Microwave Drying Characteristics, Kineticsand Drying Quality of Onion Slices (Allium CepaL.)

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1 AENSI Journals Advances in Environmental Biology ISSN EISSN Journal home page: Microwave Drying Characteristics, Kineticsand Drying Quality of Onion Slices (Allium CepaL.) 1,2 Hany S. EL-Mesery and 1 HanpingMao 1 Key Laboratory of Modern Agriculture Equipment and Technology, School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang , PR China 2 Department of Crop Handling and Processing, Agricultural Engineering Research Institute, Agricultural Research Center, Dokki, 12311, Giza, Egypt Address For Correspondence: HanpingMao, Key Laboratory of Modern Agriculture Equipment and Technology, School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang , PR China maohp@ujs.edu.cn This work is licensed under the Creative Commons Attribution International License (CC BY). Received 28 January 2016; Accepted 20 March 2016; Available online 24 March 2016 ABSTRACT The effect of microwave power ranging from 200 W to 400 W was studied on onion slices (Allium cepal.). Drying continued until slices moisture reduced to 0.07 g water/g dry solids. The various factors of the sample that were investigated were drying time, drying rate, kinetic rate constant and quality of dry product. It was observed that the drying rate was increasing with increase in microwave power and drying time decrease with increase in microwave power. The rehydration ratio and total colour difference between fresh and dried onion slices was found to increase with increasing microwave output power while shrinkage ratio decreased with increase microwave power. Various thin layer drying models were considered for the evaluation of parameters of drying kinetics, out of which semi empirical Midilli et al. model presented the best fit for all conditions of drying, giving a high value of R 2 (> 0.998) and low value of SEE, RMSE and chi-square (χ 2 ). KEYWORDS: Microwave, Drying, Onion, Quality, Modelling. INTRODUCTION Drying of materials having high moisture content is a complicated unit operation process involving simultaneous, coupled heat and mass transfer, particularly under transient conditions [1]. It is one of the oldest methods of preservation and one of the most important in post-harvest processing of fruits, vegetables and other agricultural products [2]. Also, it brings about substantial reduction in weight and volume thereby minimizing packaging, storage and transportation costs [3]. Drying not only affects the moisture content of the product, but also alters other physical, biological and chemical properties such as enzymatic activity, microbial spoilage, viscosity, hardness, aroma, flavor and palatability of foods [4]. Onion (Allium cepa L.) is considered one of the second most important horticultural crops worldwide and has always been most widely traded than most vegetables [5] as a seasoning, a food component as well as in medical applications. Dried onions are a product of great significance in world trade produced either as flaked, minced, chopped or powdered forms [6].Generally, onion is dried for efficient storage and processing [7] but also to reduce bulk handling, facilitate transportation and to allow for its use during the off-season [8]. However, Copyright 2016 by authors and Copyright, American-Eurasian Network for Scientific Information (AENSI Publication).

