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Convective drying characteristics of azarole red (Crataegus monogyna Jacq.) and yellow (Crataegus aronia Bosc.) fruits
Journal of Food Engineering 78 (2007) 1471–1475 www.elsevier.com/locate/jfoodeng
Research note
Convective drying characteristics of azarole red (Crataegus monogyna Jacq.) and yellow (Crataegus aronia Bosc.) fruits Turhan Koyuncu *, Yunus Pinar, Fuat Lule University of Ondokuz Mayıs, Faculty of Agriculture, Department of Agricultural Machinery, 55139 Samsun, Turkey Received 6 May 2005; received in revised form 9 August 2005; accepted 30 September 2005 Available online 20 February 2006
Abstract In this research, drying characteristics and energy requirement for drying of two different genotypes (Crataegus monogyna Jacq. and Crataegus aronia Bosc.) of azarole (Crataegus azarolus L.) red and yellow fruits were reported. Azarole fruits were dehydrated in a computer connected convective hot air dryer. Freshly harvested two different genotypes of azarole fruits were dried at 60 and 70 °C temperatures and drying air velocity was selected as 0.25 m/s for both temperatures. Azarole fruits were dehydrated from the initial moisture content of 211% and 273% (percentage dry basis) to a final moisture content of 8–9% for red and yellow fruits, respectively. During experiments, drying product were weighted automatically by the balance per 10 min. Data were transferred to the computer and processed by a software. The results indicated that drying air temperature significantly influenced the total drying time and total energy requirement for drying of both genotype azarole fruits. The minimum specific energy consumption for drying of red and yellow fruits were determined as 42.80 kWh/kg and 27.68 kWh/kg for 70 °C, respectively. In order to reduce drying energy consumption, it can be recommended that the drying temperature must not be less than 70 °C for this application. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Azarole fruits; Drying kinetics; Energy requirement for drying of azarole fruits; Convective hot air dryer; Drying temperatures; Drying air velocity
1. Introduction The drying or dehydration is the oldest method in food conservation, and its object is to remove by evaporation most of the water present in the product. The reduction of moisture content inhibits or decrease microbial and enzymatic activity, which otherwise would produce food damage. Besides, dehydration makes food product handling easier owing to the volumetric shrinkage and weight losses products undergo during process (Ochoa, Kesseler, Pirone, Marquez, & De Michelis, 2002). Natural open-air sun drying is practiced widely in hot climates and tropical countries. Considerable savings can be obtained with this type of drying, since the source of energy is free and renew-
*
Corresponding author. Tel.: +90 362 3121919; fax: +90 362 4576034. E-mail address: [email protected] (T. Koyuncu).
0260-8774/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2005.09.036
able. However, this technique is extremely weather, dependent and has the problems of contamination, infestation, microbial attack, etc. In addition, the required drying time for a given load is approximately 2–4 times longer than greenhouse, cabinet and convective hot air type dryers (Koyuncu & Pınar, 2001; Koyuncu & Sessiz, 2002; Tog˘rul & Dursun, 2003). Azarole (Crataegus azarolus L.) tree is a deciduous tree growing up to 3–4 m high and cultivated for centuries in the Mediterranean area. It is in flower in April and May. The plant can grow in light, medium and heavy soils. It requires moist or wet soil and can tolerate drought. Its fruits are very variable in size and colour, it is up to 25 mm in diameter. There are 1–3 large seeds in the centre of the fruit. Fruit weight 2.16–7.58 g, flesh/pit ratio 2.55– 6.86, pit weight 0.77–1.16 g. The fruits contain 15.9 g total sugar, 1.38 mg total acidity, 27.58 mg vitamin C, 11.0 mg Ca, 9.80 mg P, 1.16 mg Fe, 158 mg K, 7.10 mg Mg,
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T. Koyuncu et al. / Journal of Food Engineering 78 (2007) 1471–1475
Nomenclature A c Dt Ekg Et Fd Fl
