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Study on Drying Kinetics of Summer Onion

MD. MASUD ALAM
Senior Scientific Officer
Spices Research Center, Bangladesh Agricultural Research Institute (BARI), Bogra.

MD. NURUL ISLAM
Professors
Department of Food Technology and Rural Industries, Bangladesh Agricultural University (BAU), Mymansingh, Bangladesh.

MD. NAZRUL ISLAM
Professor
Department of Food Technology and Rural Industries, Bangladesh Agricultural University (BAU), Mymansingh, Bangladesh.

Abstract/ Executive Summary:

The present study was concerned with the kinetics of drying of summer onion. Drying was done in a mechanical dryer at constant air flow using blanched and unblanched onion with variable temperature (52, 60 and 68⁰C) and thickness (3, 5 and 7 mm). Drying rate was increased with increase of temperature and decreased with the increase in thickness in blanched and unblanched onion. Blanched onion showed higher drying rate than unblanched onion. Drying rate constant and thickness can be expressed as power low equations. The value of index “n” were found to be 1.277 and 0.845 for onion indicating that the external resistance to mass transfer was highly significant. The effect of temperature on diffusion co-efficient follows an Arrhenius type relationship. The activation energy (Ea) for diffusion of water was found 5.781 Kcal/g-mole for unblanched and 2.46 Kcal/g-mole for blanched onion when onions were dried in mechanical dryer.

Keywords: Onion; Drying rate constant; Diffusion coefficient; Activation energy

Location: Department of Food Technology and Rural Industries, under the Faculty of Agricultural Engineering and Technology, Bangladesh Agricultural University, Mymensingh.

Start Date:

End Date:

Program Area: Crop-Soil-Water Management

Commodity: Onion

Objectives:

To study the drying behavior of summer onion slices using Cabinet drying system and to examine the effect of slice thickness of onion and temperature on different drying parameter

Methodology:

The study was conducted in the laboratory of the Department of Food Technology and Rural Industries, under the Faculty of Agricultural Engineering and Technology, Bangladesh Agricultural University, Mymensingh. Materials required Fresh summer onion were collected from the Spices Research Center of BARI for conducting the laboratory study. Cabinet dryer (model OV-165 Gallenkamp Company) was used for the dehydration of summer onion. The dryer consists of several chambers in which trays of sample was dried. The velocity of air was recorded (0.6 m/s) by an Anemometer. Mechanical drying of onion slices For determining the effect of the temperature and the thickness on the rate of the drying onion were peeled and sliced by a slicer and samples were taken for the determination of moisture content. In some cases the onion slices were blanched in steam for five minutes using perforated wire pot. Fresh onion slices (3, 5 and 7 mm thick) were placed in trays in single layer and drying commenced in the dryer at constant air velocity (0.6 m/sec) and at a specified air dry bulb temperature (52, 62 or 68°C). Moisture content at each time interval was determined gravimetrically on the basis of initial moisture content and weight loss was used as a measure of the extent of drying. From the semi-theoretical equation as shown in equation (3), it may be noted that the drying rate constant, m is a function of the square of thickness of the product being dehydrated, as seen in equation 4, Symbolically, this may represented as: m = A(L)^-n Or log(m) = log A – n log (L)---------------(6) Where, A = π² De, n = 2 The above relationship shows that if external resistance to mass transfer is negligible and if simultaneous heat and mass transfer effects are taken into account, the value of the exponent of the power law equation should be 2. But the above conditions are not always satisfied and experimentally determined "n? value is found to be less than 2 (Islam, 1980).

Source of Information: Bangladesh J. Agril. Res. 39(4): 661-673, December 2014 ISSN 0258-7122

Article Link (Online Journal):

Funding Source:

Total Budget (Tk.) :

Achievement/Findings:

From the above study it appeared that mechanical dryer can be used for commercial production of dehydrated onion. The rate of drying depends on air velocity, temperature, thickness etc. Drying time of onion to a specific moisture ratio (eg. MR=0.1) increased with increase in sample thickness, while the rate constant has a power law relationship with thickness and the value of index of the equation was quite below 2 indicating presence of significant external resistance to mass transfer and increased airflow resulted in higher drying rate. Diffusion coefficient (calculated from rate constants) follow an Arrhenius type relationship with temperature and the activation value found was higher for unblanched onion than blanched onion (5.78 vs 2.46 Kcal/g-mole). Developed equations based on Fick?s 2nd law together with activation energy values could be used for process design and optimization and high quality nutritious product. Thus, large number of skilled and semi-skilled persons would be employed in the related industries.

Knowledge Management: Journal