Thin-layer infrared radiation drying of onion slices G.P. Sharma a, * , R.C. Verma a , P.B. Pathare b a Department of Processing and Food Engineering, Maharana Pratap University of Agriculture and Technology, College of Technology and Engineering, Udaipur 313 001, India b CPHET, PAU Campus, Ludhiana, 141 004, India Received 19 February 2004; accepted 4 May 2004 Abstract A laboratory scale infrared-convective dryer was developed and single layer drying of onion slices was carried out at infrared power levels of 300, 400 and 500 W, drying air temperatures of 35, 40 and 45 °C and inlet drying air velocities of 1.0, 1.25 and 1.5 m/s. The drying time reduced by about 2.25 times on increasing infrared power from 300 to 500 W, air temperature 35–45 °C and air velocity from 1.0 to 1.5 m/s. Effective moisture diffusivity varied from 0.21 · 10 10 to 1.57 · 10 10 m 2 /s and was signifi- cantly influenced by infrared power and air temperature.. The rehydration ratio of dehydrated onion slices was found to be in the range of 4.5 and 5.3. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Infrared; Convective; Drying rate; Effective moisture diffusivity; Rehydration ratio 1. Introduction Onion (Allium cepa) has been widely used even in an- cient times as seasonings, for medical uses and as foods. In current times, onion is an important vegetable to serve as ingredients in dishes, as toppings on burgers, in seasonings, as chip coatings, and in a variety of other food products including ramen noodles and canned foods. India is one of the leading countries in the world producing high quality onion. It ranks first in the world with over 480 thousand hectares i.e. about 21% of the world area planted under onion. The use of dried onions, which have decreased in mass, instigate the necessity of developing an efficient and effective method of dehydrating the onion products. Dehydration of foods is aimed at producing a high density product, when adequately packaged has a long shelf life, after which the food can be rapidly and simply reconstituted without substantial loss of flavour, colour, taste and aroma. Direct solar drying has traditionally been used but in areas where sunshine is irregular or insufficient and when strict sanitation is required, a conventional con- vection dryer is often used. The convective drying met- hod is being commercially used in the dehydration of onion in India. There is tremendous loss of thermal en- ergy in the convective drying, making it a less efficient process. When infrared radiation is used to heat or dry moist materials, the radiation impinges the exposed material, penetrates it and the energy of radiation con- verts into heat (Ginzburg, 1969). Since a material is heated intensely, the temperature gradient in the mate- rial reduces within the short period. Therefore, energy consumption in infrared drying process is relatively les- ser. IR energy is transferred from the heating element to the product surface without heating surrounding air (Jones, 1992). The use of infrared radiation technology in dehydrat- ing foods has several advantages. These may include de- creased drying time, high energy efficiency, high quality 0260-8774/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2004.05.002 * Corresponding author. E-mail address: gpsharma@mailcity.com (G.P. Sharma). www.elsevier.com/locate/jfoodeng Journal of Food Engineering 67 (2005) 361–366