drying air flow surface area, m2 specific heat of air under adiabatic conditions, kJ/(kg K) total drying time, h energy requirement for drying 1 kg of product, kWh/kg total energy requirement for a charge of the dryer, kWh geometric mean diameter, mm length of fruit, mm
Fp Fs Fw PMdb v Wd W0 DT q
sphericity surface area, mm2 width of fruit, mm the moisture content on dry basis expressed as percentage, % drying air speed, m/s weight of dry matter in product, kg initial weight of undried product, kg temperature differences, K air density, kg/m3
0.16 mg Cu, 0.24 mg Mn, 2 mg Na, 0.95 g protein and 13.12 g carbohydrate for 100 g fruits. Azarole fruits are extensively used especially in rural areas of Turkey for different aims. However, for the present, it is not easy to see enough information regarding the production and consumption in the literature even if many studies made about azarole fruits. The fruits are not only consumed fresh and dried but also used to produce jam, marmalade and syrup in Turkey. Fruits and flowers are also used for medicinal purposes (Asma & Birhanlı, 2003; Bignami, Paolocci, Scossa, & Bertazza, 2003; Karadeniz, 2004; Yesilada et al., 1997). In order to store azarole fruits, it is possible to use different methods such as traditional method, cold storage and drying depending on the technical opportunities, food consumption and food processing ways. In different several literatures, it is possible to see a little part of information about nutritional and physical properties, ingredients and characteristics of azarole fruits (Asma & Birhanlı, 2003; Bignami et al., 2003; Karadeniz, 2004). However, no report concerning the drying kinetics, drying characteristics and heat energy requirement of azarole fruits during our literature survey. Therefore, two different genotypes of azarole fruits (Crataegus azarolus L.) such as azarole red fruits (Crataegus monogyna Jacq.) and azarole yellow fruits (Crataegus aronia Bosc.) were dehydrated in a computer connected convective hot air dryer at various temperatures and selected most suitable velocity to determine the drying characteristics and energy requirement for drying in this experimental investigation.
and a 0.01 g sensitive balance. The geometric mean diameter (Fd), sphericity (Fp) and surface area (Fs) were calculated from Eqs. (1) to (3) (Demir & Kalyoncu, 2003). Freshly harvested azarole fruits that physical properties given in Table 1 were dried in a computer connected convective hot air dryer. The dryer equipped with an electric heater (air heating duct), temperature adjuster, centrifugal fan (blower), air speed adjuster (regulator of variable transformer), corrosion resistant chromium mesh, corrosion resistant cromium sheet, glass wood insulator, a 0.01 g sensitive Precisa BJ 600 D digital balance, RS232 connection, a PC, specially designed Balint data processing software, drying air inlet and outlet channels as well as thermostat, temperature indicators
2. Materials and methods
F d ¼ ðF l F 2w Þ1=3
Ripe azarole red and yellow fruits grown in Malatya Region of Turkey were harvested manually and used for the investigation. The fruits were cleaned in an air screen to remove all foreign material such as dust, dirt, pieces of branches and leaves. To establish the physical properties of the fresh fruits, approximately 20% samples were randomly taken out and width, length and weight were measured by the help of a 0.01 mm sensitive digital caliper
Table 1 Physical properties of azarole fruits
Width of fruit (mm) Length of fruit (mm) Number of fruit/kg Number of fruit/m2 Weight of fruit (g)/m2 Average weight of each fruit (g) Geometric mean diameter of fruit (mm) Sphericity of fruit Surface area of fruit (mm2) Fruit Fl/Fw
Fp ¼
1=3 ðF l F 2w Þ
Azarole red fruits
Azarole yellow fruits
19.96 20.10 296 2422 8186 3.38 20.00
19.12 19.83 264 2556 9687 3.79 19.35
0.99 1256 1.01
0.97 1176 1.04
ð1Þ ¼
Fd Fl
Fl ðPF 2w F 2l Þ ¼ PF 2d Fs ¼ 6ð2F l F w Þ
ð2Þ ð3Þ
Wattmeter and free wheels (Fig. 1). The products were placed on the cromium mesh as a thin layer and at the density of 8.20 and 9.69 kg/m2 for red and yellow fruits, respectively. The drying air at the inlet of the dryer was
T. Koyuncu et al. / Journal of Food Engineering 78 (2007) 1471–1475
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Fig. 1. Schematic presentation of the computer connected convective hot air dryer.
