) and heat of sorption for monolayer (C) were found to decrease with increasing temperature, whereas the heat of sorption for multilayer (K) was found to increase with increasing temperature. A hysteresis effect was observed at all three temperatures studied Universal Research Publications. All rights reserved Keywords: Sorption isotherms; Mango; Equilibrium moisture content; Hysteresis; GAB model; Multilayer. 1. Introduction The mango (Mangifera indica L.) is a popular fruit on the world market because of its unique and attractive flavour, colour and nutritional value. In spite of its excellence, the perishable nature of this fruit and its short harvest season severely limit its utilisation. Drying may be an interesting method in order to prevent fresh fruit deterioration. Water activity (a w ) has long been considered as one of the most important quality factors, especially for long-term storage. All chemical and microbial deterioration reactions are directly affected by changing water activity; therefore, the determination of the relationship between water activity and moisture content is significant. Moisture sorption isotherms describe the relationship between the equilibrium moisture content (EMC) and the water activity at constant temperatures and pressures. For food material, these isotherms give information about the sorption mechanism and the interaction of food biopolymers with water. The moisture sorption isotherms are extremely important in modelling the drying process, in the design and optimisation of drying equipment, in predicting shelf-life stability, in calculating moisture change which may occur during storage and in selecting appropriate packaging material (Gal, 1987). Moisture sorption isotherms are either measured during desorption (starting from the wet state) or during adsorption (starting from the dry state). Several researchers have determined water sorption isotherms for various products and temperatures and at selected ranges of water activity (Labuza, 1984; Maroulis et al., 1988; McLaughlin & Magee, 1998; Gabas et al., 1999). Empirical and semi-empirical equations have been proposed to correlate the EMC with the water activity of food products. The GAB (Guggenheim-Anderson-deBoer) equation has been suggested as being the most versatile sorption model available in the literature and has been applied successfully to various dehydrated foods (Kiranoudis et al., 1997). It is based on the BET (Brunauer- Emmett-Teller) theory (Brunauer et al. 1938) and involves three coefficients which have physical significance, two of them being functions of temperature (Maroulis et al., 1988). Moreover, the GAB equation has been recommended by the European project group COST90 on the physical properties of foods (Wolf et al., 1985) as the fundamental equation for the characterisation of water sorption of food materials. Therefore the objectives of this study were: (1) to investigate the moisture sorption isotherms (desorption and adsorption) of mango fruit and (2) to evaluate the applicability of the GAB sorption model for predicting the EMC of mango fruit. 2. Materials and methods 2.1. Raw Material Fresh mangoes, var. Kent, from Mali, were purchased at a 164

2 local supermarket in Goettingen, Germany. The mangoes were left for 5 days for post-harvest ripening at 25±2ºC and 50% relative humidity (Pott et al, 2005). The fruit was then washed, manually peeled using a stainless steel knife, and sliced using an electric food-slicer (Krups variotronic, Germany) to a thickness of 2 mm Sorption isotherms The sorption isotherms (desorption and adsorption) of the mango slices were determined gravimetrically using the standard static gravimetric method standardised in the European COST project (Wolf et al., 1985). The method is based on the use of saturated salt solutions to maintain a fixed relative humidity which corresponds with the tissue s water activity. All the chemicals used were of reagent grade. Seven salts were selected to give different water activities in the range of The water activities of the respective saturated solutions (a w = ) were based on the values given by Greenspan (1977) and Bell & Labuza (2000) (see Table 1). Each saturated salt solution was prepared following the recommendations of Labuza (1984). Each solution was placed into two separate glass jars so that it covered the Table 1. : Water activity exhibited by the seven saturated salt solutions at different temperatures Water activity, decimal Salt solution Temperatures C References Lithium chloride (LiCl) Greenspan (1977) Magnesium chloride (MgCl 2 ) Greenspan (1977) Potassium carbonate (K 2 CO 3 ) Greenspan (1977) Magnesium nitrate (Mg(NO 3 ) 2 ) Greenspan (1977) Sodium nitrite (Na NO 2 ) Bell &Labuza (2000) Sodium chloride (NaCl) Greenspan (1977) Ammonium sulphate (NH 4 ) 2 SO Greenspan (1977) bottom to a depth of 1 cm. To avoid direct contact with the mango slices, a tripod was placed in each jar. Representative samples of the mango slices (2g for adsorption and 4g for desorption) were prepared as described above. For the adsorption isotherms, the sliced samples were dried in a convective air oven at 60 C to constant weight (Telis-Romero et al., 2005). Then duplicate samples were weighed into small brass wire mesh containers, which were placed on the tripods in the jars. The jars were then closed tightly. At water activities (a w ) >0.70, a small glass bottle with crystalline thymol was placed near the samples in each glass jar to prevent mould growth (Quirijns et al., 2005). Each of the two sets of seven jars was kept in a controlled temperature incubator set at 20, 30 or 40 C for equilibration of the samples. Equilibrium was considered to be reached when the moisture content (dry basis) did not change by more than 1 mg/g of sample (0.1%) during three consecutive readings (Lim et al., 1995). The total weighing time was less than 30s to minimise any atmospheric moisture sorption (McLaughlin and Magee, 1998). The time required for the mango slices to reached equilibrium varied from 4-5 weeks depending on their water activity and the temperature. The EMC of the samples was determined after each sorption experiment using a precision air-oven method, at a temperature of 135 C for 2 hours until constant weight was reached, according to the standard method of AOAC (2000). The mean values of two replicates were obtained for each experiment. To establish the moisture sorption isotherms, the EMCs were plotted against water activity at constant temperature Development of the GAB mathematical model for sorption isotherms The EMC data were processed using Microsoft Excel (2007) software and analysed using the polynomial regression method on transformed binomial equations for the GAB parameters Calculation of GAB parameters The GAB equation is given by the following equation (Rizvi, 1986) MoCKa w ( 1 Kaw )(1 Kaw CKaw ) M (1) Where: M = equilibrium moisture content, %, d.b. Mo = GAB monolayer moisture content C = constant related to the monolayer heat of sorption K = factor related to the heat of sorption of the multilayer This equation has a similar form to BET, but has an extra constant K. BET is actually a special case of GAB, where K = 1. The three GAB parameters were determined following the method of the transformed form of the GAB isotherm (Timmermann et al., 2001). The GAB model can be rearranged to a polynomial equation as follows: a K C 2 w 2 1 aw aw (2) M Mo(1/ C 1) MoC MoCK If the three GAB parameters are given the values K C 2 1,, and, Mo( 1/ C 1) MoC MoCk then the GAB equation takes the form: a w a 2 w 165 a w M A polynomial direct nonlinear regression method of a w /M versus a w was carried out using Microsoft Excel (2007) in order to determine the values of the coefficient of the quadratic term α, the linear term coefficient β and the constant ε. Then the GAB parameters were calculated as follows: f 1/ 2 2 (3) K (4)

3 ( 1 2K Mo (5) ) C 2 (6) K Where: f β 2-4α ε The suitability of model fit was determined in terms of mean relative percentage deviation (P %) value (Boquet et al 1978; Lomauro et al, 1985). A model is considered to be suitable if P is < 10% (Lomarou et al 1985; Wang and Brennan, 1991). The mean relative percentage deviation was calculated using the following equation: N 100 M exp, i Mpred, i P(%) (7) N i 1 M exp, i Where: M exp,i = experimental EMC value M pred,i = predicted EMC value N = number of experimental observations 3. Results and Discussion 3.1. Sorption characteristics of mango fruit The desorption and adsorption isotherms obtained from mango slices at 20, 30 and 40 C are given in Figs. 1 and 2. The sorption isotherms of mango fruit follow the characteristic shape of high-sugar foods according to the BET classification [namely, type III (J-shaped)] with the EMC increasing sharply at the high water activities. These findings are in agreement with Pott et al (2005) for the adsorption characteristics of dried Kent variety mango slices at 20 C, Telis Romero et al. (2005) for the desorption and adsorption isotherms of Haden variety mango pulp at different temperatures (30-70 C), and Falade & Aworh (2004) for the adsorption isotherms of osmo-oven dried African mango. Figure1. Desorption isotherms of mango fruit at different temperatures In general, the moisture content is