IJFS#1697_bozza Ital. J. Food Sci., vol. 32, 2020 - 251 PAPER DRYING CHARACTERISTICS OF ‘ANKARA’ PEAR SLICES Y.B. ÖZTEKIN*1 and K. SACILIK2 1Department of Agricultural Machinery and Technologies Engineering, Faculty of Agriculture, Ondokuz Mayis University, 55139, Samsun, Turkey 2Department of Agricultural Machinery and Technologies Engineering, Faculty of Agriculture, Ankara University, 06130, Ankara, Turkey *Corresponding author: Tel.: +90 3623121919; Fax: +90 362 4576034 Email: yurtlu@omu.edu.tr ABSTRACT This study evaluated the effects of drying temperature and pre-treatment on the rehydration capacity and color parameters of sliced pears (cv. Ankara). Drying trials were conducted at 55, 65, and 75°C. Pre-treatment consisted of immersion of pear slices in a citric-acid solution or blanching in hot water. Pre-treatment was found to have a significant effect on both rehydration capacity and color, with higher temperatures and pre-treatment resulting in decreases in drying time and increases in rehydration capacity. Effective diffusivity values ranged between 1.12×10-10 and 2.94×10-10 m2/s. Blanched pear slices had the lowest Ea values (15.51 kJ/mol), followed by the samples immersed in citric acid (28.03 kJ/mol) and the untreated samples (33.48 kJ/mol). The Midilli et al. model displayed the best fit to the drying data of five models tested based on the statistical criteria evaluated. Natural color of fresh pear was best preserved with lower drying temperatures and pre-treatment with citric acid. Keywords: color properties, convective drying, moisture content, rehydration capacity Ital. J. Food Sci., vol. 32, 2020 - 252 1. INTRODUCTION Pear is one of the most important fruits in Turkey and around the world. In 2017, Turkey accounted for approximately 503,004 tons of the 24.17 million tons of pears produced world-wide, making it the 5th largest pear producer behind China (16.53 million tons), Argentina (930,340 tons), the United States (677,891 tons) and Italy 772,577 tons) (FAOSTAT, 2019). Turkey is also a center of genetic diversity, with over 600 of the more than 5,000 varieties found throughout the world (KARADENİZ, 1999). One of the most important pear varieties in Turkey is the ‘Ankara’ pear, which originated in Ankara and is grown mainly in Turkey’s Central Anatolia region, especially in the province of Ankara (ERDOĞAN et al., 2007). ‘Ankara’ pear trees produce medium-sized, green fruit with smooth surfaces, thin skins, short, thick stalks and juicy, fragrant flesh that melts in the mouth. The fruit are also easy to store (DUMANOĞLU et al., 2006; ERDOĞAN et al., 2007). Vegetables and fruits contain basic nutrients that are important for human health. Because fruits and vegetables are cultivated on a seasonal basis and have a high-water content that makes them easily perishable, various preservation techniques have been developed so that fruits and vegetables can be consumed throughout the year (QUILES et al., 2005). Dehydration, although a highly complicated product-processing technique (MASKAN, 2000), is the basic method used for reducing moisture levels in order to minimize on-going microbial reactions, prevent deterioration (KROKIDA and MARINOS-KOURIS, 2003), and increase the shelf life (DAS et al., 2001) of agricultural products. Of the many drying methods available, convective drying, which represents one of the most common of all postharvest technologies, allows for high-quality products that preserve close to their original color (DOYMAZ, 2004). Pears are consumed in various forms, both fresh and dried. Dried pears are consumed directly as snacks and are also widely used as inputs in the food industry. The design, operation, and maintenance of fruit-drying systems require a good