Study the effect of sun, oven and microwave drying on quality of onion slices Derya Arslan * , Mehmet Musa Özcan Selçuk University, Faculty of Agriculture, Department of Food Engineering, 42031 Konya, Turkey article info Article history: Received 3 April 2008 Received in revised form 25 February 2010 Accepted 27 February 2010 Keywords: Drying Onion Mineral Colour Phenolics abstract Sun, oven (50 and 70  C) and microwave oven (210 and 700 W) drying of onion slices were carried out to monitor the drying kinetics and quality degradation of the product. Page, “Modified Page” and “Midilli and Küçük” models exhibited high coefficient of determination (R 2 ) values, ranging between 0.994 and 0.999. The calculated effective diffusivity (D eff ) values (m 2 /s) of onion slices for the sun, oven 50  C and oven 70  C, microwave 210 W and microwave 700 W drying process were 8.339  10 10 , 7.468  10 10 , 1.554  10 9 , 4.009  10 8 and 4.869  10 8 , respectively. Fresh and dried onion slices had high amounts of K (696.82e16357.55 mg/kg), Ca (69.64e340.03 mg/kg), Na (37.72e1895.43 mg/kg), Mg (3.31e964.77 mg/kg) and P (46.47e3384.07 mg/kg) minerals. The highest mineral values were determined in oven dried samples. Sun (L * 58.00  4.83, a * 0.27  0.10, b * 14.36  2.40) and microwave oven drying (210 W) (L * 54.78  7.54, a * 0.71  0.09, b * 13.17  1.05) revealed better colour values in the dried products. The phenolic contents of microwave oven dried samples (1664.39  134.12 and 1623.59  140.02 for 210 W and 700 W, respectively) were higher than those of the other dried onion slices. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Onion (Allium cepa) has been widely used even in ancient times as seasonings, foods and for medical uses. In current times, onion is an important vegetable to serve as ingredients in dishes, as toppings on burgers, in seasonings, as chip coatings etc (Sharma, Verma, & Pathare, 2005). Onion finds widespread usage in both fresh and dried forms. Dried onions are a product of considerable importance in world trade and are made in several forms: flaked, minced, chopped and powdered. It is used as flavor additives in a wide variety of food formulations such as comminuted meats, sauces, soups, salad dressings and pickle relishes (Kumar, Hebbar, & Ramesh, 2006). Onion ranks third highest in production in the world among seven major vegetables, namely onion, garlic, cauli- flower, green peas, cabbage, tomato and green beans. The four major onion producing countries in the world are China with largest production of 3.93 million tones, followed by India with 3.35 million tones, USA 2.45 million tones and Turkey 1.55 million tones (Kumar & Tiwari, 2007; Sarsavadia, Sawhney, Pangavhane, & Singh, 1999) Although the influence of hot air drying on food quality is well recognized the understanding of processes caused by dewatering and adversely affecting material properties is limited. The major challenge during drying of food materials is to reduce the moisture content of the material to the desired level without substantial loss of flavor, taste, colour and nutrients. Beside the adverse influence of drying on food quality the process is indispensable in many food industry sectors because of the increased shelf-life of the product, reduced packaging cost, lower shipping weights and environmental advantages. Moreover, drying used properly can result in unique properties not achievable by other technological procedures. Today's consumer expectation for better quality, safety and nutri- tional value drives research and improvement of drying technolo- gies. One of the most important ways to reduce the adverse influence of drying on food quality or to ensure basic quality properties of the final product is to carefully design the process and implement it consistently (Lewicki, 2006). Sun drying allows for the production of a product rich in colour and translucent in appear- ance, but it has some disadvantages (e.g. it is a time-consuming process, weather depended, labour demanded and it is greatly exposed to possible environmental contamination (Maskan, 2001). Nevertheless, sun and oven drying methods are still widely used to produce dried products because of their low costs (Soysal & Öztekin, 2001). New and innovative techniques that increase the drying rate and enhance product quality have achieved considerable attention in the recent past. Microwave drying is one of them, gaining popularity because of its inherent advantages over conventional heating such as reducing the drying time of biological material without quality loss. In an industrial level, food processing using this technique has been reported to be both cost-effective and feasible (Mongpraneet, Abe, & Tsurusaki, 2002; Nindo, Sun, Wang, Tang, & Powers, 2003; Ren & Chen,1998; Roa, Zanjad, & Mathur, 1995). * Corresponding author. Tel.: þ90 332 2232946; fax: þ90 332 2410108. E-mail address: dears@selcuk.edu.tr (D. Arslan). Contents lists available at ScienceDirect LWT - Food Science and Technology journal homepage: www.elsevier.com/locate/lwt 0023-6438/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.lwt.2010.02.019 LWT- Food Science and Technology 43 (2010) 1121e1127