Hot air and sun drying of grape leather (pestil)

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1 Journal of Food Engineering 54 (2002) Hot air and sun drying of grape leather (pestil) Aysun Maskan, Sevim Kaya *, Medeni Maskan Department of Food Engineering, Engineering Faculty, University of Gaziantep, Gaziantep, Turkey Received 28May 2001; accepted 19 September 2001 Abstract Pestil, a well known fruit leather in Turkey, was prepared from boiled grape juice and starch mixture by using traditional technique. Drying of pestil was carried out by hot air drying and sun drying. The factors investigated in hot air drying were air temperature (55, 65 and 75 C), sample thickness (S 1 ¼ 0:71, S 2 ¼ 1:53, S 3 ¼ 2:20 and S 4 ¼ 2:86 mm) and air velocity (V 1 ¼ 0:86, V 2 ¼ 1:27 and V 3 ¼ 1:8 2m=s). The effects of drying time, temperature and slab thickness on moisture content of pestil during drying were significant (P < 0:05) and of air velocity was not (P > 0:05). Depending on sample thickness and air temperature, the drying time ranged between min to achieve the commercial moisture content of pestil (0:12 kg H 2 O=kg DS) in air drying. Whereas, sun drying took min. Almost all samples dried in the falling rate period, except S 2, S 3, S 4 of sun drying and S 4 at 55 C. The latter had a short (negligible) constant rate period. Effective moisture diffusivity values were estimated from Fick s diffusion model. These values were between 3:00 37: m 2 =s for hot air drying and 1:93 9: m 2 =s for sun drying. Activation energy value of water diffusion was calculated using an Arrhenius-type equation. The estimated values were 21.7, 16.5, 12.0 and 10.3 kj/mol for S 1, S 2, S 3 and S 4, respectively. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: Grape; Leather; Pestil; Drying 1. Introduction Food drying is one of the oldest methods of preserving food for later use. It is a complex operation involving heat and mass transfer which may cause changes in product quality. Physical changes that may occur include shrinkage, puffing and crystallisation. In some cases, desirable or undesirable chemical or biochemical reactions may occur leading to changes in colour, texture, odour or other properties of the food product. Drying can either be an alternative to canning and freezing or complement these methods. Drying occurs by vaporisation of the liquid by supplying heat to the wet feedstock. Heat may be supplied by conduction (contact or indirect dryers), by convection (direct dryers), by radiation or volumetrically by placing the wet material in a microwave or radio frequency electromagnetic field. Over 85% of industrial dryers are of convective type with hot air or direct combustion gases as the drying medium (Chirife, 1983; Mujumdar, 2000). * Corresponding author. Tel.: ; fax: addresses: skaya@gul1.bim.gantep.edu.tr, skaya@gantep.edu.tr (S. Kaya). The drying of fruit on a commercial scale has received a resurgence of interest during the past years few especially in the production of fruit leathers. Leathers are made by removing moisture from a large flat tray of wet puree until the desired cohesive leathery composition is obtained (Moyls, 1981). In many processes, improper drying may lead to irreversible damage to product quality and hence a non-saleable product. With modern dehydrators, whole fruits, fruit leathers, fruit chips and pieces can all be dried at any time of the year. Dried fruits and fruit products taste sweeter because the water has been removed thus concentrating the fruit s flavour and calories. They can be eaten as a snack food or added to various food preparations (Karathanos & Belessiotis, 1997; Mujumdar, 2000). Because of the short harvest season and the sensitivity to storage even at refrigerated conditions, most fresh grapes and grape derivatives should be preserved in some form. Drying is among the commonly used preservation methods. Sun dried fruits and fruit products are the best known of all dried foods. In Turkey, raisins, apricots, figs and fruit leathers (known as pestil) are the most popular sun dried foods. Sun drying permits one to produce a product with a rich colour, a translucent appearance and a desirable gummy texture; however, it /02/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII: S (01)