understanding of drying characteristics. Studies have evaluated drying characteristics of different varieties of pears, such as ‘d’Anjou’ (PARK et al., 2002) and ‘Deveci’ (DOYMAZ, 2013), as well as different techniques, including convective drying (GONZÁLEZ-MARTÍNEZ, 2006) air- drying (DOYMAZ, 2013; DOYMAZ and İSMAIL, 2012), osmo-vacuum drying (AMIRIPOUR et al., 2015), mid-infrared-freeze drying (ANTAL et al., 2017), and microwave-vacuum drying (TASKIN et al., 2019). However, the literature includes no data on the drying behavior of the ‘Ankara’ pear variety, whose texture varies greatly from that of other varieties, especially the ‘Deveci’ pear. Thus, this study was carried out to examine how drying temperature and pre-treatment by either immersion in a citric acid solution or blanching in hot water affect the drying characteristics and quality parameters (i.e. moisture content, rehydration capacity, color) of ‘Ankara’ pear. 2. MATERIALS AND METHODS 2.1. Material The pears used in this study (cv. Ankara) were obtained from a local market in Ankara, Turkey. Pears were kept refrigerated at 5ºC and removed 12 hours prior to the trials to obtain equilibrium. Pears were then sliced into sound, homogenous samples of 5±0.5 mm thickness and randomly distributed among 3 groups according to pre-treatment, as follows: Citric Acid: pear slices were immersed in a citric-acid solution (5 g/L) for 3 min at Ital. J. Food Sci., vol. 32, 2020 - 253 room temperature; Blanching: pear slices were blanched in 85°C water for 3 min and then rinsed with running water; Untreated: pear slices received no pre-treatment. 2.2. Drying Pears were dried according to SACILIK et al. (2010) using a convective hot-air dryer (57 x 68 x 57 cm) comprised of a perforated basket (576 cm2 x 12 cm), an adjustable fan, an electric heater, and a load-cell system attached to a PC (Fig. 1). Drying runs were carried out at 55, 65 and 75 °C, with a constant air velocity of 1 m/s (IZLI et al., 2019). A minimum of 250 g of pear slices was used for each run. Pear slices were dried with tissue paper and then placed uniformly into the basket, which was positioned in the drying system after it had been allowed to idle for 20 min to reach thermal stabilization. Initial moisture content of pears was measured at 120 ºC using an HB43-S Halogen Moisture Analyzer (Mettler Toledo, Switzerland) and recorded as 572.04% d.b. (85.12% w.b.). During the drying process, moisture loss from samples in the drying basket was measured using a load cell and continuously recorded using specially developed software connected to a PC. Once moisture-loss measurements were completed, dried samples were evaluated for rehydration capacity and color. 2.3. Effective diffusivity and activation energy A falling-rate drying period can be observed in drying pear slices, with moisture and/or vapor migration controlled by diffusion. In this case, Fick’s second law can be derived as follows (CRANK, 1975; SACILIK and UNAL, 2005): )( leff l MD t M ∇∇= ∂ ∂ (1) Figure. 1. The diagram of drying system. where Ml is the local moisture content in % d.b., t is the drying time in min, and Deff is the effective diffusivity in m2/s. Assuming moisture migration to be realized through diffusion, shrinkage to be negligible, and diffusion coefficients and temperatures to be constant (CRANK, 1975) yields the following equation: Ital. J. Food Sci., vol. 32, 2020 - 254 ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ +− + = − − = ∑ ∞ = 2 22 0 22 0 4 π)12( exp )12( 1 π 8 h tDn nMM MM M eff ne e R (2) For long drying periods, by considering only the first term in the series and, given the relatively small size of Me as compared to M and M0, reducing moisture ratio (MR) to M/M0 Equation 2 can be