Drying characteristics and kinetics of okra

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Abstract

Air-drying characteristics of okra were investigated in a laboratory scale hot-air dryer for a temperature range 50–70 °C, and a relative humidity range 8–25%. Results indicated that drying took place in the falling rate period. Moisture transfer from okra was described by applying the Fick’s diffusion model, and the effective diffusivity was calculated. Effective diffusivity increased with increasing temperature. An Arrhenius relation with an activation energy value of 51.26 kJ/mol expressed the effect of temperature on the diffusivity. Two mathematical models, the Page and the simple exponential models, available in the literature were fitted to the experimental data. The performance of these models is evaluated by comparing the coefficient of determination (R2) and reduced chi-square (χ2) between the observed and predicted moisture ratio. The Page model gave the best results for describing single layer drying of okra.

Introduction

Okra probably originates is east African, quite possibly in Ethiopia. It is widely grown in all the regions of the world with a tropical or Mediterranean climate for its immature pods. It grows best in hot weather (temperatures above 26 °C). Nowadays, the most important okra-producing countries are India, Nigeria, Pakistan, Ghana and Egypt (FAO, 2003). Okra can be consumed as a fresh vegetable, a cooked vegetable or as an additive for soups, salads and stews. Because of its sensitivity to storage, most fresh okras are preserved in some form. Freezing may prolong the life of this vegetable. In some countries fruits are dried for later use. Okra provides some amount of vitamins, dietary fiber, energy and minerals (Adom, Dzogbefia, & Ellis, 1997).

Drying is one of the most widely used primary methods of food preservation. The purpose of drying food products is to allow longer periods of storage with minimized packaging requirements and reduced shipping weights (Okos, Narasimhan, Singh, & Witnauer, 1992). Okra is traditionally preserved by drying on various surfaces such as the ground, racks, trays and concrete floors. Although sun drying is the most common method used to preserve agricultural products in tropical and sub-tropical countries, this technique is extremely weather dependent, and has the problem of contamination with foreign matter. Also, the required drying time can be quite long and the sensory qualities of the final product can deteriorate. Therefore, an effective means of overcoming these problems is to dry the okra and other vegetables with solar and hot-air dryers (Adom et al., 1997, Doymaz and Pala, 2002, Gogus and Maskan, 1999).

Recently there have been many studies on the drying characteristics of various vegetables such as red pepper and red chilli (Doymaz and Pala, 2002, Gupta et al., 2002), sweet potato (Diamante & Munro, 1993), eggplant (Ertekin & Yaldiz, 2004), green bean (Roselló et al., 1997, Senadeera et al., 2003), pigeon pea (Shepherd & Bhardwaj, 1988), carrot (Doymaz, 2004a), green peas (Simal, Mulet, Tarrazo, & Roselló, 1996), and okra (Adom et al., 1997; Gogus & Maskan, 1999). The objective of this research was to observe the effect of drying temperature on drying characteristics of okra, to evaluate a suitable drying model for describing the drying process and to compute effective moisture diffusivity and activation energy of okra during drying.

Section snippets

The laboratory dryer

Drying experiments were performed in laboratory scale hot-air dryer installed in the Chemical Engineering Department of Yildiz Technical University. The schematic diagram of the dryer is shown in Fig. 1. The dryer basically consists of a centrifugal fan, an electric heater, an air filter and an electronic proportional controller. The air temperature was controlled by means of a proportional controller. Air velocity was kept at a constant value of 1.0 m/s with an accuracy of ±0.03 m/s for all

Influence of air temperature

The effect of three temperatures on the drying curve of okra is shown in Fig. 2, Fig. 3. It can be seen that there is no constant rate drying period in the drying of okras. All the drying in case of okra takes place in the falling rate period. This shows that diffusion is the dominant physical mechanism governing moisture movement in the samples. Similar results were obtained by Roselló et al. (1997) for green bean, Gogus and Maskan (1999) for okra, and Gupta et al. (2002) for red chilli.

It is

Conclusions

Drying kinetics of okra was investigated in a laboratory scale hot-air dryer, at constant air velocity 1.0 m/s and a temperature range 50–70 °C. Drying process took place only in the falling rate period for okras. Both the simple exponential and Page models can be used to describe the drying behavior, but the Page model showed a better fit than the simple exponential model. The values of calculated effective diffusivity ranged from 4.27 × 10−10 to 1.30 × 10−9 m2/s. The effective diffusivity increases

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