Drying Characteristics of Okra Slices on Drying in Hot ... - CyberLeninka

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Procedia Engineering 51 (2013) 371 – 374

Chemical, Civil and Mechanical Engineering Tracks of 3rd Nirma University International Conference on Engineering (NUiCONE 2012)

Drying Characteristics of Okra slices on drying in Hot Air Dryer P.K.Wankhadea*, Dr.R.S.Sapkala, Dr.V.S.Sapkalb a

Chemical Technology Department, Sant Gadge Baba Amravati University, Amravati. INDIA

b

V.C., R.T.M. Nagpur University, Nagpur. INDIA.

ABSTRACT: Drying is an essential process in the preservation of agricultural products. Various drying methods are employed to dry different agricultural products. Care must be taken in choosing the drying system. Study comparing traditional drying and other drying methods for the reduction of the drying time and to a significant improvement of the product quality in terms of color texture and taste. Reduces the possibilities, the contamination by insects and microorganisms so that product is prevented. An experimental study was performed to determine the drying characteristics of okra using hot air dryer. For the hot air drying, the test samples were dried in a laboratory scale hot air dryer at a constant air velocity of 1 m/s and air temperature in the range of 40–90 . Drying characteristics of okra slices were investigated in a hot air dryer for a temperature range 60 to 90°C at constant air velocity 1.0 m/s. Results indicated that drying took place in the falling rate period. The sample dried at 40°C was found better in color texture and taste as compared to the samples obtained at 60 and 90°C. The drying data were fitted to different drying models. The performance of these models was investigated by comparing the determination of coefficient, reduced chi-square and root mean square error between the observed and predicted moisture ratios. Keywords: Drying, product quality, moisture content, hot air dryer, mathematical model, air velocity .

1. INTRODUCTION Drying of fruit and vegetables is one of the oldest forms of food preservation method known to man and is the most important process to preserve food since it has great effect on the quality of the dried products. The major objective in drying agricultural products is the reduction of the moisture content to a level, which allows safe storage over an extended period. The removal of moisture prevents the growth and reproduction of microorganisms which cause decay, and minimizes many of the moisture-mediated deteriorative reactions. It brings about substantial reduction in weight and volume, minimizing packing, storage and transportation costs and enables storability of the product under ambient temperatures (Mujumdar, 1995). During drying many changes take place; structural and physic-chemical modifications affect the final product quality, and the quality aspects involved in dry conversation in relation to the quality of fresh products and applied drying techniques. Currently hot air drying is the most widely used method in post-harvest technology of agricultural products. Using this method, a more uniform, hygienic and attractively colored dried product can be produced rapidly. Vegetables and fruits are indispensable part of human diet and can be regarded as the fuel for physiological processes. 25-30% of total produce is being wasted during handling from point of production to consumer’s plates. This wastage can be effectively reduced by applying appropriate method of processing and preservation. In this regard some of the vegetables have been traditionally processed by drying to extend their storage life well beyond few weeks and make it available in off season. * Corresponding author. Tel.: 0721-2530376. E-mail address: [email protected]

1877-7058 © 2013 The Authors. Published by Elsevier Ltd. Selection and peer-review under responsibility of Institute of Technology, Nirma University, Ahmedabad. doi:10.1016/j.proeng.2013.01.051

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P.K.Wankhade et. al / Procedia Engineering 51 (2013) 371 – 374