20 °C (±1) and 60% (±3) relative humidity. This air was heated by the electric heater. In order to produce different temperatures and air velocities, the electric current of the heater and the rotation of the fan were adjusted manually. The system was also controlled by the thermostat automatically. To measure the power consumption, air speed, relative humidity and drying air temperatures at different points, several digital devices such as Wattmeter, hot-wire anemometer having in the measurement sensitive of 0.1 m/s, Testo AG 309 type relative humidity, temperature sensors and thermocouple were connected to the drying system. Besides, the experimental drying studies we conducted showed us that the maximum length of the drying chamber must be approximately 1 m depending on the drying air temperature distribution during the length of the dryer. When the length of the drying chamber was constructed more than 1 m, important temperature differences and relative humidity were found between the beginning and the end of the drying chamber (Koyuncu, Tosun, & ¨ stu¨n, 2003; Koyuncu, Serdar, & Tosun, 2004). Therefore, U the drying chamber was selected less than 1 m long. The moisture content (percentage dry basis) of fresh fruits, at harvest were found approximately 211% and 273% for azarole red and yellow fruits, respectively (Eq. (4)) (Ekechukwu, 1999). Moisture content of the fruits were determined by using an air oven set at 105 °C, and kept until reaching constant weight (AOAC, 1984; Ochoa et al., 2002). For safe long-term storage, the moisture content should preferably be less than 10%. For that reason, the fresh products with moisture content of 211% and 273% was dehydrated until the moisture content of 8–9% in the dryer. During drying time, the mass of the fruit samples were weighted automatically by the balance per 10 min and all test were replicated three times. The dryer was installed in conditions that were a relative humidity of 60% (±3) and a temperature of 20 °C (±1). This air was heated by the heater and directed to the drying chamber. In addition, the preliminary studies we conducted showed that the temperature less than 60 °C and the air speed more than 0.25 m/s increase the drying time and energy requirement, extremely for these fruits. Thus, two different temperatures such as 60
and 70 °C and a selected air velocity of 0.25 m/s were used for experimentation. During experiments, total drying time, total energy requirement for drying of one charge of the dryer and energy needed for drying 1 kg of wet product were determined for different temperatures and for various azarole fruit genotypes (Eqs. (5) and (6)) (Holman, 1994). W0Wd PM db ¼ 100 ð4Þ Wd Et ¼ AvqcDTDt
ð5Þ
Et W0
ð6Þ
Ekg ¼
3. Results and discussion During a drying process, two periods can be distinguished. The first is called constant drying rate period. The second drying stage is also called the falling drying rate period. During the first period, the surface of the product behaves as a surface of the water. The rate of moisture removal during this period is mainly dependent on the surrounding conditions and only affected slightly by the nature of the product. The end of the constant drying rate period is marked by a decrease in the rate of moisture migration from within the product below that sufficient to replenish the moisture being evaporated from the surface. The falling drying rate period is dependent essentially on the rate of diffusion of moisture from within the product to the surface and also on moisture removal from the surface. For agricultural products, the duration of each of the drying regimes depends on the initial moisture content and the safe storage moisture content. Especially, for fruits and most vegetables, the drying would take place within both the constant and falling rate periods that can be seen easily. Both the external factors and the internal mechanisms controlling the drying processes in the two main rate regimes are important in determining the overall drying rate of products (Ekechukwu, 1999; Gigler, Loon, van Seres, Meerdink, & Coumans, 2000). For these reasons, the
T. Koyuncu et al. / Journal of Food Engineering 78 (2007) 1471–1475
changing of the moisture content of azarole fruits must have two periods depending on the drying time. The moisture content of the products as a function of drying time are presented in Figs. 2–5 for 60 and 70 °C. As seen from these figures, all lines have two stages. The moisture content rapidly reduces and then slowly decreases with rising of the drying time. Constant and falling rate periods are changing as a function of drying time depending on the drying temperatures. The lines of constant rate periods extend from beginning of the drying time to the drying time of approximately 2000, 1500, 1750 and 1500 min for drying temperature of 60 and 70 °C and for red and yellow fruits,
Moisture content dry basis (%)
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300 250 200
y = 173.77e -0.0008x R 2 = 0.8995
150 100 50 0 0
1000
2000
3000
4000
5000
Drying time (min)
Fig. 5. Moisture content as a function of drying time for azarole red fruits and temperature of 70 °C.
160
200
140
y =151.14e-0.0003x
150
Total drying time (h)
Moisture content dry basis (%)
250
2
R =0.9607
100 50 0 0
2000
4000
6000
8000
10000
Moisture content dry basis (%)
250 200 y = 182.65e-0.0008x R 2 = 0.9383
100 50
0
1000
2000 3000 Drying time (min)
4000
5000
Fig. 3. Moisture content as a function of drying time for azarole red fruits and temperature of 70 °C.
