Food Chemistry xxx (2015) xxx–xxx Contents lists available at ScienceDirect Food Chemistry journal homepage: www.elsevier.com/locate/foodchem Effect of different drying methods on moisture ratio and rehydration of pumpkin slices Liliana Seremet (Ceclu) ⇑, Elisabeta Botez, Oana-Viorela Nistor, Doina Georgeta Andronoiu, Gabriel-Danut Mocanu Food Science and Engineering Faculty, ‘‘Dunarea de Jos’’ University of Galati, 111 Domneasca Street, 800201, Romania a r t i c l e i n f o Article history: Received 24 July 2014 Received in revised form 9 March 2015 Accepted 12 March 2015 Available online xxxx Keywords: Moisture Drying combined method Microwave Pumpkin a b s t r a c t This study was carried to determine the influence of hot air drying process and combined methods on physicochemical properties of pumpkin (Cucurbita moschata) samples. The experiments in hot air chamber were lead at 50, 60 and 70 °C. The combined method consists of a triple combination of the main drying techniques. Thus, in first stage the samples were dried in hot air convection at 60 °C followed by hot air ventilation at 40 °C simultaneous with microwave. The time required to reduce the moisture content to any given level was highly dependent on the drying conditions. So, the highest value of drying time in hot air has been 540 min at 50 °C, while the lowest time has been 189 min in hot air combined by microwave at 40 °C and a power of 315 W. The samples dried by hot air shows a higher rehydration capacity than samples dried by combined method. Ó 2015 Elsevier Ltd. All rights reserved. 1. Introduction Pumpkin (Cucurbita moschata) is one of the most important fruit grown in the world because of its nutritional qualities and health protective value of the seeds (especially oil) (Yang, Zhao, & Lv, 2007), but also by fleshy shell. Pumpkin (C. moschata) is one of the vegetables that meet the requirements of healthy nutrition. It is a tasty and valuable vegetable crop, containing a lot of biologically active substances and distinguished for dietary qualities (Gajewski, Radzanowska, Danilcenko, Jariene, & Cerniauskiene, 2008). It is a good source of carotene and water soluble vitamins (Arévalo-Pinedo & Murr, 2006). It is rich in phenolics, flavonoids (Que, Mao, Fang, & Wu, 2008), polysaccharides, mineral salts, vitamins, and other substances beneficial to health (Yang et al., 2007). Abbreviations: MW, microwave drying; HA, hot air drying; MR, moisture ratio, dimensionless; DR, drying rate, (g water/g dry matter)/min; RR, rehydration ratio, dimensionless; RC, rehydration capacity, %; Wi, initial weight of pumpkin samples, g; Wd, weight of pumpkin samples after drying, g; Wr, weight of rehydrated pumpkin samples, g; M, moisture content, g water/g dry matter; M0, initial moisture content, g water/g dry matter; Me, equilibrium moisture content, g water/g dry matter; RH, relative humidity, %; t, drying time, min. ⇑ Corresponding author. Tel./fax: +40 236 460165. E-mail address: florika05@mail.ru (L. Seremet (Ceclu)). Because of the high level of water (96%) from pumpkin the product can be easily spoiled (Perez & Schmalko, 2009). Drying is used to remove water from foods. As a consequence, it prevents (or inhibits) development of microorganisms, improves food preserving and reduces the weight and bulk of food for cheaper transport and storage. As well, the reduction in moisture content below certain level can reduce the microbial damages of dried food materials and accompanied by proper treatment (Jangam, Law, & Mujumdar, 2010). So, drying is an excellent way to preserve pumpkin flesh that can add variety to meals and provide delicious and nutritious snacks. Dried and rehydrated fruits and vegetables, as pumpkin, are key ingredients in dairy products, breakfast cereals, dietetic foods formulated for people suffering from physiological disorders or for healthy people with additional needs and traditional foods such as puddings, desserts, cakes, biscuits. Product rehydration behavior must be known as total or partial reconstitution of water (Contreras, Martín-Esparza, & Martínez-Navarrete, 2012). Therefore, dried