2 82 A. Maskan et al. / Journal of Food Engineering 54 (2002) Nomenclature ANOVA analysis of variance D 0 constant in Arrhenius equation (m 2 =s) D eff effective moisture diffusivity (m 2 =s) DS dry solids E a activation energy (kj/mol) k rate constant (min 1 ) in Eq. (1) L film/slab thickness (m) LSD least-squares difference MR moisture ratio (dimensionless) r correlation coefficient (dimensionless) R g gas constant (8.314 kj/mol K) R drying rate (kg water/min) S 1 sample with a thickness of 0.71 mm S 2 sample with a thickness of 1.53 mm S 3 sample with a thickness of 2.20 mm S 4 sample with a thickness of 2.86 mm T absolute temperature (K) t time (min) V air velocity (m/s) X moisture content at any time (kg water/kg DS) X 0 initial moisture content (kg water/kg DS) X e equilibrium moisture content (kg water/ kg DS) has many disadvantages. Notable among these are the slowness of the process, the exposure to environmental contamination, the dependency on weather and the hand labour requirement. Due to these difficulties of sun drying, a rapid, safe and controllable drying operation is required. Hot air drying is an alternative drying method. This drying method both decreases drying time and improves the quality of dried product (Abdelhaq & Labuza, 1987; Kostaropoulos & Saravacos, 1995; Mahmutoglu, Emir, & Saygı, 1996). One of the important agricultural products of Turkey is grape. All of the grapes produced cannot be consumed, immediately, at harvesting time as fresh. Therefore, they must be preserved in some form for further use, i.e., their processing into other grape derivatives is necessary. Grape juice has been used for many years in Anatolia for production of local products such as pekmez (a concentrated grape juice product), pestil, sucuk (coating tree nuts with pestil) and some other typical products for improving utilisation of excess grape produced. Pestil is almost the same as fruit leathers except it includes starch. Drying of fruit juice concentrate starch mixture is the most important step in the pestil production. Nevertheless, drying characteristics of pestil has not been studied for solar and hot air. Therefore, the aim of this study was the determination of drying characteristics of pestil by air drying and sun drying, and the effect of process parameters such as drying temperature, air velocity and sample thickness. 2. Materials and methods 2.1. Materials For pestil production, grapes with an initial total soluble solids of 20 g/100 g, were obtained from a farm in Gaziantep (Turkey) in September 1999 and stored in a refrigerator at 4 0:5 C for 48h. Prior to pestil preparation, grapes were taken out of storage for juice extraction, clarification and concentration. All grapes used for pestil production were from the same batch. A natural white earth was used for fruit juice clarification. It was obtained from grape producers in Gaziantep Preparation of pestil Flow sheet for pestil production is shown in Fig. 1. The first step was washing of grapes to remove dirt, leaves, and foreign materials. Then, the grapes were crushed and pressed manually. The seeds were separated from the juice by filtration using a cheese cloth. The juice obtained had a ph of 4.4 at 19 C. In order to reduce acidity and clarify the juice 7 g natural earth (70% CaCO 3 ) was added per litre of juice. The mixture obtained was boiled for 3 5 min in order to inactivate enzymes which cause colour change (Eksßi & Artık, 1984; Nas & Nas, 1987). The foam formed on the surface of the juice during boiling was removed. Then, it was waited until tartaric acid precipitated as calcium tartarate (Kokini, Lai, & Chedid, 1992). The juice was separated from the precipitate by filtration using the cheese cloth and centrifugation. This final clarified juice had a ph of 7.6 and a concentration of 20 Brix. The total juice was divided into two parts. A 3/4 part of juice was boiled for 30 min at constant stirring to obtain concentrated juice with 40 Brix. Then, freshly prepared wheat starch juice mixture (starch dissolved in 1/4 part of juice left) was added to the boiling juice. It was boiled for 4 min more. The concentration of starch was 4 g/100 g of the total amount of juice Sample preparation and thickness measurements Cooked grape juice starch mixture was evenly spread on 8cm diameter disc of a cloth. The sample thickness

3 A. Maskan et al. / Journal of Food Engineering 54 (2002) The hot air drying experiments were performed in a pilot plant tray dryer (UOP 8tray dryer, Armfield, UK). Overall dimensions of the equipment are as follows: height: 1.40 m, width: 2.95 m and depth: 0.73 m. The dryer consisted of a temperature controller. Air was blown into the dryer through a mesh guard by a motor driven axial flow fan impeller whose speed can be controlled in the duct. The air flow was parallel to the drying surface of the sample. Dry bulb and wet bulb temperatures of drying air were measured at 30 min intervals during drying using an aspirated psychrometer which is powered from a socket outlet on the control panel mounted