simplified to yield Equation 3: ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ −= 2 2 2 0 4 π π 8 lnln h tD M M eff (3) where MR is the dimensionless moisture ratio, M is the moisture content at any time in % d.b., Me is the equilibrium moisture content in % d.b., M0 is the initial moisture content in % d.b., h is the half-thickness of the slab in sample in m, and n is a positive integer. Effective diffusivity and drying air temperature are correlated using the Arrhenius equation (Equation 4): ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ −= a a eff RT E DD exp0 (4) where D0 is the pre-exponential factor of the Arrhenius equation in m2/s, Ea is the activation energy in kJ/mol, R is the universal gas constant in kJ/mol.K, and Ta is the absolute air temperature in K. 2.4. Modelling of drying data Drying data were fitted to five selected models (Table 1). Moisture ratios were determined using the following equation: e e R MM MM M − − = 0 (5) where MR is the moisture ratio, M, Me and Mo are, respectively, the moisture content at any time, the equilibrium moisture content, and the initial moisture content in % d.b. MR was further reduced to M/M0 , given the continuous fluctuation of relative humidity during the drying processes, (DIAMENTE and MUNRO, 1993). Data were analyzed by using Statistica 6.1 (StatSoft Inc., USA) software package. Drying rate constants and model coefficients were calculated according to Levenberg-Marguardt, and the statistical validity of the selected drying models was assessed according to the criteria put forth in Equations 6, 7 and 8 (SACILIK et al., 2010; YURTLU, 2011): ∑ = − = N i iR ipreRiexR M MM N P 1 exp,, ,,,,100 (6) Ital. J. Food Sci., vol. 32, 2020 - 255 ( ) 2/1 1 2 ,,,, 1 ⎥ ⎦ ⎤ ⎢ ⎣ ⎡ −= ∑ = N i ipreRiexR MMN RMSE (7) zN MM N i ipreRiexR − − = ∑ =1 2 ,,,, 2 )( χ (8) where MR,ex,i is the ith experimental moisture ratio, MR,pre,i is the ith predicted moisture ratio, N is the number of observations, and z is the number of constants. R2 was used as the primary comparison criteria. Goodness of fit was also examined based on P, RMSE and χ2 (YURTLU, 2011). Table 1. Selected drying models. Model no Model name Model References 1 Page MR = exp(-kt m) Agrawal and Singh (1977) 2 Logarithmic MR = a exp(-kt) + c Yagcioglu et al. (1999) 3 Two-term MR = a exp(-kt) + b exp(-k0t) Henderson (1974) 4 Approximation of diffusion MR = a exp(-kt) + (1 - a) exp(-kbt) Yaldiz and Ertekin (2001) 5 Midilli et al. MR = a exp(-kt m) + bt Midilli et al. (2002) 2.5. Rehydration capacity and color parameters of pear slices Rehydration capacity is of paramount importance for dried products. In this study, rehydration capacity was determined by immersing 10 g of dried pear slices into 85 °C water for 3 min, drying the pear surfaces with paper towels, and measuring the mass of the rehydrated sample using an electronic digital scale (±0.001 g), with rehydration capacity expressed as the ratio of the mass of the rehydrated sample to the mass of the dried sample (PRAKASH et al., 2004). Color properties are also among the important quality parameters of dried fruits (ELICIN and SACILIK, 2005). In this study, color measurements were obtained from 5 points on the surface of each pear sample using a Minolta CR-300 Chromameter, and the average measurement was calculated. Hue angles and color differences between raw and dried samples were calculated with the help of Equation 9 and Equation 10 (SACILIK and UNAL, 2005): )(tan * * 1 a b H −= (9) 20 2 0 2 0 )()()( fff bbaaLLE −+−+−=Δ (10) where H is the hue angle°, ΔE is the color difference, L0, a0 and b0 are the color lightness, green-red and blue-yellow values of raw pear slices, and Lf, af and bf are the color lightness, green-red and blue-yellow