pumpkin may be a finished product or a halffinished product, subject to further processing. Properly selected drying method of the raw material may increase the quality of the finished product (Sojak, & Głowacki, 2010). Hot air drying of agricultural products is one of the most popular preservation methods because of its simplicity and low cost (Diamante, Ihns, Savage, & Leo Vanhanen, 2010). However, it causes the degradation of sensitive components leading to the losses of sensorial and other http://dx.doi.org/10.1016/j.foodchem.2015.03.125 0308-8146/Ó 2015 Elsevier Ltd. All rights reserved. Please cite this article in press as: Seremet (Ceclu), L., et al. Effect of different drying methods on moisture ratio and rehydration of pumpkin slices. Food Chemistry (2015), http://dx.doi.org/10.1016/j.foodchem.2015.03.125 2 L. Seremet (Ceclu) et al. / Food Chemistry xxx (2015) xxx–xxx important properties of the dried products. For improving drying conditions combined drying techniques can be used, such as vacuum or convective drying using high frequency radiations like microwave, radio frequency, and infrared heating (Contreras, Martín-Esparza, Chiralt & Martínez-Navarrete, 2008; Kassem, Shokr, El-Mahdy, Aboukarima, & Hamed, 2011). Microwave drying has several advantages over conventional hot air drying, such as high drying rate and short drying time (Sharma & Prasad, 2004; Wang, Wang, & Yu, 2007). Heat is generated when microwave interacts with the polar water molecules in fruits and vegetables and significantly high drying rate was achieved compared to air drying (Workneh, Raghavan, & Gariepy, 2011). Heating is immediate due to irradiative energy transfer; hence the surface-to-center conduction stage is largely eliminated due to gradual vapor pressure differences. Under microwave drying, internal heat generation leads to an increase in internal temperature and vapor pressure, both of which help liquid flow towards the surface, thus increasing the drying rate. MW drying offers opportunities to shorten the drying time and improves the final quality of the dried products (Zhang, Tang, Mujumdar, & Wang, 2006). The variance of moisture content in pumpkin is important for process design and conditions. This was calculated using mathematical models, empirical, semi-empirical or theoretical, which can optimally describe the drying mechanism (Akgun & Doymaz, 2005). Heat and moisture transfer, induce stresses inside food materials, which consequently lead to many physical changes such as cell wall collapse and shrinkage. Such deformations are the main determinant of textural properties in foods (Prachayawarakorn, Tia, Plyto, & Soponronnarit, 2008). Some parameters are related to the properties of the sample such as thickness, shape, particle size, drying air temperature and relative humidity. Therefore, modeling of a drying process is a complex task, because pumpkin exhibits a capillary porous structure and kinetics of this liquid removal depends on the material properties of the solid phase as well as on cellular structure. The principle of modeling is based on a set of mathematical equations which can satisfactorily explain the drying system. The aim of the present study was to determine the influence of different drying methods, convective drying and combined drying methods on the drying curves and rehydration capacity, due to its highest technological efficiency. The experimental results were interpreted by statistical analysis. 2. Materials and methods 2.1. Raw material Pumpkin (C. moschata) is cultivated all over the Romanian territory. This pumpkin variety is characterized by some specific issues like: pear shape, thin peel and flesh colored in yellow to dark orange, which varies due to the carotenes percentage content. The pumpkin is well known for the main characteristics of flesh, an important source of b-carotene, vitamins and minerals. This crop is also valuable for seeds and flowers. Pumpkins were purchased from the local market, from Galati, Romania hand peeled and washed in running tap water. The pumpkins were cut into cylinders with 5 mm thickness and diameter 25 mm, weighting 50 ± 1 g. The zone near the peel (<10 mm) was removed because of its different texture. 