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Drying Rate Modeling Through Variable Diffusivity Approach for Vacuum Drying of Onion Slices

Published by the American Society of Agricultural and Biological Engineers, St. Joseph, Michigan

Citation:  2010 Pittsburgh, Pennsylvania, June 20 - June 23, 2010  1009363.(doi:10.13031/2013.29862)
Keywords:   Vacuum Drying, Onion Slices, Drying Rate, Modeling, Variable Diffusivity

Onion is the largest vegetable produced and consumed not only in India but also in the world. It contain generous amounts of a flavonoid called quercetin, in addition to a variety of other naturally occurring chemicals known as organosulfur compounds that have been linked to lowering blood pressure and cholesterol levels. Dehydrated onion can be used as such or it can be converted into powder, granules, flakes, kibbled forms or it can be used to prepare products such as french fried onion rings, salt toasted onion pieces, encased products etc. The key benefits of vacuum drying of onion include lower process temperatures, improved drying rates, and in some case less shrinkage of the product. Onion slices of different thickness viz., 1, 3 and 5 mm were subjected to various temperatures of 50, 60 and 70C dried under constant vacuum level of 50 mm Hg absolute in order to produce a good quality dehydrated onion. During the drying process, a very short constant rate period was found existing followed by a prominent first and second falling rate periods. The drying rate versus moisture content data fitted well to 3rd degree polynomial with a reasonable accuracy (>0.96). Variable diffusivity approach was applied instead of constant diffusivity and it explained the drying data quite efficiently. Moisture diffusivity of the slices varied from 4.05E-10 to 6.14E-09 m2/s initially and changed continuously with varying moisture content throughout the drying period. However, towards the end of drying it changed from 9.01E-10 to 2.25E-08 m2/s owing to increase in temperature of slice. While concentration dominated the earlier stages of moisture diffusivity and shrinkage played a vital role in the later part.

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