values of dried pear slices. Ital. J. Food Sci., vol. 32, 2020 - 256 3. RESULTS AND DISCUSSION 3.1 Hot-air drying curves of pears Pear (cv. Ankara) drying characteristics are presented in Figs. 2, 3 and 4 according to drying temperature and pre-treatment procedures. As the Figs show, pear moisture content was observed to decrease continuously over time from 572.04% d.b. to between 4.43% d.b. and 19.22% d.b. Moisture content was significantly affected by drying temperature, citric-acid treatment, and blanching. Untreated pears required drying times of 1,560, 1,080 and 900 min at 55, 65 and 75 ºC, respectively, to reach their final moisture content, as compared to 1,140, 900 and 660 min for pear samples pre-treated with citric acid and 840, 720 and 600 min for samples blanched in hot water. These Figs. – representing decreases in drying time of 46% at 55ºC and 33% at 65ºC and 75ºC for blanched pears as compared to untreated pears – demonstrate that water diffusion increases with pre-treatment. Similar results have been reported by DOYMAZ (2010) for Amasya red apples, by DOYMAZ (2013) for pear, by VARDIN and YILMAZ (2018) for pomegranate arils, and by PANDEY et al. (2019) for green peas. Figure 2. Drying curves for ‘Ankara’ pear at 55°C. 0 100 200 300 400 500 600 0 5 10 15 20 25 30 M oi st ur e co nt en t (% d .b .) Drying time (h) Untreated Blanch Citric Acid Ital. J. Food Sci., vol. 32, 2020 - 257 Figure 3. Drying curves for ‘Ankara’ pear at 65°C. Figure 4. Drying curves for ‘Ankara’ pear at 75°C. 3.2. Effective diffusivity and activation energy From Equation 3, a plot of ln(MR) vs. the time provides a straight line with a slope s of: 2 2 4 π h D s eff= (11) The highest Deff values were obtained for the blanched pear samples, followed by the citric- acid treated and the untreated samples (Table 2). Deff values were observed to increase with increases in air temperature due to accelerated moisture diffusion, which could be due to an increase in water permeability caused by cracks in the sample surfaces. The Deff values obtained for ‘Ankara’ pear slices in the present study are comparable to values ranging from 1.59×10−10 to 7.64×10−10 m2/s obtained for ‘d’Anjou’ pear at 40 ºC - 80 ºC (PARK et al., 2002), from 2.27×10-10 to 4.97×10-10 m2/s for “organic apple” at 40 ºC - 60 ºC (SACILIK and 0 100 200 300 400 500 600 0 5 10 15 20 M oi st ur e co nt en t (% d .b .) Drying time (h) Untreated Blanch Citric Acid 0 100 200 300 400 500 600 0 5 10 15 20 M oi st ur e co nt en t (% d .b .) Drying time (h) Untreated Blanch Citric Acid Ital. J. Food Sci., vol. 32, 2020 - 258 ELICIN, 2006), from 2.66×10-10 to 4.56×10-10 m2/s for Üryani plum at 50ºC - 70ºC (SACILIK et al., 2006), from 0.85×10−10 to 2.18×10−10 m2/s for pear slices at 55ºC - 75ºC (DOYMAZ, 2012), and from 8.56×10-11 to 2.25×0-10 m2/s for ‘Deveci’ pear slices at 50ºC - 71ºC (DOYMAZ, 2013). Activation energy values were obtained by plotting ln(Deff) vs. 1/T (Fig. 5), which yielded a straight line indicating an Arrhenius dependence on temperature. Using Equation 4, activation energy values for untreated pear samples, pear samples treated with citric acid, and blanched pear samples were obtained using Equations 12, 13 and 14, respectively, as follows: Table 2. Effective diffusivity for ‘Ankara’ pear at various air temperatures. Air temperature, °C Deff x10 10, m2 /s R2 Untreated 55 1.12 0.9747 65 1.56 0.9878 75 2.26 0.9713 Citric acid 55 1.45 0.9951 65 1.90 0.9839 75 2.61 0.9821 Blanched 55 2.12 0.9709 65 2.44 0.9756 75 2.94 0.9825 Figure 5. Arrhenius-type relationship between Deff and Ta. Untreated samples: ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ −×= − T Deff 23.4026 exp1036.2 5 (12) (R2=0.9979), Citric acid-treated samples: -23,0 -22,6 -22,2 -21,8 0,0028 0,0029 0,003 0,0031 ln D ef f 1/Ta, (K-1) Untreated Blanch Citric Acid Ital. J. Food Sci., vol. 32, 2020 - 259 ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ −×= − T Deff 19.3371 exp1015.4 6 (13) (R2=0.9965), Blanched samples: ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ −×= − T Deff 72.1865 exp1019.6 8 (14) (R2=0.9916). The highest value of activation energy was obtained for the untreated samples (Ea=33.48 kJ/mol), followed by the citric-acid treated samples (Ea=28.03 kJ/mol) and blanched samples (Ea=15.51 kJ/mol). These values are in line with the range (15-40 kJ/mol) specified by Rizvi (1986) for various foods. 3.3. Parameter estimation Estimated values of drying models and comparison criteria (R2, P, RMSE and χ2) are given in Table 3. Selected models offered a good fit to data. Of the 5 models examined, the MIDILLI et al. had highest R2 and lowest P, RMSE and χ2 values, indicating it to be the best model in terms of fitness to data. Comparisons of the experimental data and the predicted moisture ratios obtained using the MIDILLI et al. model for ‘Ankara’ pear slices at 55, 65 and 75°C are presented in Fig 6. As the Fig. show, there is very good conformity between the actual and the predicated data, confirming the goodness of fit of the MIDILLI et al. model. 3.4. Quality parameters (rehydration and color retention) Air temperature as well as pre-treatment, either with a citric-acid solution or by blanching, significantly affected the rehydration capacity of ‘Ankara’ pears (Table 4). The highest rehydration values were observed for the blanched pear slices dried at 75ºC. At every temperature examined, the blanched pear slices showed the greatest rehydration capacity, followed by the samples treated with citric acid and the untreated samples. Increases in air temperatures during drying resulted in increases in rehydration capacity, with increases of 5.43%, 4.64% and 10.54%, respectively, for untreated samples, samples treated with citric acid, and blanched samples when temperatures were increased from 55 to 75 ºC. This finding can be explained by an increase in the rate of moisture removal with increases in air temperature, which leads to less shrinkage and thus an accelerated rate of rehydration. Similar results have been reported by AMIRIPOUR et al. (2015), HEBDA et al. (2019) and SINGH et al. (2006). Table 5 shows the Hunter color values for pears by air temperature and pre-treatment procedures. The lowest a* values and the highest L* and H values were observed at 55°C regardless of pretreatment. H and L* values decreased with increases in temperature, whereas a* values increased with increases in temperature, demonstrating that browning occurred as a result of temperature increases. Similar results were reported by WANG and CHAO (2003), ELICIN and SACILIK (2005) and SACILIK and ELICIN (2006). Ital. J. Food Sci., vol. 32, 2020 - 260 Table 3. Statistical criteria of the models for ‘Ankara’ pear. °C Model no Model coefficients R2 P (%) RMSE χ2 Untreated 55 1 k=0.0985; m=1.2518 0.9974 25.54 1.84x10-2 3.65x10-4 2 a=1.0627; k=0.164; c=-0.0126 0.9932 23.20 3.06x10-2 10.15x10-4 3 a=-0.0405; k0=0.5464; b=1.4754; k1=0.2142 0.9980 21.22 1.69x10-2 3.11x10-4 4 a=6.901; k=0.113; b=0.9935 0.9915 28.90 3.42x10-2 12.67x10-4 5 a=0.99; k=0.082; m=1.3664; b=0.0015 0.9993 9.65 1.02x10-2 1.14x10-4 65 1 k=0.0982; m=1.3124 0.9988 10.84 1.33x10-2 1.97x10-4 2 a=1.12; k=0.1527; c=-0.0858 0.9942 26.81 2.98x10-2 9.92x10-4 3 a=05257.; k0=0.1863; b=0.5265; k1=0.1863 0.9894 29.24 4.14x10 -2 19.24x10-4 4 a=-7.4478; k=0.3419; b=0.908 0.9920 10.02 1.34x10-2 1.99x10-4 5 a=0.9795; k=0.0836; m=1.395; b=0.00061 0.9992 7.24 1.15x10-2 1.49x10-4 75 1 k=0.156; m=1.3316 0.9962 44.90 2.24x10-2 