2.2. Equipments The drying process was carried in a convection microwave oven (SHARP R-94ST Inverter Germany). Humidity/Temperature Transmitter for High Humidity and Chemical Applications (EE33 Series) was used for measurement of the temperature (°C) and RH (%) inside the drying oven. 2.3. Drying process In this study two different drying methods were analyzed as follows: 2.3.1. Hot air drying Pumpkin drying kinetics was examined for a hot-air dryer using air with temperature ranging between 50 and 70 °C and RH between 30.6% and 47.1%. Pumpkin samples were dried from moisture content (M0) of 90.06 ± 0.3% until (Me) 8.40 ± 0.11%. The final moisture content of each sample was measured in order to calculate the moisture content at each weighing interval. Drying tests were replicated three times at each inlet air temperature and averages are reported. 2.3.2. Combined method The combined method consists of a triple combination of the main drying techniques. Thus, in first stage the samples were dried in hot air convection at 60 °C, 180 min, followed by hot air ventilation at 40 °C simultaneous with microwave at different powers (105 W – 30 min, 210 W – 15 min and 315 W – 9 min). From the starting of the drying the change in the sample weight was recorded at the time intervals of 30 min for hot air drying and 3 min for microwave drying. The drying method labels for these set are (60 °C/C40 °C + MW105 W), (60 °C/C40 °C + MW1210 W), (60 °C/C40 °C + MW315 W). 2.4. Mathematical modeling of drying Drying curves obtained under controlled conditions provide important information regarding the water transportation mechanisms, and they are used in the determination of the effective diffusion coefficient (Corrêa et al., 2011). The moisture content (M) at any time of drying (%), was calculated according to Eq. (1): M¼ Wi  Wd  100; % Wi ð1Þ The reduction of moisture ratio with drying time was used to analyze the experimental drying data. Moisture ratio (MR) represents the amount of moisture remaining in the pumpkin samples reported to the initial moisture content. It was calculated using Eq. (2): MR ¼ M  Me M0  Me ð2Þ The equilibrium moisture contents (Me) were determined by drying until no further change in weight was observed for the pumpkin samples in each treatment and drying conditions (Tunde-Akintunde & Ogunlakin, 2013). The drying rate (DR) of pumpkin samples can be determined by Eq. (3): DR ¼ Mtþdt  Mt dt ð3Þ where dt is a time dependent derivative and varies depending on the nature of the heat treatment, such as: the time interval for convection drying is 30 min, while for MW using the treatment time is 3 min. Please cite this article in press as: Seremet (Ceclu), L., et al. Effect of different drying methods on moisture ratio and rehydration of pumpkin slices. Food Chemistry (2015), http://dx.doi.org/10.1016/j.foodchem.2015.03.125 3 L. Seremet (Ceclu) et al. / Food Chemistry xxx (2015) xxx–xxx 2.5. Rehydration analysis (a) 1 Hot Air 50°C Hot Air 60°C Hot Air 70°C 0.8 Moisture ratio, MR The rehydration characteristics of dried products are known as quality parameters and indicate if physical and chemical changes occurred during the drying process due to process conditions, pre-treatments and sample composition (Apati, Furlan, & Laurindo, 2010). Rehydration means refreshing the dehydrated or dried products in water. Dried samples were put into 50 ml of cold water in Petri dishes. Samples were out into a dish, after their surface was covered with a piece of filter paper to soak the excess water. The samples’ weight was recorded and the rehydration ratio was calculated according to Eq. (4): 0.6 0.4 0.2 0 Wr Wd The rehydration capacity, described as percentage water gain, was calculated from the samples weight difference before and after the rehydration (Maskan, 2001) as follows (Eq. (5)): Mr RC ¼  100; % M0  Me 0 ð4Þ ð5Þ 2.6. Scanning electron microscopy (SEM) analysis (b) 60 120 180 240 300 360 1 420 480 540 Time, min Hot Air 60˚C 60˚C/C40˚C+MW105W 0.8 Moisture ratio, MR RR ¼ 60˚C/C40˚C+MW210W 60˚C/C40˚C+MW315W 0.6 0.4 0.2 The structure of the dried pumpkin slices was examined using a scanning electron microscope SEM Quanta 250 FEI with X-ray and WetSTEM detector. To analyze the changes produced by the drying temperatures which can affect the cellular structure of dried products, the samples were identically prepared (same size). Thin slices of about 1 mm thick were cut from the dried samples, fixed on the SEM stub and studied under the same conditions: High Vacuum mode at a pressure 100 kPa, mag 600. 