Sirirattana Chimplee and Utai Klinkesorn* Thin-Layer Drying Model of Rambutan (Nephelium lappaceum L.) Kernel and Its Application in Fat Extraction Process Abstract: Rambutan (Nephelium lappaceum L.) kernels contain a high amount of fat and it has a potential to be used as a new alternative source of edible vegetable fat. Drying of fat seed can improve the fat yield and prolong the seed storage shelf life. Therefore the drying behaviour of rambutan kernels was investigated using a hot air dryer for 6 h at each of three temperatures, 45°C, 55°C and 65°C. Based on the coefficient of determination (R 2 ) and root mean square error (RMSE), modified Henderson and Pabis model was found to be the best thin-layer drying model for rambutan kernels (R 2 > 0.99 and RMSE < 0.02). The effective moisture diffusivity of rambutan kernels, estimated from a modified equation of Fick’s second law of diffusion, was 2.56  10 –10 to 3.68  10 –10 m 2 s –1 . The result of fat extraction signifi- cantly indicates the effect of low moisture rambutan ker- nels on the fat yield enhancing (P < 0.05). Keywords: rambutan kernel, thin-layer drying, mathema- tical modelling, fat extraction, fat yield DOI 10.1515/ijfe-2014-0209 1 Introduction Rambutan or Nephelium lappaceum L. is a tropical fruit, mostly found in Southeast Asia countries such as Indonesia, Malaysia, Philippines, Vietnam and Thailand. It is usually eaten fresh but it is also preserved and pro- cessed into other products, e.g. dried rambutan, rambutan in syrup and rambutan with pineapple in syrup. During processing of the above the seeds, that comprise approximately 4–9% by weight of the rambutan fruit, are wasted, discarded by industry in the past [1]. However, since rambutan seeds and kernels (unshelled seeds) are high in fat, with around 30–40% of dry basis, they are useful [2, 3]. Moreover, the fat from rambutan kernels is non-toxic for oral and skin irritation test in vivo [4, 5]; thus, it could be applied to food or non-food products. Therefore, at the present time, many researchers are studying utiliza- tion of rambutan kernels as an alternative natural source of vegetable fat [1–3, 6–8]. Generally, fats and oils can be extracted from raw materials by two main processes, solvent extraction and pressing. The main factor that affects oil yield in both processes is moisture content of the sample, the reduction of material moisture at an optimal level can improve the quantity of oil yield significantly [9–11]. Therefore, drying is the important pre-treatment step of fat extraction pro- cess. According to this reason, drying behaviour of seed oil should be studied to predict the drying time for the desired moisture content, a significant parameter of fat extraction [12]. Moreover, drying seed materials below certain moist- ure content, generally 14% wet basis, could also extend the storage stability of such seeds [13]. From this point of view, it is important and will be useful to study the drying kinetic of rambutan kernels, new alternative fat seed, that have not previously been reported. Drying is the removing or reduction process (eva- poration) of moisture content through heat and mass transfer between water in the product and the air by changing the liquid state to a vapour state [14–19]. The hot air dryer or convective dryer or tray dryer (the name is determined by the user) is the drying system most often used in industry at present time. Although some researchers avoid this machine because of low energy efficiency and long drying time, even at high tempera- tures that can cause a reduction in quality [20–22]. This system is controlled by circulating air transfer, sweeping the water out of a solid surface in a single layer move- ment during the falling rate period [23], also known as thin-layer drying. The thin-layer drying model is a good supporting equation for studying drying behaviour. It was first *Corresponding author: Utai Klinkesorn, Department of Food Science and Technology, Faculty of Agro-Industry, Kasetsart University, 50 Ngam Wong Wan Road, Chatuchak, Bangkok 10900, Thailand, E-mail: utai.k@ku.ac.th, utai27@yahoo.com Sirirattana Chimplee, Department of Food Science and Technology, Faculty of Agro-Industry, Kasetsart University, 50 Ngam Wong Wan Road, Chatuchak, Bangkok 10900, Thailand, E-mail: srcfstku@gmail.com Int. J. Food Eng. 2015; 11(2): 243–253 Brought to you by | New York University Bobst Library Technical Services Authenticated Download Date | 5/28/15 9:20 AM