on the duct. Access points for the psychrometer are provided both upstream and downstream of the drying trays. The details of this dryer have been published elsewhere (Maskan, 2000). was measured with a dial micrometer. At least six measurements of the thickness were made at different points. Only 5% deviation of the average thickness was allowed for the samples Drying equipment Fig. 1. Flow sheet of pestil production Drying procedure The samples were dried from one side (top surface). The initial moisture content of the samples was 64.09% (wet basis). The drying regimes were as follows: (1) Hot air drying. In this study, various factors were investigated. These factors were temperature (55, 65 and 75 C dry and 27, 30 and 33 C wet bulb temperatures, respectively), air velocity (V 1 :0:86 0:03, V 2 :1:27 0:04 and V 3 : 1:82 0:09 m=s) and sample thickness (S 1 : 0:71 0:035, S 2 :1:53 0:070, S 3 :2:20 0:110 and S 4 : 2:86 0:071 mm). Moisture loss was recorded at 10 min intervals during drying for determination of drying curves by a digital balance (Avery Berkel, CC062D10- ABAAGA). The concentrated grape juice starch mixture samples were dried until equilibrium (no weight change) was reached. (2) Sun drying. The samples were dried under direct sunlight and during night in September Moisture loss was recorded at 1 h intervals up to sixth hour of drying, no measurement was made during night, and 3 h intervals at the final stage of drying. All the rest was the same as hot air drying. The samples were exposed to sunlight for 14 h. Intensity of sunlight was J=cm 2 min; the percent relative humidity of the air 43:5 11:4; temperature at day 21:4 9:2 and at night 14:9 5:3 C; air velocity at day 0:530:34 and at night m/s. These values are the average values, where indicates standard deviation from the average. They were obtained from Meteorology Unit established in Gaziantep Statistical analysis Analysis of variance (ANOVA) was conducted to determine the effect of variable factors on drying parameters using Statgraphics software (Statgraphics, 1991). LSD multiple range test method was used when difference in means detected. The parameters of Fick s model (Eq. (1)), Arrhenius equation (Eq. (2)) were estimated by the linear regression procedure of SigmaPlot (Scientific Graph System, version 4.00, Jandel). The experiments both for hot air and sun drying were repeated twice. 3. Results and discussion In this study, neglecting shrinkage, the effect of drying method (hot air/sun) on drying rate of pestils was evaluated. It was found that visual appearance (attractive colour) and texture (elasticity) of sun dried pestil samples were better than air dried samples.

4 84 A. Maskan et al. / Journal of Food Engineering 54 (2002) Hot air drying of pestil samples Fig. 3. Effect of air temperature on drying of pestil slab (S 3 ¼ 2:20 mm, V ¼ 1:27 m/s). The effects of drying time, air temperature, sample thickness and air velocity on moisture content of pestil during drying were analysed using analysis of variance (ANOVA). Three-way ANOVA clearly showed that drying time, temperature and pestil thickness were significant (P < 0:05) factors in drying, affecting moisture content of pestil samples. The influence of air velocity through the product were statistically insignificant (P > 0:05) on moisture content. The change in moisture content vs drying time, for various air velocities at 75 C is shown in Fig. 2 for S 3 as an example. This is similar to that observed in potato drying by Yusheng and Poulsen (1988), on sugar beet root by Vaccarezza, Lombardi, and Chirife (1974) and for thin layer drying of garlic slices by Madamba, Driscoll, and Buckle (1996). It was found that the trend was the same for the other sample thicknesses (S 1, S 2 and S 4 ) and temperatures (65 and 55 C). However, the numerical data were different. The main study was limited to only an intermediate velocity V 2 (1.27 m/s) at all the temperatures and thicknesses defined for the sake of simplicity. In general, the time required to reduce the moisture content to desired level was dependent on drying conditions and sample thickness, being the highest in sun drying and the thickest sample, and lowest with 75 C and the thinnest sample. When the experimental data of moisture content vs drying time were plotted, concave downward curves were obtained. These are typical of the drying curves obtained during drying. Fig. 3 shows the effectiveness of increasing the drying air temperature in accelerating the dehydration of 2.20 mm thick sample. It can be seen that the increase in temperature, at constant sample thickness, reduced the time needed to reach equilibrium moisture