Article Real-time color change monitoring of apple slices using image processing during intermittent microwave convective drying Nahid Aghilinategh, Shahin Rafiee, Soleiman Hosseinpour, Mahmoud Omid and Seyed Saeid Mohtasebi Abstract An intermittent microwave convective drying method combined with a real-time computer vision technique was employed to detect the effect of drying parameters on color properties of apple slices. The experiments were performed at air temperature of 40 to 80  C, air velocities of 1–2 m/s, microwave powers of 200–600 W, and pulse ratios (PRs) of 2–6. Drying rate and drying time varied from 0.014 to 0.000001 min 1 and 27 to 244 min, respectively. The normalized lightness values had ascending and descending parabolic trends with decrease in product moisture content. With descending dimensionless moisture content, redness, yellow- ness, color change, hue angle, and chroma were enlarged. The normalized redness values changed from 4 to 3. Models relating drying parameters with drying time, drying rate, and lightness were obtained and found to be significant (P < 0.01). Results indicated that microwave power and PRs had more influence on lightness and color change than other parameters. Keywords Apple slices, intermittent microwave convective drying, computer vision system, online monitoring, color properties Date received: 17 September 2015; accepted: 11 January 2016 INTRODUCTION Apple is the fourth largest horticultural product grown worldwide (Forsline et al., 2003). Antioxidants are abundant in apples. One of the oldest methods of pre- serving food is drying (Ertekin and Yaldiz, 2004). The method of using hot air to dry food is the oldest and most extensively used drying method. The disadvantage of this technique is its high-energy consumption and resultant low product quality. Hence, finding a fast, safe, and controllable drying method is needed (Kavak Akpinar et al., 2005; Motevali et al., 2011). The application of microwave energy can be a good alternative to overcome the existing problems with con- ventional drying methods (Vadivambal and Jayas, 2007; Wang et al., 2004). However, the major drawback associated with microwave heating is the uneven tem- perature and moisture distribution, resulting in hot and cold spots in the heated product. One of the possible solutions to reduce non-uniformity is to employ micro- wave intermittently (Gunasekaran and Yang, 2007). On the other hand, if microwave power is not properly applied, a low-quality product is obtained. Wang et al. (2004) found that it is better to combine micro- wave drying with other drying methods, such as con- vective hot-air, vacuum, or freeze drying to achieve Faculty of Agricultural Engineering and Technology, Department of Agricultural Machinery Engineering, University of Tehran, Karaj, Iran Corresponding author: Shahin Rafiee, Faculty of Agricultural Engineering and Technology, Department of Agricultural Machinery Engineering, University of Tehran, Karaj, Iran. Email: shahinrafiee@ut.ac.ir Food Science and Technology International 0(0) 1–13 ! The Author(s) 2016 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/1082013216636263 fst.sagepub.com by guest on April 9, 2016 fst.sagepub.com Downloaded from