Temperature and power control in microwave drying Zhenfeng Li a,b, * , G.S.V. Raghavan b , Valérie Orsat b a College of Mechanical and Electronic Engineering, Northwest A&F University, Yangling, Shaanxi 712100, China b Department of Bioresource Engineering, McGill University, 21111 Lakeshore Road, Ste-Anne-de-Bellevue, QC, Canada H9X 3V9 article info Article history: Received 8 August 2009 Received in revised form 28 October 2009 Accepted 6 November 2009 Available online 13 November 2009 Keywords: Temperature Microwave Drying Power Control Apple abstract A microwave drying system with the ability of automatic temperature and power control was developed for apple drying. The samples’ mass and the moisture content could be obtained online. Four different drying modes were attempted with different combination of temperature and power controls. Feedback temperature control with predefined variable power profiles resulted in the best temperature control and product quality. All other modes proved their own pros and cons. The study also verified that it was dif- ficult to maintain a constant temperature during microwave drying process when a feedback tempera- ture control was not included. The results can be used to determine microwave power levels in microwave drying practices both with and without temperature control. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Drying temperature and microwave power are the two most important factors in microwave drying of agricultural products. These two factors significantly influence the drying parameters such as drying time, drying curve, drying speed, drying efficiency, and the final product quality. To improve microwave drying, a number of studies have been conducted to investigate the effects of different microwave power levels and drying temperatures, and different prediction models have been established (Adedeji et al., 2009; Andres et al., 2004; Changrue et al., 2008; Cui et al., 2005; Clary et al., 2005; Lu et al., 1999). In a typical microwave drying application, a fixed microwave power level is applied throughout the entire drying process, but a temperature control is usually not included. With this arrangement, a microwave drying process can be divided into three periods according to temperature variations: a warming-up period in which sample temperature increases with little moisture removal; a con- stant temperature period in which most of the drying takes place; and a heating-up period when the drying rate decreases and sample temperature increases rapidly (Lu et al., 1999). While the drying ef- fects in the first and second periods are acceptable, product char- ring often occurs in the last period when the temperature reaches an undesirably high value. The problem becomes complex when microwave drying is combined with other drying methods, such as hot air drying, vacuum drying, freeze drying, etc., where the tem- perature variations are either enhanced or weakened. To achieve an ideal drying effect over the entire microwave dry- ing process, sample temperature must be controlled and micro- wave power must be adjusted, especially in the last drying stage. Unfortunately, such study is seldom reported in the literature (Li et al., 2009a), although it had been recommended by some researchers (Clary et al., 2005; Zhang et al., 2006). The reason might be the high cost of temperature measurement in a micro- wave environment, the lack of convenient power and temperature control and recording techniques, or other engineering difficulties in microwave drying applications. To address the problem, a new microwave drying system was developed in this study. The developed system can automatically and continuously adjust the power levels, control the product tem- perature, and measure the samples’ mass online. Hence it becomes possible for the first time to monitor and control the power, temper- ature and moisture contents in real time. The specific objectives in- cluded: investigating the relationship amongst microwave power, drying temperature, and moisture contents; optimizing tempera- ture and power control in a feedback control loop; and attempting a temperature control without temperature measurement. 2. Materials and methods 2.1. Microwave drying system A domestic microwave oven with a nominal power output of 600 W (Beaumark 02314, Matsushita Electric Ind. Co. Ltd., Yama- 0260-8774/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2009.11.004 * Corresponding author. Address: Department of Bioresource Engineering, McGill University, 21111 Lakeshore Road, Ste-Anne-de-Bellevue, QC, Canada H9X 3V9. Tel.: +1 514 398 4400/7632; fax: +1 514 398 8387. E-mail address: zhenfeng.li@mail.mcgill.ca (Z. Li). Journal of Food Engineering 97 (2010) 478–483 Contents lists available at ScienceDirect Journal of Food Engineering journal homepage: www.elsevier.com/locate/jfoodeng