2.7. Statistical analysis and evaluation Empirical modeling of the drying behavior of agricultural products often requires the statistical methods of regression and correlation analysis. Linear and nonlinear regression models are important tools to find the relationship between different variables, especially those for which no established empirical relationship exists (Omid, Baharlooei, & Ahmadi, 2009). A non-linear multiple regression analysis was performed using the drying mathematical models and the experimental data. The experimental data were interpreted by means of a non-linear regression and statistical analysis modeled with the DataFit 9.0.59 – program. The drying models which are expressing the best the experimental data were selected based on the mean relative error (MRE), the standard error of estimate (SEE) and coefficient of multiple determinations (R2). Time series analyses are forecasted by including an automatic selection model procedure. Models include exponential smoothing, moving averages, random walks, linear and nonlinear trends. 3. Results and discussions 3.1. Drying data Fig. 1 provides information on the hot air drying curves for pumpkin samples and the combined methods curves. These figures contain the experimental results of pumpkin having an average initial moisture content of 90.06 ± 0.3%. The drying process is characterized by a progressive decrease in moisture content versus time. The necessary time to achieve the moisture equilibrium content (8.40 ± 0.11%) for pumpkin samples dried by hot air was 540 min 0 0 60 120 180 240 300 360 420 Time, min Fig. 1. Effect of drying air temperature and drying time on the moisture ratio of pumpkin samples: (a) hot air drying; (b) combined method drying. (50 °C), 420 min (60 °C) and 330 min (70 °C). Regarding the combined method, the drying time was 210 min (60 °C/ 40 °C + 105 W), 195 min (60°/40 °C + 210 W) and 189 min (60 °C/ 40 °C + 315 W). Drying curves (Fig. 1.) were experimentally obtained by plotting the moisture content versus drying time. Eq. (3) is a derivative time and has the role of calculating the drying rate from the curves determined by process kinetics. Drying curves describe two distinct stages with a short constant drying rate time for hot air drying (hot air 50 °C and hot air 60 °C) (Fig. 2(a)). In first stage, when moisture content was high (0.8200– 0.9085 g water/g dry matter), drying rate increased with drying temperature, reaching the highest value at 70 °C – 0.483 (g water/g dry matter)/min, in which the unbound water is removed. Water evaporates as if there is no solid present, and its rate of evaporation is not dependent on the material being dried. In the second stage, the drying rate is decreasing, up to 0.0083–0.0056 due to water linkage. In the first stage, the drying rate reached its maximum level, 0.3476–0.483 (g water/g dry matter)/min. Then falling drying rate period occurred after 30 min. It can be noticed, from the curves that the drying temperature had a significant effect on the DR. These results are in agreement with the observation of earlier researchers (Akpinar, 2006; Therdthai & Zhou, 2009; Workneh et al., 2011). For the combined method the classical shape of the drying curves can be observed as a variance of drying rate with a constant drying rate stage (Fig. 2(b)) (Mujumdar, 2006). Drying rate increased with high power level (from 0.2123 g water/g dry matter/min for 105 W to 0.4057 g water/g dry matter/min for 315 W) at the same moisture content, 0.3247 (g water/g dry matter). The result indicated that mass transfer within Please cite this article