5.47x10-4 2 a=1.0545; k=0.273; c=0.0065 0.9904 35.97 3.65x10-2 14.54x10-4 3 a=2.2259; k0=0.3951; b=-1.2366; k1=0.6568 0.9965 42.11 2.24x10-2 5.51x10-4 4 a=9.9874; k=0.1899; b=0.9696 0.9878 50.74 4.11x10-2 18.46x10-4 5 a=0.9826; k=0.1336; m=1.4489; b=0.00165 0.9994 9.72 0.96x10-2 1.03x10-4 Citric acid 55 1 k=0.1352; m=1.1923 0.9991 12.76 1.09x10-2 1.32x10-4 2 a=1.0648; k=0.1897; c=-0.0213 0.9970 13.48 2.08x10-2 4.83x10-4 3 a=0.5246; k0=0.2011; b=0.5246; k1=0.2009 0.9965 8.83 2.31x10 -2 5.99x10-4 4 a=-5.7317; k=0.121; b=1.0676 0.9955 17.59 2.57x10-2 7.36x10-4 5 a=1.0034; k=0.131; m=1.2332; b=0.0012 0.9997 3.73 0.64x10-2 0.45x10-4 65 1 k=0.1254; m=1.3216 0.9987 12.84 1.40x10-2 2.23x10-4 2 a=1.1207; k=0.1869; c=-0.0836 0.9937 29.66 3.20x10-2 11.74x10-4 3 a=0.4054; k0=0.2271; b=0.6489; k1=0.2271 0.9891 30.74 4.35x10 -2 21.85x10-4 4 a=-6.8642; k=0.4215; b=0.899 0.9987 11.83 1.44x10-2 2.39x10-4 5 a=0.9798; k=0.1081; m=1.4065; b=0.0007 0.9991 8.66 1.24x10-2 1.78x10-4 75 1 k=0.198; m=1.3054 0.9982 13.01 1.69x10-2 3.41x10-4 2 a=1.1148; k=0.2576; c=-0.0823 0.9931 30.98 3.49x10-2 14.66x10-4 3 a=0.5247; k0=0.3119; b=0.5247; k1=0.3119 0.9886 31.51 4.67x10 -2 26.72x10-4 4 a=-5.384; k=0.147; b=1.1161 0.9929 32.32 3.53x10-2 14.97x10-4 5 a=0.9738; k=0.1699; m=1.4089; b=0.0011 0.9988 8.65 1.48x10-2 2.69x10-4 Blanched 55 1 k=0.1757; m=1.2674 0.9987 17.28 1.41x10-2 2.29x10-4 2 a=1.0863; k=0.2434; c=-0.0475 0.9944 32.44 3.03x10-2 10.62x10-4 3 a=-0.0407; k0=0.2745; b=1.01; k1=0.2746 0.9922 26.57 3.71x10 -2 16.11x10-4 4 a=-5.789; k=0.4965; b=0.8918 0.9988 16.57 1.41x10-2 2.29x10-4 5 a=0.9843; k=0.1584; m=1.3390; b=0.00098 0.9990 9.79 1.32x10-2 2.03x10-4 65 1 k=0.2161; m=1.2946 0.9987 17.90 1.44x10-2 2.45x10-4 2 a=1.0827; k=0.2986; c=-0.0391 0.9927 28.95 3.56x10-2 15.06x10-4 3 a=-0.4799; k0=0.3298; b=0.5734; k1=0.3298 0.9911 21.92 4.11x10-2 20.21x10-4 4 a=-4.5469; k=0.6298; b=0.859 0.9989 16.06 1.37x10-2 2.21x10-4 5 a=0.9813; k=0.1921; m=1.3988; b=0.0021 0.9995 2.65 9.61x10-2 1.11x10-4 75 1 k=0.281; m=1.2235 0.9996 11.58 0.83x10-2 0.82x10-4 2 a=1.08; k=0.3377; c=-0.0442 0.9963 24.31 2.55x10-2 7.96x10-4 3 a=-2.7851; k0=0.7071; b=3.7845; k1=0.5749 0.9996 11.08 0.86x10-2 0.93x10-4 4 a=0.1349; k=0.3568; b=0.9989 0.9914 28.21 3.91x10-2 18.68x10-4 5 a=0.9936; k=0.2729; m=1.257; b=0.00103 0.9996 7.70 0.78x10-2 0.75x10-4 Ital. J. Food Sci., vol. 32, 2020 - 261 (a) (b) (c) Figure 6. Conformity of the Midilli et al. for ‘Ankara’ pear at 55°C (a), at 65°C (b) and at 75°C (c). 0,0 0,2 0,4 0,6 0,8 1,0 0 5 10 15 20 25 30 M R Drying time (h) Untreated, experimental Citric Acid, experimental Blanch, experimental Midilli et al. model 0,0 0,2 0,4 0,6 0,8 1,0 0 5 10 15 20 M R Drying time (h) Untreated, experimental Citric Acid, experimental Blanch, experimental Midilli et al. model 0,0 0,2 0,4 0,6 0,8 1,0 0 5 10 15 20 M R Drying time (h) Untreated, experimental Citric Acid, experimental Blanch, experimental Midilli et al. model Ital. J. Food Sci., vol. 32, 2020 - 262 Table 4. Rehydration capacity for ‘Ankara’ pear at various temperatures. Air temperature, °C Rehydration capacity Untreated 55 3.31 65 3.35 75 3.49 Citric acid 55 3.45 65 3.54 75 3.61 Blanched 55 3.51 65 3.69 75 3.88 Table 5. Color values for ‘Ankara’ pear at various temperatures. Air temperature Hunter color values °C L* a* b* ΔE H° Untreated 55 72.69 5.04 33.27 10.09 81.40 65 68.04 4.46 26.33 14.62 80.13 75 66.38 6.57 29.22 15.43 77.50 Citric acid 55 78.16 2.91 34.03 8.97 85.25 65 75.69 5.79 34.96 9.41 80.58 75 67.71 5.20 31.98 12.85 80.56 Blanched 55 69.45 6.26 30.73 12.54 78.43 65 64.72 6.99 30.77 16.61 77.65 75 63.98 7.69 34.37 16.29 77.60 In terms of pre-treatment, the present study found samples pre-treated with citric acid had higher H and L* values as compared to blanched and untreated samples at each air temperature tested. Moreover, the samples treated with citric acid had smaller ∆E values than both the blanched and untreated samples, indicating that pre-treatment with citric acid helped to preserve the original color of pear slices. Overall, the natural color of fresh pear was best preserved when slices were pre-treated with a citric-acid solution and dried at the lowest air temperature (55°C). 