2 179 Hany S. EL-Mesery and HanpingMao, 2016 the use of dried onion, which has a decreased mass compared to fresh onion, requires that an efficient and effective dehydration method be developed and employed. The colour and flavour of dried onions are considered the most important quality attributes that affect its acceptance by consumers. The non-enzymatic browning reactions, measured in terms of an optical index and the loss in pungency, measured in terms of thiolsulphinate or pyruvate concentration, are considered the dominant factors in quality deterioration during drying and storage of dried onion [9]. Many conventional thermal drying methods such as hot air drying result in slow drying rates in the falling rate period of drying [10]. The long drying times at relatively high temperatures during the falling rate periods often lead to undesirable thermal degradation of the finished products and consume more energy and yield low drying efficiency [11]. Microwave drying is having advantage of high drying rates, high energy efficiency, better product quality and efficient space utilization. Microwave heating is a result of dipolar interaction of water molecules inside the food materials. The polar water molecule tend to align themselves according to change in electrical field and heat is produced due to friction between oscillating molecules. This rapid internal energy generation causes the pressure build up and results in rapid evaporation of water [12]. Several studies have been carried out by researchers to investigate the microwave drying characteristics of agricultural materials. For example, Cabbage [13], Potato [14], Mushroom [15], carrot [16], basil [17], white mulberry [18], celery leaves [19], mango ginger [20]. The objective of present investigation is (a) to study the effect of microwave power on dying kinetics of onion slices. (b) to select the best model among several thin layer drying models to describe the moisture removal behaviour during microwave drying of onion slices. MATERIALS AND METHODS 2.1.Materials: Fresh onions procured in bulk from a local market and stored in a refrigerator at 4 o C were used in the present investigation. To prepare the onions for the drying experiments, they were removed from the refrigerator and allowed to equilibrate in an ambient environment before being hand peeled. The onions were then cut into slices of approximately 5 ±0.1 mm thick using a sharp stainless steel knife. The direction of cut was perpendicular to the vertical axis of onion bulbs. A micrometer was used to check the thickness and uniformity of each slice at three different locations, and acceptance was based on consideration of the average value and the deviation of each value from the desired thickness. A sample of approximately 100 g of onion slices ranging from 5 to 8 cm in diameter and 5±0.1 mm in thickness was then carefully setup as a single layer on the drying tray for use in the drying experiment. The initial moisture content of the onion slices, expressed in gwater /gdry matter was measured by the oven drying method at an air temperature of 105 o C and a drying period of 24 h [21]. The initial moisture content of the onion slices was found to be about7.3 gwater /gdry matter Drying equipment : Microwave drying was performed in a 230-V, 50-Hz, 2900-W laboratory digital microwave oven (WEG- 800A, Jinan, China). The microwave oven has the capability of operating at different microwave stages, from 100 to 1000 W. The area on which microwave drying was performed was cm in size and consisted of a rotating glass plate of 28 cm in diameter at the base of the oven. The glass plate rotates 5 times per min and the direction of rotation can be changed by pressing the on/off button. Time adjustment was performed with the aid of the oven s digital clock Drying procedures: Drying trials were carried out at three microwave generation power levels: 200, 300, and 400 W. The onion slices (100 g) selected from uniform and healthy plants. Three drying trials were conducted at each power level. The values obtained from these trials were averaged and the drying parameters determined. The rotating glass plate was removed from the oven every 30 s during the drying period and moisture loss determined by weighing the plate using a digital balance (Mettler Toledo PM30, Germany) with 0.01 g precision Quality evaluation: For quality evaluation purposes, similar drying experiments were conducted separately under the same microwave drying conditions Colour assessment: Sample colour was measured before and after drying using a Hunter Lab Colour Flex A model colorimeter (Hunter Lab., Reston, VA). The total colour difference between fresh and dried onion slices δewas defined in equation (1) as