in press as: Seremet (Ceclu), L., et al. Effect of different drying methods on moisture ratio and rehydration of pumpkin slices. Food Chemistry (2015), http://dx.doi.org/10.1016/j.foodchem.2015.03.125 4 L. Seremet (Ceclu) et al. / Food Chemistry xxx (2015) xxx–xxx (a) Hot Air 50°C 0.5 DR, (g water/g dry matter)/ min Hot Air 60°C Hot Air 70°C 0.4 0.3 0.2 0.1 0 0 DR, (g water/g dry matter)/min (b) 100 200 300 400 500 Time, min 0.45 0.3 0.15 60°C/C40°C+MW105W 60°C/C40°C+MW210W 60°C/C40°C+MW315W 0 0 30 60 90 120 150 180 210 Time, min Fig. 2. Drying rate of pumpkin samples changes with drying time. (a) Hot air drying; (b) combined method drying. the sample is rapid for greater microwave power heating, 315 W/ 9 min, because more heat is generated within the sample, creating a larger vapor pressure differential between the center and the surface of products (Soysal, Öztekin, & Eren, 2006; Therdthai & Zhou, 2009; Wang et al., 2007; Workneh et al., 2011; Özbek & Dadali, 2007). 3.2. Rehydration Removal of water from a cellular structure induced variations in the physico-chemical properties of the system. The rehydration characteristics of pumpkin samples dried at different temperatures and combined method are shown in Fig 3. Rehydration is a complex process and indicates the physical and chemical changes induced by drying treatments. It can be also observed from Fig. 3 that the rehydration ratio of pumpkin samples is increasing with the decrease of temperature and microwave power (hot air drying – 79.81 ± 0.3% at 70 °C, 88.97 ± 0.3% at 60 °C, 92.92 ± 0.3% at 50 °C, combined method – 77.68% at 315 W, 85.44% at 210 W, 87.05% at 105 W). The lower rehydration values (77.68–79.81%) are evidence of higher temperature (70 °C) and microwave power (315 W), which have the capacity to break the cellular structure (Fig. 4). Such a behavior is reported in a few similar studies regarding microwave assisted air drying of apple and mushrooms (Singh & Pandey, 2011). When placing the dried samples into water, the cell walls absorb water. Then due to the natural elasticity of the cellular structure, the cells returned to their original shape by drawing water into inner cavities. The volume of absorbed water increased with increasing rehydration time, 77.6 ± 0.3–93.0 ± 0.3% in 210 min in water at 25 °C irrespective of the air temperature/microwave power. This was manifested in a relatively rapid rate of reconstitution (half from total moisture content) during the early stages – in first 30 min increased up to 40.5 ± 0.3–54.2 ± 0.3%, followed by a more gradual increase in rate, tended towards a maximal rehydration ratio, 79.81–92.92% for pumpkin samples dried by hot air and 77.6 ± 0.3–87.1 ± 0.3% for pumpkin samples dried by combined method. This rapid moisture uptake is very likely due to surface and capillary suction (Singh & Pandey, 2011; Souzaa, Pimentela, Pradob, Marquesb, & Naraina, 2011). 3.3. Scanning electron microscopy (SEM) results Effects of different drying methods and conditions on the structure of dried pumpkin samples was observed under scanning electron microscopy. From the obtained images (Fig. 5) can be observed temperature effects on tissue structure by comparing fresh sample with dried samples. At fresh sample can be seen a fraction of cell where vacuoles are well defined by cell walls (lighter areas to a pale white) as well as parts similar to pockets filled with cellular juice (darker areas). More noticeable are the results for the dried samples, which are no longer indicating these vacuoles inside cell due to water evaporation and merge cell walls or even to breakage the cell walls, enlightening some severe contraction of tissue. Thus, it is obvious that the increasing of tissue dehydration is accompanied by heightened deformation of cell texture and it is related to collapse as a result of loss of cell turgor. 