Characterization of microwave vacuum-dried durian chips Swittra Bai-Ngew, Nantawan Therdthai ⇑ , Pisit Dhamvithee Department of Product Development, Faculty of Agro-Industry Kasetsart University, 50 Phaholyothin Rd., Chatuchak, Bangkok 10900, Thailand article info Article history: Received 27 September 2010 Received in revised form 29 November 2010 Accepted 1 December 2010 Available online 9 December 2010 Keywords: Durian Microwave Vacuum Drying Model abstract Durian CV. Monthong was subjected to microwave vacuum drying (at 13.33 kPa) to produce durian chips. Various levels of microwave power (3.88 W g 1 , 5.49 W g 1 and 7.23 W g 1 ) were used. Prior to the microwave vacuum drying, the sliced durian was either chilled at 4 °C or frozen at 18 °C. Both pretreat- ments yielded non-significant difference in dissipation factor (p > 0.05). Among several thin layer models, the Page model was found to be the best for explaining the drying characteristics of durian chips. An increase in the microwave power intensity produced a clear increase in the drying rate and did not affect lightness and yellowness of the durian chips (p > 0.05). The structure and hardness of the dried durian chips were comparable to that of conventionally fried durian chips. In addition, microwave vacuum dry- ing reduced the fat content of the durian chips by at least 90%, compared with conventionally deep fried durian chips. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Durian (Durio zibethinus Murr) is a tropical fruit grown widely in Southeast Asia (Subhadrabandhu and Ketsa, 2001). It is nutrition- ally rich in carbohydrates, protein, fat, phosphorous, iron and vita- min A. To overcome the problem of oversupply during the harvesting season, various value-added durian products have been developed. Durian chips are one of the most popular durian prod- ucts, made conventionally by deep frying. Based on health issues, fried durian chips may not be accepted by some consumers (Jamradloedluk et al., 2007). Drying is one of the alternative methods to produce oil-free snack products. However, conventional hot air drying produces poor quality chips that have problems associated with shrinkage, dark color and hard texture. To improve the quality of the dried products and the process efficiency, microwave vacuum drying could be used (Figiel, 2009). As microwaves can penetrate deep into food material, the entire food product is heated up quickly from the inside to the outside. As a result, rapid water evaporation and the outward flux of escaping vapor are noticeable. Therefore, a possible reduction in the drying time has been suggested (Hu et al., 2006). Giri and Prasad (2007) reported a reduction of 70–90% in the drying time of mushrooms, when hot air drying was replaced with microwave vacuum drying. In addition, Therdthai and Zhou (2009) observed an increase in the drying kinetic rate constant with in- creased microwave power intensity. Therefore, the drying time of mint leaves was reduced from 2 h in a conventional hot air drier at 60–70 °C to 15 min in a microwave vacuum drier. Decreasing the pressure during microwave heating reduced the boiling point of water and thereby the drying temperature (Durance et al., 2002). In addition, there was a possible decrease in the number of burning spots (Zhang et al., 2007). Microwave drying should be suitable for drying heat-sensitive materials, such as fruit and vegetables (Hu et al., 2006). Several studies reported that the color of microwave vacuum-dried products was superior to that of hot air-dried products, such as potatoes (Bondaruk et al., 2007), honey (Cui et al., 2008) and mint leaves (Therdthai and Zhou, 2009). Moreover, microwave vacuum drying can be used to create a desirable, crispy texture for dried foods that can be consumed without the need for rehydration, such as snack foods. The crispy texture was obtained by tissue consisting of air cavities sur- rounded by a brittle structural phase (Scaman and durance, 2005). Sham et al. (2001) found that a decrease in the chamber pressure during drying increased the puffing and crispness of apple chips. Likewise, Zhang et al. (2007) reported improved puffing and crispness in fish slides, when the pressure was reduced. A number of authors (Kompany et al., 1993; Arévalo-Pinedo et al., 2004) suggested that prior to drying, physical pretreatments (such as freezing, chilling or blanching) of solid food materials could be used as a method to modify the food structure and thereby affect the drying kinetics. Arévalo-Pinedo and Murr (2006) investigated the kinetics of vacuum drying pumpkin samples that had been pretreated by freezing and blanching. The freezing played a more significant role than the blanching did on the rate of moisture transfer in the samples. However, both pretreatments improved the heat transfer coefficient, compared with untreated samples. 0260-8774/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2010.12.003 ⇑ Corresponding author. Tel.: +66 2 562 5010; fax: +66 2 562 5005. E-mail address: faginwt@ku.ac.th (N. Therdthai). Journal of Food Engineering 104 (2011) 114–122 Contents lists available at ScienceDirect Journal of Food Engineering journal homepage: www.elsevier.com/locate/jfoodeng