expected to decrease with increasing temperature at a given water activity. This trend occurred in both the desorption and adsorption experiments with mango slices with low and intermediate water activities, and even with mango slices with a high water activity (a w >0.75) for desorption. However, at such high water activity the trend was reversed for adsorption, resulting in a crossing of the moisture isotherms. Such crossing points have also been reported for the isotherms of raisins (Saravacos et al., 1986), figs, prunes and apricots (Maroulis et al., 1988; Tsami et al., 1990). This is due to the dissolution of sugars or an increased solubility of sugars and thus more water being held at higher temperatures as result of an increase in the number of adsorption sites upon the breakage of the crystalline structures of sugar (Ayranci et al., 1990). Telis-Romero et al. (2005) observed a tendency for this crossing-over at even higher water activities in Haden variety mango, and Falade and Aworh (2004) reported a crossing of the isotherms at water activities above 0.8 in their study on osmo-oven dried African mango. Figure 2. Adsorption isotherms of mango fruit at different temperatures The intersection point of isotherms depends on the type of sugar present, sugar size distribution and composition of the food (Weisser, 1985). Since this process is known to be endothermic, more sugar is dissolved and thus more water is held by food products at higher temperatures. The isotherm crossing behaviour was not observed for fresh fruit (desorption Fig.1) since the sugars are already in a dissolved form throughout the whole water activity range in fresh fruit cells. However, Gabas et al. (1999) found an isotherm-crossing behaviour for grapes even during desorption and at water activities as low as The characteristic shape of moisture isotherms depends upon the variety and total amount of hygroscopic materials present in the particular heterogeneous mixture of hydrophilic substances (Falade et al., 2003). At low water activities, water can be adsorbed only onto the surface-oh sites of crystalline sugars. Wiesser (1985) reported that at low water activities there was a local dissolution of sugar, a swelling of the biopolymers and the appearance of new active sites. At high water activities, dissolution of sugar occurs and crystalline sugar is converted into amorphous sugar. The amount of water to be absorbed increases greatly after this transition because of the increase in the number of adsorption sites arising upon the breakage of the sugars crystalline structure (Ayranci et al., 1990). Figures 3-5 show the sorption isotherms obtained for mango at 20, 30 and 40 C, respectively. These figures clearly show that the phenomena of hysteresis (differences in water activity versus EMC between desorption and adsorption) occur at all temperatures, with hysteresis becoming more evident as temperature decreases. These 166

4 Table.2.: GAB model parameters and mean relative percentage deviations obtained for the sorption isotherms of mango fruit Temperature ( C) Desorption Adsorption C K Mo P% C K Mo P% C and K = constants in the sorption isotherms of the GAB model, Mo = monolayer moisture content (% d.b.); and P% = mean relative percentage deviations results are in contrast with MacLaughlin and Magee (1998) for potato. Hysteresis is not fully understood, although there is general agreement that some thermodynamically irreversible processes must occur during desorption or adsorption or both. One theory used to explain hysteresis suggests that in the wet condition the polar site onto which water is sorbed is not entirely satisfied. Upon drying, the water-holding sites are drawn close enough together with shrinkage to satisfy each other. This results in a reduction of the water-binding capacity during adsorption (Moshsenin, 1986). Several theories have been formulated to explain the phenomena of hysteresis. The principle factors affecting hysteresis are the composition of the product, its temperature, storage time, drying temperature and the number of successive adsorption and desorption cycles. (P %) for both desorption and adsorption over the temperature range used are presented on Table 2. Figure 5. Desorption and adsorption characteristics showing a hysteresis effect at 40 C Figure. 3. Desorption and adsorption characteristics showing a hysteresis effect at 20 C Figure 6. EMCs predicted by the GAB model compared with the experimental EMCs for adsorption data at 20ºC Figure 4. Desorption and adsorption characteristics showing a hysteresis effect at 30 C 3.2. Fitting of the GAB model to the sorption isotherms data of mango fruit The three GAB parameters obtained from each set of sorption data and the mean relative percentage deviations Figure 7. EMC predicted by the GAB model compared with the experimental EMC for desorption data at 20 ºC 167