3.4. Statistical analysis and evaluation Response Surface Methodology (RSM) was used to investigate the main effects of drying methods. DataFit was used to fit response surfaces and optimize the drying process. From the 79 9 Hot Air 1 100 Rehydration ratio, RR I II 3 6 75 RC, % Hot Air 50˚C 50 Hot Air 60˚C 3 Combined method 2 III 1 - Hot Air 50°C 2 - Hot Air 60°C 3 - Hot Air 70°C I - 60°C/40°C+105W II - 60°C/40°C+210W III - 60°C/40°C+315W Hot Air 70˚C 60˚C/C40˚C+MW105W 25 60˚C/C40˚C+MW210W 60˚C/C40˚C+MW315W 0 0 50 100 150 200 Time, min Fig. 3. Influence of drying temperature/drying treatments on rehydration ratio. 0 Fig. 4. Influence of drying temperature/drying treatments on rehydration capacity. Please cite this article in press as: Seremet (Ceclu), L., et al. Effect of different drying methods on moisture ratio and rehydration of pumpkin slices. Food Chemistry (2015), http://dx.doi.org/10.1016/j.foodchem.2015.03.125 5 L. Seremet (Ceclu) et al. / Food Chemistry xxx (2015) xxx–xxx Fresh samples HA 50°C HA 60°C HA 70°C 60°C/40°C+105W 60°C/40°C+210W 60°C/40°C+315W Fig. 5. SEM micrographs for pumpkin fresh and dried samples. models which were generated by Data Fit program, for moisture ratio (MR) and drying rate (DR), the equation which gave the best fitting curve was of the dried samples of pumpkin compared with the other equations (Figs. 6 and 7). Moisture ratio data 3 5 Y ¼ a þ bX 1 þ cX 21 þ dX 2 þ eX 22 þ fX 2 þ gX 42 þ hX 2 Equations of regression represent the prediction of a dependent variable (moisture content) based on independent variables (drying temperature and drying conditions). The moisture content values for both simple and combined drying are folded on 4 different mathematical models (Newton’s and Page’s exponential models, a logarithmic model, Pabis and Handerson modified model). Prediction is done by fitting a surface to the experimental data points that minimizes the standard error of estimation (Table 1). The standard error of estimated values is the sum of the differences between each obtained value and its value as predicted by the regression equation. From Table 1 it can be observed that the coefficient of multiple determinations for moisture content variation during time and temperature evolution are strongly positive reaching values of 0.97–0.98. The strong correlation indicates that more than 97% of the total variation in Y can be explained by the linear relationship between X and Y (as it was described by the regression equation). As expected, the other 3% or less of the total variation in Y remains unexplained (Abraham & Ledolter, 2006). The mathematical formula or the analytical expression pattern was chosen from a set of non linear approximation models offered by the soft DataFit 9.0.59: Table 1 Fit information. Coefficient of multiple determination (R2) Adjusted coefficient of multiple determination (Ra2) Standard error of the estimate Proportion of variance explained MR hot air MR combined method 0.9831 0.9799 0.9773 0.9722 4.6828 98.31% 0.0566 97.74% 3 5 Y ¼ a þ bX 1 þ cX 21 þ dX 2 þ eX 22 þ fX 2 þ gX 42 þ hX 2 where: Y is the value predicted for the moisture content (dependent variable); X1 represents a time-dependent vector; X2 is temperature/drying period dependent vector; a is the constant term of the regression line (when the value for X = 0) – the independent variable; b, c, d, e, f, g, h are coefficients which are indicating the amount by which the Y value is decreasing/increasing when X value is modified by one unit. The prediction of the dependent variable Y, can be mathematically formulated by determining the quantities a, b, c, d, e, f, g, h from the equation generated by the program in order to describe the mathematical model. Multiple non linear regression equations describes the prediction of the dependent variable (moisture content), depending on the independent variables (temperature of drying/the combination of drying conditions and time). From the graphs (Figs. 6 and 7) it can be seen that the multiple non linear regression equation has the same form for both cases (drying in hot air convection and convection combined with microwave). 