60 40
Red-60°C
Red-70°C
Yellow-60°C
Yellow-70°C
Fig. 6. Total drying time of azarole red and yellow fruits at different temperatures.
respectively. The lines of falling rate periods of these experiments are also extend from the end of the constant rate period lines to the maximum drying time (Figs. 2–5). In addition, it is obvious from the figures that drying temperature has an important role on the total drying time (Fig. 6). The least drying time (59.60 h) was obtained at 70 °C for yellow fruits. The highest drying time (141.50 h) was also found at 60 °C temperature for red fruits. The total energy consumption for a charge of the dryer and energy needed for drying 1 kg of fruits can be seen from Figs. 7 and 8, respectively. There is a strict correlation between these two figures. This is because of the fact that the values of Fig. 8 were obtained from value of Fig. 7 30 Total energy needed (kWh)
300 Moisture content dry basis (%)
80
0
Fig. 2. Moisture content as a function of drying time for azarole red fruits and temperature of 60 °C.
0
100
20
Drying time (min)
150
120
250 y = 297.92e-0.0011x R 2 = 0.9586
200 150 100 50
25 20 15 10 5
0 0
500
1000
1500
2000
2500
3000
3500
4000
Drying time (min)
Fig. 4. Moisture content as a function of drying time for azarole yellow fruits and temperature of 60 °C.
0 Red-60°C
Red-70°C
Yellow-60°C
Yellow-70°C
Fig. 7. Total energy requirement for a charge of the dryer at different temperatures.
Specific energy requirement (kWh/kg)
T. Koyuncu et al. / Journal of Food Engineering 78 (2007) 1471–1475
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References
70 60 50 40 30 20 10 0 Red-60°C
Red-70°C
Yellow-60°C
Yellow-70°C
Fig. 8. Energy requirement for drying 1 kg of product at different temperatures.
by calculation (Eq. (6)). As it is understood from these figures, the minimum heat energy (27.68 kWh/kg) is needed for drying of 1 kg fruits at temperature of 70 °C for yellow fruits. The maximum energy (62.12 kWh/kg) is also needed at 60 °C for red fruits. As a result, it can be said from the figures that 70 °C temperature must be selected for drying freshly harvested azarole fruits. 4. Conclusions Two genotypes of freshly harvested azarole fruits were successfully dried in a computer connected convective hot air dryer at different temperatures of 60 and 70 °C and air speed of 0.25 m/s. It is found from the results of the experimental investigation that the drying air temperature has an important role on the total drying time. It is also seen from the results that the drying air temperature significantly affects the energy needed for drying of azarole fruits. The main conclusion of this study is that azarole red and yellow fruits must be dried at temperature of 70 °C and air velocity of 0.25 m/s to minimize the energy consumption for drying of azarole red and yellow fruits.
AOAC (1984). Official methods of analysis of the association of official analytical chemists. 14th Ed. Arlington: Virginia. USA, 22.013. Asma, B.M., Birhanlı, O. (2003). Malatya c¸evresinde dog˘al olarak yetisßen alıc¸larda seleksiyon c¸alısßmaları (in Turkish). Tu¨rkiye 4. Ulusal Bahc¸e Bitkileri Kongresi, 08–12 Eylu¨l, Antalya, pp. 61–62. Bignami, C., Paolocci, M., Scossa, A., & Bertazza, G. (2003). Preliminary evaluation of nutritional and medicinal components of crataegus azarolus fruits. Acta Horticulture (ISHS), 597, 95–100. _ H. (2003). Some nutritional, pomological and Demir, F., & Kalyoncu, I. physical properties. Journal of Food Engineering, 60, 335–341. Ekechukwu, O. V. (1999). Review of solar-energy drying systems I: an overview of drying principles and theory. Energy Conversion and Management, 40, 593–613. Gigler, J. K., Loon, W. K. P., van Seres, I., Meerdink, G., & Coumans, W. J. (2000). Drying characteristics of willow chips and stems. Journal of Agricultural Engineering and Research, 77(4), 391–400. Holman, J. P. (1994). Experimental methods for engineers. New York: McGraw-Hill, Inc. ¨ . Ordu Ziraat Karadeniz, T. (2004). S ß ifalı Meyveler (in Turkish). K.T.U Faku¨ltesi Bahc¸e Bitkileri