3 180 Hany S. EL-Mesery and HanpingMao, 2016 where the subscript o refers to the colour reading of fresh onion slices and L, a and b indicate the brightness, redness and yellowness of the dried sample, respectively. Fresh onion slices were used as a reference and a larger δe denotes a greater change incolourdue to drying. The browning index (BI), indicating the purity of the brown colour of the onion slices was calculated using equations (2) and (3)according to [22] Shrinkage ratio (Sr): The shrinkage ratio of the dried onion was evaluated by a volumetric displacement method using n-heptane as the working liquid. Ten slices were tested and the average values were reported. The shrinkage ratio (Sr) was calculated as presented in Eq. 4. where V o and V d is the average volume of slices before and after drying, respectively Rehydration ratio (Rr): The rehydration ratio of dried onion slices was determined according to [23]by immersing 10 g of dried sample in 50 ml of water at a temperature of 35 o C andafter 5 h,samples were drained and weighed. The rehydration ratio (Rr) was calculated as the ratio of the mass of the rehydrated sample to that of dry sample using equation (5) Analysis: A number of theoretical, semi-theoretical and empirical drying models have been reported in the literature. The most frequently used type of model for thin layer drying is the lumped parameter type, such as the Newton equation [24,25]. The moisture ratio during drying is determined using equation (6) i.e. where M is the moisture content of the product at any time, M e is the equilibrium moisture content, M i is the initial moisture content all in kg water /kg dry matter, k is the drying constant (in units of 1/min) and t is the drying time in min. In this analysis, it was assumed that the moisture gradient driving force during drying is a liquid concentration gradient, meanwhilethe effect of heat transfer was neglected as a simplifying assumption. For all experimental conditions, the value of (M-M e )/ (M i -M e ), a dimensionless moisture content was obtained. Because samples were not exposed to uniform relative humidity and temperature continuously during drying, the moisture ratio was simplified as recommended by [26,27,28] and expressed as 2.5. Mathematical modelling of the drying curves: For mathematical modelling, the equations in Table 1 were tested to select the best model for describing the drying curve equation of the onion slices. The moisture ratio of the onion slices during drying was calculated using equation (7). The goodness of fit of the tested mathematical models on the experimental data was evaluated using coefficient of determination (R 2 ) [equation (8)];modelling efficiency (EF), [Eq. (9)]and chisquare test (χ 2 ) [equation (10)] with higher R 2 and EF values and lower χ 2 values indicating a better fit [29].

4 181 Hany S. EL-Mesery and HanpingMao, 2016 MR exp.i is the i th experimental moisture ratio,mr pre.i is the i th predicted moisture ratio, while N and n are the number of observations constants, respectively. RESULTS AND DISCUSSION Onion slices with initial moisture content of 7.31 g water/g dry matter were dried microwave drying to a final moisture content of 0.07 g water/g dry matter. Microwave drying trials were conducted at output power levels of 200, 300 and 400 W andthe influence of each microwave power level on moisture ratio over drying time presented in Fig. 1. The drying time decreased as microwave power level was increased. The times required for the moisture content of onion slices to decrease from 85 to 7 % (w.b) were 25, 20 and 17 min at microwave output power levels of 200, 300 and 400 W, respectively. The effect of microwave power level of decreasing drying time was observed by [30]. The results indicated that mass transfer is more rapid at higher microwave power levels as more heat is generated within the sample, creating a larger vapour pressure differential between the interior and the surface of the product [31,32]

5 182 Hany S. EL-Mesery and HanpingMao, W 300 W 400 W Moisture ratio Time (min) Fig. 1: Variation in moisture ratio overtime of microwave drying of onion slices at different microwave power levels Fitting of mathematical models to the drying curves: The moisture content data observed at the drying experiment were converted into the moisture ratio (MR) and fitted to the 11 models listed in Table 1. The statistical results of the different models, including the drying model coefficients and the comparison criteria used to evaluate goodness of fit, namely, R 2, χ 2 and EF, are listed in Table 2.In all cases, R 2 and EF values for all models under study were higher than 0.98 and 0.97, respectively and χ 2 values were lower than For all drying methods, the Midili et al model gives the best fit of experimental values since the highest values of R 2 and are EF and 0.999, respectively, and the lowest values of χ 2 (0.0004) were obtained (Table 2). Accordingly, the Midili et al model is selected as a suitable model to represent the thin layer drying behaviour of onion slices under different microwave power. The Midilli et al. model has also been suggested by others in the drying of apple and red bell pepper [33]; saskatoon berry [34]; cape gooseberry [35]. Table 2: Average values of statistical parameters of selected models filled to thin-layer drying of onion slices under different microwave drying. Microwave power, W Model R 2 EF χ 2 R 2 EF χ 2 R 2 EF χ 2 Newton Henderson and Pabis Page Modified Page Logarithmic Two-term Modified Hend &Pabis Midilli et al Verma et al Wang and Sing Thompson Rehydration ratio: The rehydration characteristics of a dried product are widely used as indicators of quality. Rehydration is a complex process that is influenced by both physical and chemical changes associated with drying and the treatments preceding dehydration [36]. The rehydration ratio was found to increase with increasing microwave