4. CONCLUSIONS In conclusion, drying temperature and pre-treatment with either a citric-acid solution or by blanching in hot water significantly affected the moisture content, rehydration capacity and color parameters of ‘Ankara’ pear slices. Blanched pear slices required shorter drying times than samples treated with citric acid as well as untreated samples. When compared to untreated pears, blanched pear slices required 46% less time for drying at 55ºC and 33% less time at 65ºC and at 75ºC. Deff values were observed to decrease with decreases in Ital. J. Food Sci., vol. 32, 2020 - 263 temperature and were lower for untreated pears than for pre-treated pears. Ea values were highest for untreated samples (33.48 kJ/mol), followed by citric acid-treated (28.03 kJ/mol) and blanched samples (15.51 kJ/mol). Based on the evaluated statistical criteria, the MIDILLI et al. model showed the best fit to the drying data of all the models tested. Rehydration capacity of pear slices was seen to decrease with decreases in drying temperature. The natural color of fresh pear slices was best retained when the samples were pre-treated with citric acid and dried at the lowest air temperature. REFERENCES Agrawal Y.C. and Singh R.D. 1977. Thin layer drying studies on short grain rice. ASAE Paper No: 3531 ASAE, St. Joseph, MI. Amiripour M., Habibi-Najafi M.B., Mohebbi M. and Emadi B. 2015. Optimization of osmo-vacuum drying of pear (Pyrus communis L.) using response surface methodology. J Food Meas. Charact. 9(3):269-280. Antal T., Tarek-Tilistyák J., Cziáky Z.and Sinka L. 2017. Comparison of drying and quality characteristics of pear (Pyrus communis L.) using mid-infrared-freeze drying and single stage of freeze drying. Int. J. Food Eng. 13(4):125-146. Crank J. 1975. “Mathematics of Diffusions” 2nd ed. Oxford University Press, London. Das S., Das T., Rao P.S. and Jain R.K. 2001. Development of an air recirculating tray dryer for high moisture biological materials. J. Food Eng. 50(4):223-227. Diamente L.M. and Munro P.A. 1993. Mathematical modeling of the thin layer solar drying of sweet potato slices. Sol. Energy. 51(4):271-276. Doymaz I. 2004. Effect of dipping treatment on air drying of plums. J. Food Eng. 64(4):465-470. Doymaz I. 2010. Effect of citric acid and blanching pre-treatments on drying and rehydration of Amasya red apples. Food Bioprod. Process. 88(2-3):124-132. Doymaz İ. and İsmail O. 2012. Experimental characterization and modelling of drying of pear slices. Food Sci. Biotechnol. 21(5):1377-1381. Doymaz İ. 2013. Experimental study on drying of pear slices in a convective dryer. Int. J. Food Sci. Technol. 48(9):1909- 1915. Dumanoğlu H., Güneş N.T., Erdoğan V., Aygün A. and Şan B. 2006. Clonal selection of a winter-type European pear cultivar 'Ankara' (Pyrus communis L.). Turk. J. Agric. For. 30(5):355-363. Elicin A.K. and Sacilik K. 2005. An experimental study for solar tunnel drying of apple. J Agr. Sci. 11(2), 207-211. Erdoğan V., Aygün A., Şan B., Koltarla A., Güneş N. and Dumanoğlu H. 2007. ‘Ankara’ armudu (Pyrus communis L.) klonlarının RAPD tekniği ile moleküler analizi. Türkiye V. Ulusal Bahçe Bitkileri Kongresi, Bildiriler Kitabı, Cilt 1, pp. 56-59, Türkiye. FAOSTAT. 