3.6. Drying rate data 3 5 Y ¼ a þ bX 1 þ cX 21 þ dX 2 þ eX 22 þ fX 2 þ gX 42 þ hX 2 Prediction of drying rate was based on a range of process parameters that were determined in the preliminary investigations including the optimum values. For both methods, the graphs have a linear behavior which means that the experimental data for drying rate are similar to the predicted ones (Fig. 8(a) and (b)). The graphic is based on the residual probability, which is obtained as a difference between the predicted and the experimental data. The differences of the values have a negligible level of significance and arranged after a linear model. (Abraham & Ledolter, 2006; Wang, 2011). Following the obtained results, it can be concluded that by combined method with microwaves, the drying time is reduced with almost 50% relative to classical convection. But, due to the thermal shock, a profound destruction of cell walls occurs, leading to a lower capacity of hydration compared to the samples dried by convection. Please cite this article in press as: Seremet (Ceclu), L., et al. Effect of different drying methods on moisture ratio and rehydration of pumpkin slices. Food Chemistry (2015), http://dx.doi.org/10.1016/j.foodchem.2015.03.125 6 L. Seremet (Ceclu) et al. / Food Chemistry xxx (2015) xxx–xxx 4. Conclusions Water removal during drying of the pumpkin slabs occurs in the falling rate period. The reconstitution attributes are indicative of the degree of structural modification occurred during drying. The rehydration characteristic is correlated positively with the air temperature and drying treatment. The methodology we have proposed, can give a good reason to be used in case of new experimental drying ovens and/or drying products or exploratory drying processes, when the most advantageous drying schedule must be selected from a number of variants. Since the drying results can be obtained by simulation of a large number of factor combinations and the outcomes processed by using a multifactor experiment, the experimental work can be reduced, the presence and magnitude of factors interactions can be estimated and recommendations over a large variety of conditions can be outlined. Acknowledgements The morphological analyses were supported by the Laboratory of Instrumental Analysis and Analytical Chemistry (LAICA), Research Center from Alexandru Ioan Cuza University of Iasi, Romania. Data Fit 9.0.59 – program is acknowledged for providing technical support for mathematical modeling. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.foodchem.2015. 03.125. References Abraham, B., & Ledolter, J. (2006). Introduction to regression modeling. Belmot, CA: Duxbury Press. Akgun, N. A., & Doymaz, I. (2005). Modelling of olive cake thin-layer drying process. Journal of Food Engineering, 68, 455–461. Akpinar, E. K. (2006). Mathematical modelling of thin layer drying process under open sun of some aromatic plants. Journal of Food Engineering, 77, 864–870. Apati, G. P., Furlan, S. A., & Laurindo, J. B. (2010). Drying and rehydration of Oyster mushroom. Brazilian Archives of Biology and Technology, 53(4), 945–952. Arévalo-Pinedo, A., & Murr, F. E. X. (2006). Kinetics of vacuum drying of pumpkin (Cucurbita maxima): Modeling with shrinkage. Journal of Food Engineering, 76, 562–567. Contreras, C., Martín-Esparza, M. E., Chiralt, A., & Martínez-Navarrete, N. (2008). Influence of microwave application on convective drying: Effects on drying kinetics, and optical and mechanical properties of apple and strawberry. Journal of Food Engineering, 88, 55–64. Contreras, C., Martín-Esparza, M. E., & Martínez-Navarrete, N. (2012). Influence of drying method on the rehydration properties of apricot and apple. Journal of Food Process Engineering, 35, 178–190. Corrêa, P. C., Botelho, F. M., Horta Oliveira, G. H., Goneli, André Luis Duarte, Resende, O., & de Carvalho Campos, S. (2011). Mathematical modeling of the drying process of corn ears. Acta Scientiarum Agronomy, 33(4), 575–581. Diamante, L. M., Ihns, R., Savage, G. P., & Leo Vanhanen, L. (2010). A new mathematical model for thin layer drying of fruits. International Journal of Food Science and Technology, 45, 1956–1962. Gajewski, M., Radzanowska, J., Danilcenko, H., Jariene, E., & Cerniauskiene, J. (2008). Quality of pumpkin cultivars in relation to sensory characteristics. Notulae Botanicae Horti Agrobotanici