content. This is, according to kinetic theory, due to the increased energy of water molecules as temperature is increased. Hence, escaping of molecules becomes easier from the medium and faster. It has been reported by Prabhanjan, Ramaswamy, and Raghavan (1995) that the higher temperatures provide a larger water vapour pressure deficit (wvpd, the difference between the saturated water vapour pressure and partial pressure of water vapour in air at a given temperature), which is one of the driving forces for the outward moisture diffusion process (drying). Similar behaviours were observed by Vergara, Amezaga, Barcenas, and Welti (1997) for osmotically dehydrated apples, Moyls (1981) for apple purees drying and Salgado, Lebert, Garcia, and Bimbenet (1994) for sugar beet root and sugar beet pulp, Maskan and G og usß (1998) for mulberry, respectively. Drying times in order to reach moisture content of about 0.12 kg H 2 O=kg DS (11% wet basis) are given in Table 1. This moisture content was selected since it is the final moisture content of commercial grape pestils (Eksßi & Artık, 1984; Nas & Nas, 1987). It has also been speculated (Karel et al., 1994) that moisture contents at or below 15% (wet basis) for most fruits (especially for grape and its products) is a rather safe indication that there is no microbial or mould growth and the reaction rate of a number of other deteriorative reactions (sugar crystallisation, non-enzymatic browning, flavour deterioration, lipid oxidation, etc.) is significantly reduced. Fig. 4 shows the influence of sample thickness on drying at 75 C. The effects of sample thickness were Table 1 Drying time (min) for pestil samples to reach the moisture content of 0:12 kg H 2 O=kg DS Fig. 2. Drying curves at different air velocities for pestil slab (2.20 mm) at 75 C. T ( C) S 1 S 2 S 3 S

5 A. Maskan et al. / Journal of Food Engineering 54 (2002) results are quite higher than those of hot air drying (Table 1) and lower than those of sun dried sultana raisins which required 740 h of drying (Karathanos & Belessiotis, 1997). This abnormality is due to the difference in shape/size and nature of the two products and the skin on the raisins. Because the outer skin results in an overall larger resistance to the water vapour movement. The comparison of drying time for sun drying of pestil and raisins is important in order to present the difference in drying behaviour of grape and its derivative such as pestil Analysis of drying rates Fig. 4. Effect of sample thickness on air drying of pestil slabs at 75 C. similar to studies on air drying of banana slices at 60 C (Maskan, 2000), drying of potato slices (Yusheng & Poulsen, 1988) and drying of apple slices (Roman, Rotstein, & Urbicain, 1979) with all authors concluding that thinly sliced products dried faster due to the reduced distance the moisture travels. The analysis of variance showed that the effect of drying time and sample thickness on moisture content was more pronounced than that of the air temperature due to significantly higher F-values (258, 82 and 40, respectively for time, thickness and temperature) Sun drying of pestil samples The moisture content vs time curves for sun drying of pestil samples are given in Fig. 5. The thin sample (S 1 ) dried appreciably faster than the others. The time to reach the moisture content of 11% (wet basis) was 3, 5, 15 and 25 h for S 1, S 2, S 3 and S 4, respectively. These The drying rate was determined from the slopes of the moisture content vs drying time curves, at each measurement point. The variation of the drying rates against moisture content are shown in Fig. 6 for hot air drying at 75 C and Fig. 7 for sun drying. Statistically, a significant difference (P < 0:05) was found between drying rates of hot air and sun drying processes. A comparison of the drying rates for the tested conditions were as follows; according to (a) drying method: air drying > sun drying (b) sample thickness: S 1 > S 2 > S 3 > S 4 (c) temperature: 75 C > 65 C > 55 C From the results of previous works on drying of apple puree (Moyls, 1981), apple slabs (Roman et al., 1979) and banana slabs (Mowlah, Takano, Kamoi, & Obara, 1983) both constant and falling rate drying periods have been observed. In this study, at 75 and 65 C all the samples, at 55 C S 1, S 2, S 3 and in sun drying only S 1 dried in the falling rate drying period. However, a short constant drying rate period was observed in sun dried samples of S 2, S 3, S 4 (30, 40, 50 min, respectively) and Fig. 5. Typical drying curve for sun drying of pestil slabs at different thicknesses. Fig. 6. Drying rate curves for pestil slabs at air temperature of 75 C (V ¼ 1:27 m/s).