5 The monolayer moisture content (Mo) is an important parameter in food deterioration studies. The Mo values decreased with increasing temperature; these results are in agreement with Kumar and Mishra (2006), Labuza (1968), Hossain et al. (2001) and Rahman and Labuza (1999). The decrease in Mo values may be due to a reduction in the total number of active sites for water binding as a result of physical and / or chemical changes in the mango flesh induced by temperature (Iglesias and Chirife, 1976; Mazza and Le Maguer, 1978). The Mo values are also in good agreement with those stated by Van den Berg and Bruin (1984), which indicated that the monolayer moisture content in foods was lower than 10g/100g dry matter. The values for the constant K in the GAB model varied between and (slightly greater than 1). The K values increased with increasing temperature. These results are in agreement with Kumar and Mishra (2006). The GAB model fitted well over the whole range of water activity, giving a low P (%) which varied between 0.03 and This result is <10% and so it is an indication of good fit according to both Wang & Brennan (1991) and Lomauro et al (1985). Figure 6 shows the experimental EMCs compared to the predicted EMCs provided by the GAB model for the desorption isotherms. In this graph, the predicted EMCs and experimental EMCs are plotted versus water activity at 20ºC. It can be seen that the GAB model gives a very close estimate of the experimental EMC. As such good fits were also found for 30 and 40ºC, the GAB model can be recommended for characterising the moisture desorption characteristics of mango fruit in the temperature range of C and water activity range of to These results are in agreement with those of a number of authors who have used GAB for successfully describing moisture sorption isotherms and EMC data in various plant tissues [Haden variety mango (Telis-Romero et al 2005); Ataulfo mango pulp freeze-dried in the temperature range C with water activity values from 0.11 to 0.89 (Rangel Marrón et al. 2011); guar grain and guar gum splits (Vishwakarma et al. 2011); and the Nam Dak Mai variety of mango for the temperature range C and water activity from (Janjai et al 2007)]. Figure 7 shows a comparison of the experimental EMCs and the predicted EMCs provided by the GAB model for the adsorption isotherms of mango slices. In this graph, both the predicted EMCs and experimental EMCs are plotted against water activity at 20 ºC. It can be seen that the GAB model gives a very close estimate of the experimental EMC. Again, as such good fits were also found for 30 and 40 ºC, the GAB model can also be recommended for characterising the moisture adsorption characteristics of mango fruit in the temperature range of C and a water activity range of to These results are in agreement with Spiess and Wolf (1987), who applied the GAB model to experimental adsorption isotherms at 25 C, Maroulis et al. (1988) who used the GAB model on the moisture sorption isotherms for various dried fruits and Telis-Romero et al. (2005) who used the GAB model for the description of experimental EMC for mango pulp. 4-Conclusions The moisture desorption-adsorption isotherms of mango fruit (cv. Kent) at three temperatures (20, 30 and 40 ºC) and in the water activity (a w ) range ( ) were determined with standard gravimetric methods using various saturated salt solutions. The desorption-adsorption isotherms of mango fruit have a shape typical of high-sugar foods according to the BET classification; i.e. Type III (J-shaped). The equilibrium moisture content values increased with decreasing temperature at constant water activity. A reverse temperature effect occurred only for the adsorption isotherms at a w >0.75 due to the fruit s high sugar content, resulting in a crossing over of the sorption curves at all three temperatures. A hysteresis effect was observed at all three temperatures studied. The GAB model was satisfactorily used to predict the moisture sorption isotherms of mango fruit over ºC and for water activities between Acknowledgements The first author is thankful to the DAAD (German Academic Exchange service) for the 6-month scholarship grant (A/05/19861) which enabled him to conduct this experiment at the Institute of Agricultural Engineering, University of Goettingen, Germany. References 1. AOAC. (2000). Official method of analysis. 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