6 183 Hany S. EL-Mesery and HanpingMao, 2016 output power levels. As found, increasing the microwave power level from 200 to 400 W increased rehydration ratio from 5.93 to 6.87 [37,38] Shrinkage ratio: Increasing the microwave power from 200 to 400 W caused the shrinkage ratio to fall from a high of 0.25 to a low of Thus the lowest values of shrinkage ratio were found when operating at high microwave power (400 W). The low shrinkage under higher microwave power can be attributed to higher product temperatures and the lower moisture content of the external surfaces on both sides of the slice [39,40] Colour assessment: Total colour difference (δe) and browning index of onion slices, calculated from equations (1), (2) and (3) are colorimetric parameters used extensively to characterize variation in colour of foods during processing. The total colour difference and browning index data obtained following the microwave drying method are presented in Table 3. Microwave drying prevented colour change during drying. The colour difference increased with increase in infrared radiation intensity and with air velocity [41, 38]. Table 3: Colour and browning change of onion slices dried at selected processing conditions drying. Microwave power Total colour change (δe) Browning index (BI) 200 W W W Conclusions: This study characterized the influence of drying conditions on the drying behaviour of onion slices using microwave drying. Drying rate is substantially influenced by microwave power level used. Statistical results for that thin-layer drying showed that, the Midilli et al. model given by [MR = a.exp (-k.tn) + b.t] represented the drying characteristics of onion slices better than eleven other frequently used thin layer drying models. For all the conditions studied, R 2 and EF values were higher than and0.997, respectively and χ 2 value were lower than Irrespective of the power applied, microwaving is an effective method of shortening the time required for drying to the desired moisture content without charring the samples. The rehydration ratio and colour difference increased with increase in microwave power while the shrinkage ratio decreased with microwave power. ACKNOWLEDGEMENTS We acknowledge that this work was supported by grants from the National Natural Science Foundation of China (No ), fund name, Damage mechanism in picking up vegetable plug seeding and multiobjective optimization of the seeding pick-up device. the Priority Academic Program Development of Jiangsu Higher Education Institutions (SFE (2014) 37), the Jiangsu Province Synergistic Innovation Center Program of Modern Agricultural Equipment and Technology (No. NZXT ), and the Fund for Independent Innovation of the Jiangsu Province Agricultural Science and Technology (No.CX(15)1033-5). REFERENCES [1] Karimi, F., S. Rafiee, A. Taheri-garavanda and M. Karimi, Optimization of an air drying process for Artemisia absinthiun leaves using response surface and artificial neural network models. Journal of Taiwan International Chemical Engineering, 43(1): [2] Mujumdar, A.S. and C.L. Law, Drying Technology: Trends and Applications in post-harvest processing. Journal of Food Bioprocessing and Technology, 3(6): [3] Okos, M.R., G. Narsimhan, R.K. Singh and A.C. Witnauer, Food dehydration: In Heldman, DR, Lund DB (Ed). Handbook of food engineering Marcel Dekker, New York. [4] Ozbek, B. and G. Dadali, Thin-layer drying characteristics and modeling of mint leaves undergoing microwave treatment. Journal of Food Engineering, 83(4): [5] Griffiths, G., L. Trueman, T. Crowther, B. Thomas, B. Smith, Onions A Global Benefit to Health.Phytother Res., 16: [6] Arslan, D., M.M. Özcan, Study the effect of sun, oven and microwave drying on quality of onion slices. Food Sci Technol., 43: [7] Sarsavadia, P.N., Development of a solar-assisted dryer and evaluation of energy requirement for the drying of onion. Renew Energ, 15: [8] Mota, C.L., C. Lucianoa, A. Diasa, M.J. Barrocab, R.P.F. Guiné, Convective drying of onion: Kinetics and nutritional evaluation. Food Bioprod Process, 88:

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