Bo¨lu¨mu¨, Ordu, pp. 34–36. Koyuncu, T., Pınar, Y. (2001). Kırmızı biber ic¸in bir gu¨nesßli kurutucu tasarımı (Design of a solar dryer for red pepper). Tarımsal Mekanizasyon 20 Ulusal Kongresi; 13–15 Eylu¨l, S ß anlıurfa, Tu¨rkiye. _ (2004). Drying kinetics and energy Koyuncu, T., Serdar, U., & Tosun, I. requirement for dehydration of chestnuts (Castanea sativa Mill.). Journal of Food Engineering, 62, 165–168. Koyuncu, T., & Sessiz, A. (2002). Gu¨nesß enerjili kurutucular u¨zerine karsßılasßtırmalı bir arasßtırma (A comparative study on solar energy ¨ . Ziraat Fak Dergisi, 17(2), 53–60. dryer). O.M.U _ &U ¨ stu¨n, N. S. (2003). Drying kinetics and color Koyuncu, T., Tosun, I., retention of dehydrated rosehips. Drying Technology, 21(7), 1369–1381. Ochoa, M. R., Kesseler, A. G., Pirone, B. N., Marquez, C. A., & De Michelis, A. (2002). Shrinkage during convective drying of whole rose hip (Rosa rubiginosa L.) fruits. Lebensmittel–Wissenschaft und Technologie, 35, 400–406. Tog˘rul, T. T., & Dursun, P. (2003). Modeling of drying kinetics of single apricot. Journal of Food Engineering, 58, 23–32. Yesilada, E., Ustun, O., Sezik, E., Takaishi, Y., Ono, Y., & Honda, G. (1997). Inhibitory effects of Turkish folk remedies on inflammatory cytokineses: interleukin-1 alpha, interleukin-1 beta and tumor necrosis factor alpha. Journal of Ethnopharmacology, 58(1), 59–73.
Research note
Convective drying characteristics of azarole red (Crataegus monogyna Jacq.) and yellow (Crataegus aronia Bosc.) fruits Turhan Koyuncu *, Yunus Pinar, Fuat Lule University of Ondokuz Mayıs, Faculty of Agriculture, Department of Agricultural Machinery, 55139 Samsun, Turkey Received 6 May 2005; received in revised form 9 August 2005; accepted 30 September 2005 Available online 20 February 2006
Abstract In this research, drying characteristics and energy requirement for drying of two different genotypes (Crataegus monogyna Jacq. and Crataegus aronia Bosc.) of azarole (Crataegus azarolus L.) red and yellow fruits were reported. Azarole fruits were dehydrated in a computer connected convective hot air dryer. Freshly harvested two different genotypes of azarole fruits were dried at 60 and 70 °C temperatures and drying air velocity was selected as 0.25 m/s for both temperatures. Azarole fruits were dehydrated from the initial moisture content of 211% and 273% (percentage dry basis) to a final moisture content of 8–9% for red and yellow fruits, respectively. During experiments, drying product were weighted automatically by the balance per 10 min. Data were transferred to the computer and processed by a software. The results indicated that drying air temperature significantly influenced the total drying time and total energy requirement for drying of both genotype azarole fruits. The minimum specific energy consumption for drying of red and yellow fruits were determined as 42.80 kWh/kg and 27.68 kWh/kg for 70 °C, respectively. In order to reduce drying energy consumption, it can be recommended that the drying temperature must not be less than 70 °C for this application. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Azarole fruits; Drying kinetics; Energy requirement for drying of azarole fruits; Convective hot air dryer; Drying temperatures; Drying air velocity
1. Introduction The drying or dehydration is the oldest method in food conservation, and its object is to remove by evaporation most of the water present in the product. The reduction of moisture content inhibits or decrease microbial and enzymatic activity, which otherwise would produce food damage. Besides, dehydration makes food product handling easier owing to the volumetric shrinkage and weight losses products undergo during process (Ochoa, Kesseler, Pirone, Marquez, & De Michelis, 2002). Natural open-air sun drying is practiced widely in hot climates and tropical countries. Considerable savings can be obtained with this type of drying, since the source of energy is free and renew-
*
Corresponding author. Tel.: +90 362 3121919; fax: +90 362 4576034. E-mail address: [email protected] (T. Koyuncu).