Cluj-Napoca, 36(1), 73–79. Jangam, S.V., Law, C.L., Mujumdar, A.S. (2010). Drying of Foods, Vegetables and Fruits, Vol. 1, ISBN: 978-981-08-6759-1. Kassem, A. S., Shokr, A. Z., El-Mahdy, A. R., Aboukarima, A. M., & Hamed, E. Y. (2011). Comparison of drying characteristics of Thompson seedless grapes using combined microwave oven and hot air drying. Journal of the Saudi Society of Agricultural Sciences, 10, 33–40. Maskan, M. (2001). Drying, shrinkage and rehydration characteristics of kiwifruits during hot air and microwave drying. Journal of Food Engineering, 48, 177–182. Mujumdar, A. S. (2006). Handbook of Industrial Drying (3rd ed.). CRC Press, Taylor & Francis Group. Omid, M., Baharlooei, A., & Ahmadi, H. (2009). Modeling drying kinetics of pistachio nuts with multilayer feed-forward neural network. Drying Technology, 27, 1069–1077. Özbek, B., & Dadali, G. (2007). Thin-layer drying characteristics and modelling of mint leaves undergoing microwave treatment. Journal of Food Engineering, 83, 541–549. Perez, N. E., & Schmalko, M. E. (2009). Convective drying of pumpkin: Influence of pretreatment and drying temperature. Journal of Food Process Engineering, 32, 88–103. Prachayawarakorn, S., Tia, W., Plyto, N., & Soponronnarit, S. (2008). Drying kinetics and quality attributes of low-fat banana slices dried at high temperature. Journal of Food Engineering, 85, 509–517. Que, F., Mao, L., Fang, X., & Wu, T. (2008). Comparison of hot air-drying and freezedrying on the physicochemical properties and antioxidant activities of pumpkin (Cucurbita moschata Duch.) flours. International Journal of Food Science and Technology, 43, 1195–1201. Sharma, G. P., & Prasad, S. (2004). Effective moisture diffusivity of garlic cloves undergoing microwave-convective drying. Journal of Food Engineering, 65, 609–617. Singh, N. J., & Pandey, R. K. (2011). Rehydration characteristics and structural change of sweet potato cubes after dehydration. American Journal of Food Technology, 6(8), 709–716. Sojak, M., & Głowacki, Sz. (2010). Analysis of giant pumpkin (Cucurbita maxima) drying kinetics in various technologies of convective drying. Journal of Food Engineering, 99, 323–329. Souzaa, D.S., Pimentela, J.D.R., Pradob, M.M., Marquesb, L.G., Naraina, N. (2011). Rehydration characteristics of freeze-dried avocado (Persea americana). International Congress on Engineering and Food - ICEF11, Food Process Engineering in a Changing World, Athens, Greece. Soysal, Y., Öztekin, S., & Eren, Ö. (2006). Microwave drying of parsley: Modelling, kinetics, and energy aspects. Biosystems Engineering, 93(4), 403–413. Therdthai, N., & Zhou, W. (2009). Characterization of microwave vacuum drying and hot air drying of mint leaves (Mentha cordifolia Opiz ex Fresen). Journal of Food Engineering, 91, 482–489. Tunde-Akintunde, T. Y., & Ogunlakin, G. O. (2013). Mathematical modeling of drying of pretreated and untreated pumpkin. Journal of Food Science and Technology, 50(4), 705–713. Wang, J.C. (2011). Multiple Linear Regression Analysis of Real Estate Data Chapter 11. Linear Regression Multiple Linear Regression. (WMU) Stat2160 S2160. Wang, J., Wang, J. S., & Yu, Y. (2007). Microwave drying characteristics and dried quality of pumpkin. International Journal of Food Science and Technology, 42, 148–156. Workneh, T.S., Raghavan, V., and Y. Gariepy (2011). Microwave assisted hot air ventilation drying of tomato slices. International Conference on Food Engineering and Biotechnology, IPCBEE (vol. 9). Singapore: IACSIT Press. Yang, X., Zhao, Y., & Lv, Y. (2007). Chemical composition and antioxidant activity of an acidic polysaccharide extracted from Cucurbita moschata Duchesne ex Poiret. Journal of Agricultural and Food Chemistry, 55, 4684–4690. Zhang, M., Tang, J., Mujumdar, A. S., & Wang, S. (2006). Trends in microwaverelated drying of fruits and vegetables. Trends in Food Science & Technology, 17, 524–534. Please cite this article in press as: Seremet (Ceclu), L., et al. Effect of different drying methods on moisture ratio and rehydration of pumpkin slices. Food Chemistry (2015), http://dx.doi.org/10.1016/j.foodchem.2015.03.125