Okra is a flowering plant, tropical perennial crop growing 3 to 6 feet tall. It is grown throughout the tropical and sub tropical countries. According to FAO data for 2007, okra production all over the world was about 5,941 million tones. The major producer countries include India, Nigeria, Sudan, Pakistan, Iraq and Ghana [7]. Okra can be consumed as a fresh vegetable, cooked vegetable or an additive for soups, salads and stews [16]. Drying is a complex thermal process in which unsteady heat and moisture transfer occur simultaneously [3]. From an engineering point of view, it is important to develop a better understanding of the controlling parameters of this complex process. Mathematical models of the drying processes are used for the control of the drying process. Many mathematical models have been proposed to describe the drying process, of which thin-layer drying models have been widely in use. These models can be categorized as theoretical, semi-theoretical, and empirical [14, 22]. Recently, there has been a lot of research in mathematical modeling and experimental studies of the drying characteristics of various vegetables and fruits. Studies on the sun drying of okra are scarce in the literature. Therefore, the present study was undertaken to study the drying kinetics of okra in hot air dryer to evaluate a suitable drying model for describing the drying process. 2. MATERIALS AND METHODS 2.1 Material The experiment was conducted on effect of drying on storage and dried quality of okra. The fresh okra fruits were collected from local market during Feb- March. The average dimensions of fruit vegetable were 60-80 mm long and 15-20 mm average diameter, dark green colored fruits were selected for the study. The fruits vegetable were thoroughly washed and sliced into 5 mm thickness using sharp sterilized knife. 2.2 Method The experimental set ups used for determining the influence drying temperature on drying behavior of okra. The laboratory scale batch type hot air dryer is used. The slices were then weighed exactly 100 grams for each treatment. The samples, about 100 g, were distributed uniformly in a single layer in the sample tray, and then dried in hot air dryer. These were kept for drying in three replications. The hot air drying was carried by drying the samples at 40 , 60 and 90 air temperatures and a constant air velocity of 1 m/s. The hot air drying was carried out by keeping the weighed slices in steel plates. Observations on physiological loss in weight and color change in each sample were recorded at the particular interval of in 10-20 minutes for hot air drying. The change in color of slices was observed for further analysis. The texture of end produce was also tested by breaking the dried slices and the produce was categorized into different grades. Temperature and relative humidity was recorded throughout the drying period using hygro- thermometer. The hot air drying experiments were conducted at 40, 60 and 90 air temperatures and a constant air velocity of 1 m/s. In each experiment, about 100 g of okra samples were used. Moisture losses of samples were recorded at 10 min intervals for first one hour and 20 min subsequently thereafter for determination of drying curves. To determine the moisture loss of drying samples during experiments, okra samples were taken out of the dryer and weighed at various time intervals, ranging from 10 min at the beginning of the drying to 1hr during the last stage of the process. The moisture loss of samples was determined with the help of a digital electronic balance having an accuracy of 0.01 g. The experiments were repeated for obtaining more accurate results, after that average values were used. Drying was continued until the sample reached the desired moisture level (15+0.5%, w.b.). 2.3 Mathematical modelling The moisture ratio (MR) and drying rate during drying experiments were calculated using the following equations: (1) Drying rate (Rd): (2)


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Table 1. List of models Model Name Newton Page Wang and Singh Logarithmic Two-term exponential Handerson and Pabis Verma et al. Magee

Model MR = exp(-k t) MR = exp(-k tn) MR = 1 + at + bt2 MR = a exp(-k t) + c MR =a exp(-k t)+(1-a)exp(-k a t) MR = a exp(-k t) MR = a exp(-k t) + (1-a)exp(-g t) MR=a+kt1/2

References Mujumdar (1987) Diamante and Munr(1993) Wang and Singh (1978) Yagcioglu et al. (1999) Sharaf-Eldeen et al. (1980) Henderson and Pabis (1961) Verma et al. (1985) Magee et al.(1983) (3)

where, MR, Mo, Me, Mt and Mt+dt are the moisture ratio, initial moisture content, equilibrium moisture content, moisture content at t and moisture content at t + dt (kg moisture/kg dry matter), respectively, t is drying time (min). The air-drying curves were fitted with different mathematical models as given in Table 1. The regression analysis was performed using the computer program. Non-linear regression, which used to evaluate goodness of fit of the mathematical models to the experimental data are coefficient of determination (R2) and the reduced chi-square ( ), was used for data analysis. The higher value for R2 and the lower values for _2 and root mean square error analysis (RMSE) indicate the better fitness of model. These parameters were calculated as follows: RMSE =

(4)

where, MRexp.i is the ith experimentally observed moisture ratio, MRpre.i the ith predicted moisture ratio, N is the number of observations and n is the number constants. RESULTS AND DISCUSSION Drying characteristics of okra slices Moisture ratio and drying rate of the samples were calculated using Equations (1) and (2). Figure1 shows the drying curves of okra slices at different temperatures. The drying rate decreased continuously throughout the drying period. It is obvious from Figure 2 that the constant rate period was absent and the drying process of okra slices took place in falling rate period. The drying times to reach the equilibrium moisture content for the fresh sample at 40, 60, and 90°C, respectively. As the temperature was increased by difference of 10°C, from 40 to 90°C the drying time decreased by 20, 14.58 and 5.13%, correspondingly. Maximum reduction of drying time was obtained when the drying temperature was increased from 40° to 60°C as compared from 60° to 90°C, respectively. The moisture ratio reduced exponentially as the drying time increased (Doymaz, 2007). Continuous decrease in moisture ratio indicates that diffusion has governed the internal mass transfer. Experimental results showed that drying air temperature is effective parameter for the drying of okra slices. The drying process took place in a falling rate period except a very short accelerating period in the beginning. It can be seen that at higher moisture content, the increase in temperature has more considerable effect on the drying rates as compared to lower temperatures, which is almost negligible towards the end. It was further observed that the drying rate or moisture loss was faster at the beginning than that at the end. The reduction in the drying rate at the end of drying may be due to the reduction in moisture content as drying advances (Sharma and Prasad, 2001). The dried samples obtained at 40°, 60° and 90°C were evaluated for taste and color. The sample dried at 40°C was found better as compared to the samples obtained at 60° and 90°C.