0260-8774/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2005.09.036
able. However, this technique is extremely weather, dependent and has the problems of contamination, infestation, microbial attack, etc. In addition, the required drying time for a given load is approximately 2–4 times longer than greenhouse, cabinet and convective hot air type dryers (Koyuncu & Pınar, 2001; Koyuncu & Sessiz, 2002; Tog˘rul & Dursun, 2003). Azarole (Crataegus azarolus L.) tree is a deciduous tree growing up to 3–4 m high and cultivated for centuries in the Mediterranean area. It is in flower in April and May. The plant can grow in light, medium and heavy soils. It requires moist or wet soil and can tolerate drought. Its fruits are very variable in size and colour, it is up to 25 mm in diameter. There are 1–3 large seeds in the centre of the fruit. Fruit weight 2.16–7.58 g, flesh/pit ratio 2.55– 6.86, pit weight 0.77–1.16 g. The fruits contain 15.9 g total sugar, 1.38 mg total acidity, 27.58 mg vitamin C, 11.0 mg Ca, 9.80 mg P, 1.16 mg Fe, 158 mg K, 7.10 mg Mg,
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T. Koyuncu et al. / Journal of Food Engineering 78 (2007) 1471–1475
Nomenclature A c Dt Ekg Et Fd Fl
drying air flow surface area, m2 specific heat of air under adiabatic conditions, kJ/(kg K) total drying time, h energy requirement for drying 1 kg of product, kWh/kg total energy requirement for a charge of the dryer, kWh geometric mean diameter, mm length of fruit, mm
Fp Fs Fw PMdb v Wd W0 DT q
sphericity surface area, mm2 width of fruit, mm the moisture content on dry basis expressed as percentage, % drying air speed, m/s weight of dry matter in product, kg initial weight of undried product, kg temperature differences, K air density, kg/m3
0.16 mg Cu, 0.24 mg Mn, 2 mg Na, 0.95 g protein and 13.12 g carbohydrate for 100 g fruits. Azarole fruits are extensively used especially in rural areas of Turkey for different aims. However, for the present, it is not easy to see enough information regarding the production and consumption in the literature even if many studies made about azarole fruits. The fruits are not only consumed fresh and dried but also used to produce jam, marmalade and syrup in Turkey. Fruits and flowers are also used for medicinal purposes (Asma & Birhanlı, 2003; Bignami, Paolocci, Scossa, & Bertazza, 2003; Karadeniz, 2004; Yesilada et al., 1997). In order to store azarole fruits, it is possible to use different methods such as traditional method, cold storage and drying depending on the technical opportunities, food consumption and food processing ways. In different several literatures, it is possible to see a little part of information about nutritional and physical properties, ingredients and characteristics of azarole fruits (Asma & Birhanlı, 2003; Bignami et al., 2003; Karadeniz, 2004). However, no report concerning the drying kinetics, drying characteristics and heat energy requirement of azarole fruits during our literature survey. Therefore, two different genotypes of azarole fruits (Crataegus azarolus L.) such as azarole red fruits (Crataegus monogyna Jacq.) and azarole yellow fruits (Crataegus aronia Bosc.) were dehydrated in a computer connected convective hot air dryer at various temperatures and selected most suitable velocity to determine the drying characteristics and energy requirement for drying in this experimental investigation.
and a 0.01 g sensitive balance. The geometric mean diameter (Fd), sphericity (Fp) and surface area (Fs) were calculated from Eqs. (1) to (3) (Demir & Kalyoncu, 2003). Freshly harvested azarole fruits that physical properties given in Table 1 were dried in a computer connected convective hot air dryer. The dryer equipped with an electric heater (air heating duct), temperature adjuster, centrifugal fan (blower), air speed adjuster (regulator of variable transformer), corrosion resistant chromium mesh, corrosion resistant cromium sheet, glass wood insulator, a 0.01 g sensitive Precisa BJ 600 D digital balance, RS232 connection, a PC, specially designed Balint data processing software, drying air inlet and outlet channels as well as thermostat, temperature indicators
2. Materials and methods
F d ¼ ðF l F 2w Þ1=3
Ripe azarole red and yellow fruits grown in Malatya Region of Turkey were harvested manually and used for the investigation. The fruits were cleaned in an air screen to remove all foreign material such as dust, dirt, pieces of branches and leaves. To establish the physical properties of the fresh fruits, approximately 20% samples were randomly taken out and width, length and weight were measured by the help of a 0.01 mm sensitive digital caliper
Table 1 Physical properties of azarole fruits
Width of fruit (mm) Length of fruit (mm) Number of fruit/kg Number of fruit/m2 Weight of fruit (g)/m2 Average weight of each fruit (g) Geometric mean diameter of fruit (mm) Sphericity of fruit Surface area of fruit (mm2) Fruit Fl/Fw
Fp ¼
1=3 ðF l F 2w Þ
Azarole red fruits
Azarole yellow fruits
19.96 20.10 296 2422 8186 3.38 20.00
19.12 19.83 264 2556 9687 3.79 19.35
0.99 1256 1.01
0.97 1176 1.04
ð1Þ ¼
Fd Fl
Fl ðPF 2w F 2l Þ ¼ PF 2d Fs ¼ 6ð2F l F w Þ
ð2Þ ð3Þ
Wattmeter and free wheels (Fig. 1). The products were placed on the cromium mesh as a thin layer and at the density of 8.20 and 9.69 kg/m2 for red and yellow fruits, respectively. The drying air at the inlet of the dryer was
T. Koyuncu et al. / Journal of Food Engineering 78 (2007) 1471–1475
1473
Fig. 1. Schematic presentation of the computer connected convective hot air dryer.