6 86 A. Maskan et al. / Journal of Food Engineering 54 (2002) Fig. 7. Drying rate curves for sun dried pestil slabs. sample S 4 (30 min) of 55 C prior to falling rate period. It is due to, relatively, poor drying conditions. This constant rate period was neglected in our diffusivity calculations and it was assumed that the entire drying process for pestil occurred in the falling rate period for all of the test conditions, as it has been commonly found in biological products by various authors (G og usß & Maskan, 1999, 2001; Maskan, 2000, 2001; Maskan & G og usß, 1998; Maskan & Ibanoglu, 1998; Yusheng & Poulsen, 1988). Non-existence of a constant rate period either at high temperatures (65 and 75 C) and the thin samples may be explained by the fact that at high temperatures the surface of products dries out very quickly (especially of the thin samples) and a partial barrier is generated to resist moisture movement freely. This might have caused pestil to dry in the falling rate period. Also, non-existence of a strong constant rate period, as existed in apple puree (Moyls, 1981), apple slabs (Roman et al., 1979) and banana slabs (Mowlah et al., 1983), may be due to addition of starch to grape juice for pestil preparation. This may add additional hydrophilic interaction to the system. In the water starch system, each water molecule is hydrogen bonded to two hydroxyl groups either on adjacent starch chains or on the same coiled chain. In order to form this structure, the starch must be arranged in a precise manner to bind the water. The extent of water sorption depends on the availability of these sites where starch water starch hydrogen bonds can form (Xiong, Narsimhan, & Okos, 1991). These sites, probably, form during cooking of grape juice and starch mixture, hence, makes water solid interaction difficult to break. Therefore, there is no available water at the pestil surface at any time, the surface is not covered by wet regions and the evaporation of water does not take place entirely at the surface for occurrence of a constant drying rate period. Since, at that point there are no free water regions the evaporation front may be moved into the interior of the pestil. The mass transfer is then by molecular (liquid) diffusion or vapour diffusion or by capillary forces in the interior (wet) region of the pestil and the water is evaporated as it reaches the surface (negligible resistance to mass transfer at the surface). And also the starch molecules form a close-knit matrix resulting in the encapsulation of water molecules in small cavities. The diffusion process of water through the dry outside layers is activated by high temperatures (Karathanos & Belessiotis, 1997; Vagenas & Karathanos, 1993). As mentioned previously, the air velocity had no effect on the drying curves in all the cases studied. It means that the resistance to moisture movement at the surface of sample (external resistance) is negligible. From the results shown in Figs. 6 and 7 it can be seen that the internal moisture movement (internal resistance) is the main resistance to the rate of loss of moisture for which the drying rate is not affected by air velocity. Internal resistance is evident from presence of falling rate period in any drying process as reported by several authors (Geankoplis, 1993; Vaccarezza et al., 1974) Calculation of the effective moisture diffusivity and activation energy The results obtained have shown that the drying process is entirely controlled by internal mass transfer resistance (falling rate drying period), so that the experimental results can be interpreted by using Fick s diffusion model. Assuming uniform initial moisture distribution and negligible external resistance, the solution of Fick s diffusion equation developed for particles with slab geometry by Crank (1975) is applicable and is of the form of Eq. (1) MR ¼ ðx X eþ ðx 0 X e Þ ¼ 8 p 2 exp p2 D eff t L 2 : ð1þ The effective moisture diffusivity was calculated using the method of slopes. When logarithm of MR values vs drying time were plotted in accordance with Eq. (1), straight lines were obtained at all temperatures and sample thickness investigated. Linear regression analysis was employed to obtain values of diffusion coefficients for different drying conditions from the slope of the straight lines obtained. Values of D eff for different conditions together with correlation coefficient of estimation are presented in Table 2. It can be seen that (Table 2) the drying air temperature has a pronounced influence on the drying rate and as a consequence, markedly affects the value of the diffusion coefficient. As was expected, the drying rate increased greatly with increasing temperature. Drying at 75 C gave the highest D eff values. The lowest values were obtained with sun drying. The values for hot air drying ranged from 3.00 to 37: m 2 =s and for sun drying 1:93 9: m 2 =s depending on tem-