2019. Statistical database. Available: www.fao.org. González-Martínez C., Cháfer M., Xue K. and Chiralt A. 2006. Effect of the osmotic pre-treatment on the convective air drying kinetics of pear var. Blanquilla. Int. J. Food Prop. 9(3):541-549. Hebda T., Brzychczyk B., Lapczynska-Kordon B. and Styks J. 2019. Influence of pre-treatment and drying methods on process of rehydration of dried apple. Eng. for Rural Dev. 18:669-676. Henderson S.M. 1974. Progress in developing the thin-layer drying equation. T ASAE. 17:1167-1168/1172. Izli N., Taskin O. and Izli G. 2019. Drying of lime slices by microwave and combined microwave-convective methods. Ital. J. Food Sci. 31(3):487-500. Karadeniz F. 1999. Armut suyunun kimyasal bileşimi üzerine araştırma. Turk. J. Agric. For. 23(3):355-358. Ital. J. Food Sci., vol. 32, 2020 - 264 Krokida M.K. and Marinos-Kouris D. 2003. Rehydration kinetics of dehydrated products. J. Food Eng. 57(1):1-7. Maskan M. 2000. Microwave/air and microwave finish drying of banana. J. Food Eng. 44(2):71-78. Midilli A., Kucuk H. and Yapar Z. 2002. A new model for single-layer drying. Drying Technol. 20(7):1503-1513. Pandey O.P., Mishra B.K. and Misra A. 2019. Comparative study of green peas using with blanching and without blanching techniques. Information Processing in Agriculture, 6(2):285-296. Park K.J., Bin A. and Brod F.P.R. 2002. Drying of pear d’Anjou with and without osmotic dehydration. J. Food Eng. 56(1):97-103. Prakash S., Jha S.K. and Datta N. 2004. Performance evaluation of blanched carrots dried by three different driers. J. Food Eng. 62(3):305-313. Rizvi S.S.H. 1986. “Thermodynamic properties of foods in dehydration” In: M.A. Rao and S.S.H. Rizvi (Eds.), Engineering properties of foods, Marcel Dekker, New York. Quiles A., Hernando I., Pérez-Munuera I., Larrea V., Llorca E. and Lluch M.Á. 2005. Polyphenoloxidase (PPO) activity and osmotic dehydration in Granny Smith apple. J. Sci. Food Agric. 85(6):1017-1020. Sacilik K. and Unal G. 2005. Dehydration Characteristics of Kastamonu Garlic Slices. Biosyst. Eng. 92(2):207-215. Sacilik K. and Elicin A.K. 2006. The thin layer drying characteristics of organic apple slices. J. Food Eng. 73(3):281-289. Sacilik K., Elicin A.K. and Unal H.G. 2006. Drying kinetics of Üryani plum in a convective hot-air dryer. J. Food Eng. 76(3):362-368. Sacilik K., Yurtlu Y.B. and Unal H.G. 2010. Thin layer convective drying and mathematical modeling of einkorn. Conference Proceeding - 4th International Conference, TAE 2010: Trends in Agricultural Engineering, pp. 540-548, Prague; Czech Republic, September 7-10. Sing, S., Raina C.S., Bawa A.S. and Saxena D.S. 2006. Effect of pretreatments on drying and rehydration kinetics and color of sweet potato slices. Drying Technol. 24(11):1487-1494. Taskin O., Polat A., Izli N. and Asik B.B. 2019. Intermittent Microwave-Vacuum Drying Effects on Pears. Pol. J. Food Nutr. Sci. 69(1):101-108. Vardin H. and Yilmaz F.M. 2018. The effect of blanching pre-treatment on the drying kinetics, thermal degradation of phenolic compounds and hydroxymethyl furfural formation in pomegranate arils. Ital. J. Food Sci. 30(1):156-169. Wang J. and Chao Y. 2003. Effect of 60Co irradiation on drying characteristics of apple. J. Food Eng. 56(4):347-351. Yurtlu Y.B. 2011. Drying characteristics of bay laurel (Laurus nobilis L.) fruits in a convective hot-air dryer. Afr. J. Biotechnol. 10(47):9593-9599. Yagcioglu A., Degirmencioglu A. and Cagatay F. 1999. Drying characteristics of laurel leaves under different drying conditions. In Proceedings of the 7th International Congress on Agricultural Mechanization and Energy, pp. 565-569, Adana, Turkey, May 26-27. Yaldız O. and Ertekin C. 2001. Thin layer solar drying some different vegetables. Drying Technol. 19(3):583-596. Paper Received September 24, 2019 Accepted November 6, 2019