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Fig 1: Drying curve of hot air dryer temperature of dried okra slices

Fig. 2: Drying rate of dried okra slices at different temperature condition.

CONCLUSION The drying behavior okra slices in hot air dryer was investigated at three different drying air temperatures. The times to reach equilibrium moisture (10%) from the initial moisture content. In order to explain the drying behavior of okra slices models in the literature were applied. In addition to, these results showed good agreement with the experiment data. It can be concluded that the influence of air temperature on drying time cause to with increase in air temperature a decrease in drying time during falling rate period is observed. According to the results, it can be stated that drying characteristics of okra slices in the drying process at a temperature range 40-90°C, air relative humidity 20- 60% and air velocity of 1.0 m/s the effect of air temperature on drying rate and drying time, For higher values of the moisture content, increasing drying temperature resulted in increasing drying rate and, in follows, decreasing drying time. This can be explained by the increasing temperature difference between the drying air and the product and, in follows, accelerating water migration. The effect of air relative humidity on drying rate and drying time, decreasing the relative humidity, intensifies drying rate change by the drying time. In other words, at low relative humidity of air the transfer of heat and mass is high and water loss is excessive. REFERENCES [1] Adom, K.K., Dzogbefia, V.P. and Ellis, W.D. (1997), Effect of drying time and slice thickness on the solar drying of okra, Journal of Science of Food and Agriculture, 73(3), 315-320. [2] Aghbashlo M, Kianmehr M H, Khani, Ghasemi S M (2009). Mathematical modelling of thin-layer drying of carrot. International Agrophysics, 23: 313-317. [3] A.Z. Sahin, I. Dincer J. Food Eng. 71 (2005) 119-126 [4] Bala, B. K., Mondol, M. R. A., Biswas, B. K., Das Chowdury, B. L., & Janjai, S. (2003). Solar drying of pineapple using solar tunnel drier. Renewable Energy, 28(2), 183–190. [5] Bhosale, B.S. and Arya, A.B. (2004), Effect of different modes of drying on moisture content and drying time of the selected vegetables, The Indian Journal of Nutrition and Dietetics, 41, 293. [6] Ertekin, C., & Yaldiz, O. (2004). Drying of eggplant and selection of a suitable thin layer drying model. Journal of Food Engineering, 63(3), 349–359 [7] FAO, FAO Statistical Database (2009), available from:http://www.fao.org [8] Henderson S M (1974). Progress in developing the thin layer drying equation. Transactions of the ASAE, 17: 1167–1172. [9] I.Domyaz, Drying of green bean and okra under solar energy CI&CEQ 17(2) 199-205(2011) [10] Jayaraman, D.K., Das Gupta and N. Babu Rao (1991), Quality characteristic of some vegetables dried by direct and indirect sun drying, Indian food packer, 45, 18-23. [11] Kalra, S. K. and K. C. Bhardwaj (1981), Use of simple solar dehydration for drying fruits and vegetable products. Journal of Food Science Technology, 45, 18-23. [12] K.R. Ajao , A.A. Adedeji, Assessing the Drying rates of some Crops in Solar Dryer. Journal of Research information in Civil Engineering, Vol.5, No.1, 2008 [13] Maskan, A., Kaya, S., & Maskan, M. (2002). Hot air and sun drying of grape leather (pestil). Journal of Food Engineering, 54(1), 81–88. [14] M. Ozdemir, Y.O. Devres, J. Food Eng. 42 (1999) 225-233 [15] Mujumbar, A. S. (1995). Handbook of industrial drying (second ed.). New York: Marcel Dekker. [16] O. Sobukola, Int. J. Food Eng. 5(2) (2009), article 9 [17] Pangavhane, D. R., Sawhney, R. L., & Sarsavadia, P. N. (2002). Design, development and performance testing of a new natural convection solar dryer. Energy, 27(6), 579–590. [18]Sacilik, K., & Elicin, A. K. (2006). The thin layer drying characteristics of organic apple slices. Journal of Food Engineering, 73(3), 281–289. [19]Schirmer, P., Janjai, S., Esper, A., Smitabhindu, R., & Mu¨hlbauer, W. (1996). Experimental investigation of the performance of the solar tunnel dryer for drying bananas. Renewable Energy, 7(2), 119–129. [20] Tiris, C., Tiris, M., & Dincer, I. (1996). Energy efficiency of a solar drying system. International Journal of Energy Research, 20(9), 767–770. [21 23–32. [22] W.A.M. McMinn, J. Food Eng. 72 (2006) 113-123 [23] Westerman, P. W., White, G. M., & Ross, I. J. (1973). Relative humidity effect on the high temperature drying of shelled corn. Transactions of the ASAE, 16, 1136–1139.