7 A. Maskan et al. / Journal of Food Engineering 54 (2002) Table 2 Effective diffusivity values (m 2 =s) for drying of pestil slabs ðd eff Þ Slab thickness (mm) Hot air drying Temperature ( C) Sun drying 75 r 65 r 55 r D eff r S 1 ¼ 0: S 2 ¼ 1: S 3 ¼ 2: S 4 ¼ 2: perature and sample thickness. These values are within the general range m 2 =s for drying of food materials and comparable with the reported values of m 2 =s for air drying of apricots (Abdelhaq & Labuza, 1987), sun drying of differently treated grapes : m 2 =s (Mahmutoglu et al., 1996) hot air drying of okra 4: : (G og usß & Maskan, 1999), hot air drying of mulberry 2: : (Maskan & G og usß, 1998) and hot air drying of banana slices at 60 C 8: m 2 =s (Mowlah et al., 1983). Activation energy for diffusion was estimated using an Arrhenius-type equation D eff ¼ D 0 exp E a R g T : ð2þ The activation energy was calculated by plotting ln(d eff ) vs the reciprocal of the absolute temperature ð1=t Þ as presented in Fig. 8. Then, the activation energy values for water diffusion (calculated from the slopes of straight lines of Fig. 8) were found to be 21.7 ðr ¼ 0:912Þ, 16.5 ðr ¼ 0:992Þ, 12.0 ðr ¼ 0:943Þ and 10.3 ðr ¼ 0:979Þ kj/ mol for S 1, S 2, S 3 and S 4, respectively. It is obvious that activation energy values decreased with increasing sample thickness. These values are in the range or close to the E a values reported (15 40 kj/mol) by Rizvi (1986) Fig. 8. Effect of temperature on the diffusion values of water in pestil slabs. for various foods. The value of E a shows the sensitivity of the diffusivity against temperature. The greater the value of E a, the more sensitivity of the diffusivity to the temperature. The greater value for S 1 shows the sensitivity of the thin sample against temperature, i.e., a small variation in temperature during drying results in significant changes in diffusivity value compared to the thicker samples. 4. Conclusions The study of production of pestil from boiled grape juice starch mixture and drying of pestil revealed the following conclusions; drying time, sample thickness and air temperature influenced moisture content of pestil during drying. Air velocity had no effect on drying of samples. Almost all samples dried in the falling rate drying period. Sun drying took longer time than hot air drying and is therefore a risk for microbial growth. Fick s model of water diffusion fitted all experimental data with acceptable correlation coefficients. Diffusivity values followed an Arrhenius-type temperature dependence. References Abdelhaq, E-H., & Labuza, T. P. (1987). Air drying characteristics of apricots. Journal of Food Science, 52(2), (see also p. 360). Chirife, J. (1983). Fundamentals of the drying mechanism during air dehydration of foods. In A. S. Mujumdar (Ed.), Advances in drying (vol. 2) (pp ). Washington, DC: Hemisphere. Crank, J. (1975). Mathematics of diffusion. Oxford: Clarendon Press. Eksßi, A., & Artık, N. (1984). Pestil nasıl yapılır. Bilim ve Teknik, 17, Geankoplis, C. J. (1993). Transport processes and unit operations (3rd ed., pp ). Englewood Cliffs, NJ: Prentice-Hall. G og usß, F., & Maskan, M. (1999). Water adsorption and drying characteristics of okra (Hibiscus Esculentus L.). Drying Technology, 17(4&5), G og usß, F., & Maskan, M. (2001). Drying of olive pomace by a combined microwave-fan assisted convection oven. Nahrung, 45(2), Karathanos, V. T., & Belessiotis, V. G. (1997). Sun and artificial air drying kinetics of some agricultural products. Journal of Food Engineering, 31, Karel, N., Anglea, S., Buera, M., Karmas, R., Levi, G., & Roos, Y. (1994). Stability-related transitions of amorphous foods. Thermochimica Acta, 246,

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