20 °C (±1) and 60% (±3) relative humidity. This air was heated by the electric heater. In order to produce different temperatures and air velocities, the electric current of the heater and the rotation of the fan were adjusted manually. The system was also controlled by the thermostat automatically. To measure the power consumption, air speed, relative humidity and drying air temperatures at different points, several digital devices such as Wattmeter, hot-wire anemometer having in the measurement sensitive of 0.1 m/s, Testo AG 309 type relative humidity, temperature sensors and thermocouple were connected to the drying system. Besides, the experimental drying studies we conducted showed us that the maximum length of the drying chamber must be approximately 1 m depending on the drying air temperature distribution during the length of the dryer. When the length of the drying chamber was constructed more than 1 m, important temperature differences and relative humidity were found between the beginning and the end of the drying chamber (Koyuncu, Tosun, & ¨ stu¨n, 2003; Koyuncu, Serdar, & Tosun, 2004). Therefore, U the drying chamber was selected less than 1 m long. The moisture content (percentage dry basis) of fresh fruits, at harvest were found approximately 211% and 273% for azarole red and yellow fruits, respectively (Eq. (4)) (Ekechukwu, 1999). Moisture content of the fruits were determined by using an air oven set at 105 °C, and kept until reaching constant weight (AOAC, 1984; Ochoa et al., 2002). For safe long-term storage, the moisture content should preferably be less than 10%. For that reason, the fresh products with moisture content of 211% and 273% was dehydrated until the moisture content of 8–9% in the dryer. During drying time, the mass of the fruit samples were weighted automatically by the balance per 10 min and all test were replicated three times. The dryer was installed in conditions that were a relative humidity of 60% (±3) and a temperature of 20 °C (±1). This air was heated by the heater and directed to the drying chamber. In addition, the preliminary studies we conducted showed that the temperature less than 60 °C and the air speed more than 0.25 m/s increase the drying time and energy requirement, extremely for these fruits. Thus, two different temperatures such as 60
and 70 °C and a selected air velocity of 0.25 m/s were used for experimentation. During experiments, total drying time, total energy requirement for drying of one charge of the dryer and energy needed for drying 1 kg of wet product were determined for different temperatures and for various azarole fruit genotypes (Eqs. (5) and (6)) (Holman, 1994). W0Wd PM db ¼ 100 ð4Þ Wd Et ¼ AvqcDTDt
ð5Þ
Et W0
ð6Þ
Ekg ¼
3. Results and discussion During a drying process, two periods can be distinguished. The first is called constant drying rate period. The second drying stage is also called the falling drying rate period. During the first period, the surface of the product behaves as a surface of the water. The rate of moisture removal during this period is mainly dependent on the surrounding conditions and only affected slightly by the nature of the product. The end of the constant drying rate period is marked by a decrease in the rate of moisture migration from within the product below that sufficient to replenish the moisture being evaporated from the surface. The falling drying rate period is dependent essentially on the rate of diffusion of moisture from within the product to the surface and also on moisture removal from the surface. For agricultural products, the duration of each of the drying regimes depends on the initial moisture content and the safe storage moisture content. Especially, for fruits and most vegetables, the drying would take place within both the constant and falling rate periods that can be seen easily. Both the external factors and the internal mechanisms controlling the drying processes in the two main rate regimes are important in determining the overall drying rate of products (Ekechukwu, 1999; Gigler, Loon, van Seres, Meerdink, & Coumans, 2000). For these reasons, the
T. Koyuncu et al. / Journal of Food Engineering 78 (2007) 1471–1475
changing of the moisture content of azarole fruits must have two periods depending on the drying time. The moisture content of the products as a function of drying time are presented in Figs. 2–5 for 60 and 70 °C. As seen from these figures, all lines have two stages. The moisture content rapidly reduces and then slowly decreases with rising of the drying time. Constant and falling rate periods are changing as a function of drying time depending on the drying temperatures. The lines of constant rate periods extend from beginning of the drying time to the drying time of approximately 2000, 1500, 1750 and 1500 min for drying temperature of 60 and 70 °C and for red and yellow fruits,
Moisture content dry basis (%)
1474
300 250 200
y = 173.77e -0.0008x R 2 = 0.8995
150 100 50 0 0
1000
2000
3000
4000
5000
Drying time (min)
Fig. 5. Moisture content as a function of drying time for azarole red fruits and temperature of 70 °C.
