Strategy for longan drying in two-stage superheated steam and hot air Thanutyot Somjai * , Siva Achariyaviriya, Aree Achariyaviriya, Kodkwan Namsanguan Department of Mechanical Engineering, Faculty of Engineering, Chiang Mai University, 239 Huay Keaw Road, Muang District, Chiang Mai 50200, Thailand article info Article history: Received 16 December 2008 Received in revised form 3 May 2009 Accepted 4 May 2009 Available online 8 May 2009 Keywords: Color Hot air drying Mathematical model Shrinkage Superheated-steam drying abstract The aim of this research was to investigate a novel strategy and to develop a model for longan without stone drying using a two-stage SSD + HAD process (superheated-steam drying followed by hot air dry- ing). The experiments were conducted using superheated-steam at: 120, 140, 160, and 180 °C followed by hot air at: 60 and 70 °C. The moisture content of longan at the end of the first-stage drying was about 200% dry basis. The effects of superheated steam and air temperatures on the drying kinetics and quality of dried longan viz. color, shrinkage, and microstructure were determined. A mathematical model has been developed to predict the moisture content of longan, the drying temperature at the outlet of the dry- ing chamber and the drying rate. Comparisons showed that the simulated results were in agreement with experimental values. SSD + HAD using 180 °C superheated steam, followed by 70 °C hot air was the most suitable drying condition for drying longan. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Longan (Dimocarpus longan Lour.) is a sub-tropical fruit, com- monly grown in China, Taiwan, Indonesia, Thailand and Vietnam (Lapsongphol et al., 2007), of which the first three countries are the leading producers in the world. It is one of the most important commercial fruits in the north of Thailand with an export value of $75.15 million in 2008 (Office of Agricultural Economics, Thailand, 2009). The fresh products in season are in excessive supply and have very short post-harvest life of 3–4 days (Jiang, 1997; Yueming et al., 2002) causing a depreciation in market price. A number of efficient processes have been proposed to lengthen the shelf life, and to add value to the product as canned, frozen and dried longan. The dried longan is the most popular and is suitable to be eaten di- rectly as desert or snack food and it can be used to prepare refresh- ing drinks and longan tea (Choo, 2000). It is also widely used as herb medication for benefiting the mind and spleen (Chang et al., 1998) and in Chinese medicine for example as a stomachic, febri- fuge and vermifuge (Lapsongphol et al., 2007). Conventional hot air drying (HAD) is the most common process for the production of dried longan, as either peeled or unpeeled fruit. However, this method requires long periods of drying, which requires high en- ergy consumption and causes a loss in product quality. Several researchers have attempted to improve longan drying to give optimum drying performance and better product quality. Ach- ariyaviriya et al. (2001), for example, studied the drying character- istics and quality of peeled longan using hot air at 45–85 °C. They also developed a mathematical model to investigate the effects of temperature and flow rate of drying air on the specific energy con- sumption (SEC). They found that the SEC decreases with increasing temperature and with decreasing flow rate. Drying longan at 65 °C for 12.5 h was recommended to produce the desired golden brown product. Next, Achariyaviriya et al. (2003) studied the drying kinet- ics of longan without stone (the fruit which was seeded without peeling) using air temperatures of 50–90 °C, and air flow rates of 86–130 kg dry air /h. They reported that hot air drying at 60 °C for 38 h produced acceptable golden brown flesh. However, the SEC was minimized as 4.7 MJ/kg water evaporated , using hot air at 75 °C and an air flow of 130 kg dry air /h. This study was extended further by development of a drying model for unpeeled longan to evaluate the optimum conditions (Achariyaviriya and Nuthong, 2007). The SEC was optimized as 10.56 MJ/kg water evaporated for re-circulating air of 90% with specific air flow of 73 kg dry air /h-kg dry longan , drying temperature of 75 °C, bed thickness of 40 cm and drying time of 64.2 h. Varith et al. (2007) have introduced an alternative drying process for peeled longan using a combined microwave-hot air method. They found that a step-wise drying process with micro- wave power of 450 W at hot air of 40 °C for 1.7 h, followed by microwave power of 300 W at hot air of 65 °C for 3.3 h yielded the maximum drying efficiency. Superheated stream drying (SSD) is of interest to the food indus- try since it is non-polluting, offers high production rates and is a safe drying method requiring low energy consumption. An impor- tant advantage is that it provides high product qualities, for exam- ple, low shrinkage, high porosity (Li et al.,1999; Moreira, 2001), good color and high vitamin C retention (Caixeta et al., 2001). However, although drying with high-temperature superheated 0260-8774/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2009.05.005 * Corresponding author. Tel.: +66 5394146; fax: +66 5394145. E-mail address: Thanutyot_a@yahoo.com (T. Somjai). Journal of Food Engineering 95 (2009) 313–321 Contents lists available at ScienceDirect Journal of Food Engineering journal homepage: www.elsevier.com/locate/jfoodeng