160
200
140
y =151.14e-0.0003x
150
Total drying time (h)
Moisture content dry basis (%)
250
2
R =0.9607
100 50 0 0
2000
4000
6000
8000
10000
Moisture content dry basis (%)
250 200 y = 182.65e-0.0008x R 2 = 0.9383
100 50
0
1000
2000 3000 Drying time (min)
4000
5000
Fig. 3. Moisture content as a function of drying time for azarole red fruits and temperature of 70 °C.
60 40
Red-60°C
Red-70°C
Yellow-60°C
Yellow-70°C
Fig. 6. Total drying time of azarole red and yellow fruits at different temperatures.
respectively. The lines of falling rate periods of these experiments are also extend from the end of the constant rate period lines to the maximum drying time (Figs. 2–5). In addition, it is obvious from the figures that drying temperature has an important role on the total drying time (Fig. 6). The least drying time (59.60 h) was obtained at 70 °C for yellow fruits. The highest drying time (141.50 h) was also found at 60 °C temperature for red fruits. The total energy consumption for a charge of the dryer and energy needed for drying 1 kg of fruits can be seen from Figs. 7 and 8, respectively. There is a strict correlation between these two figures. This is because of the fact that the values of Fig. 8 were obtained from value of Fig. 7 30 Total energy needed (kWh)
300 Moisture content dry basis (%)
80
0
Fig. 2. Moisture content as a function of drying time for azarole red fruits and temperature of 60 °C.
0
100
20
Drying time (min)
150
120
250 y = 297.92e-0.0011x R 2 = 0.9586
200 150 100 50
25 20 15 10 5
0 0
500
1000
1500
2000
2500
3000
3500
4000
Drying time (min)
Fig. 4. Moisture content as a function of drying time for azarole yellow fruits and temperature of 60 °C.
0 Red-60°C
Red-70°C
Yellow-60°C
Yellow-70°C
Fig. 7. Total energy requirement for a charge of the dryer at different temperatures.
Specific energy requirement (kWh/kg)
T. Koyuncu et al. / Journal of Food Engineering 78 (2007) 1471–1475
1475
References
70 60 50 40 30 20 10 0 Red-60°C
Red-70°C
Yellow-60°C
Yellow-70°C
Fig. 8. Energy requirement for drying 1 kg of product at different temperatures.
by calculation (Eq. (6)). As it is understood from these figures, the minimum heat energy (27.68 kWh/kg) is needed for drying of 1 kg fruits at temperature of 70 °C for yellow fruits. The maximum energy (62.12 kWh/kg) is also needed at 60 °C for red fruits. As a result, it can be said from the figures that 70 °C temperature must be selected for drying freshly harvested azarole fruits. 4. Conclusions Two genotypes of freshly harvested azarole fruits were successfully dried in a computer connected convective hot air dryer at different temperatures of 60 and 70 °C and air speed of 0.25 m/s. It is found from the results of the experimental investigation that the drying air temperature has an important role on the total drying time. It is also seen from the results that the drying air temperature significantly affects the energy needed for drying of azarole fruits. The main conclusion of this study is that azarole red and yellow fruits must be dried at temperature of 70 °C and air velocity of 0.25 m/s to minimize the energy consumption for drying of azarole red and yellow fruits.
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