Journal of Food Engineering 302 (2021) 110559 Available online 26 February 2021 0260-8774/© 2021 Elsevier Ltd. All rights reserved. Development of a closed-loop control system for microwave freeze-drying of carrot slices using a dynamic microwave logic control Narathip Sujinda a, b , Jaturapatr Varith a, * , Rosnah Shamsudin b , Somkiat Jaturonglumlert a , Saranyapak Chamnan a a Division of Food Engineering, Faculty of Engineering and Agro-Industry, Maejo University, Chiang Mai, Thailand b Department of Process and Food Engineering, Faculty of Engineering, Universiti Putra Malaysia, Malaysia A R T I C L E INFO Keywords: Microwave freeze-drying Closed-loop control system Dynamic microwave logic control Drying characteristics Carrot slices ABSTRACT This research aimed to develop a closed-loop control (CLC) system to improve the microwave freeze-drying (MFD) process and to examine the effects of a dynamic microwave logic control (DMLC) on the drying char- acteristics of MFD. The development process consisted of two parts: (1) the MFD experiment to develop the DMLC, and (2) the implementation of the CLC with the DMLC on the MFD process. In the first part, the MFD process was examined to obtain the strategy for drying the carrot slices using microwave powers of 100 W, 200 W, and 300 W, with a temperature profile of the sample from  15 ◦ C to 40 ◦ C, and the final moisture content of 6% (wet basis). In the second part, the DMLC was strategically developed and integrated into the CLC system. The results showed that in the MFD process, the DMLC was developed based on a drying-phase configuration and dynamic control between the microwave power and real-time moisture content sensing to provide feedback to the CLC system. After applying the DMLC into the CLC system, the efficiency of the MFD process was improved by up to 62.4% by shortening the drying time, as compared with the freeze-dry (FD) process. The MFD-DMLC also resulted in the quality of carrot equivalent to that of a traditional FD process. Since the DMLC exhibited great potential to improve the MFD process, it could be developed for future industrial use for a high- performance MFD process in terms of product quality and process efficiency. 1. Introduction Carrot (Daucus carota L.) is considered a significant root vegetable owing to its medicinal properties beneficial to human health. Because of its susceptibility to spoilage, an applicable technique would be needed to preserve its nutritional value (Guo et al., 2020). Of the drying methods, freeze-drying (FD) has been suggested as a good option for storage of fruits and vegetables in food industries due to retention of organoleptic and nutritional qualities. Contrastingly, it is time- and energy-consuming and high in operational cost because of inadequate heat supply coming from the heated plate (Cao et al., 2018b; Wu et al., 2020). Hence, a process that reduces the drying period and energy usage while retaining the product’s quality resolves this issue. Microwave freeze-drying (MFD) is a drying method that applies microwave energy as a heating source to the FD process (Duan et al., 2010a). Thus, microwave energy offers a better option for reducing the drying period and energy usage (Cao et al., 2018a; Huang et al., 2009), while considerably increasing the drying rate (Ozcelik et al., 2019) due to rapid heating in materials. Microwave energy generates heat inside the materials by two mechanisms: a dipole rotation and an ionic con- duction. These mechanisms occur simultaneously as the ions are induced by electrical oscillation for alignment within the electromag- netic field. The electromagnetic energy in this process is converted to thermo-kinetic energy and is absorbed in all parts of the material (Song et al., 2018; Varith et al., 2007). For this reason, the MFD process can reduce the drying time by half (Duan et al., 2010b; Wang et al., 2010) and the energy consumption by up to 30%, while obtaining a product quality similar to that of FD (Jiang et al., 2013). Thus, MFD is an effective drying method that could address the weakness of traditional FD. In the MFD process, complex parameters, such as, microwave power levels, moisture content, material temperature, and vacuum pressure are found to affect the drying rate, product quality and energy consumption. Although the higher microwave power reduces the drying time and * Corresponding author. E-mail address: varithj@mju.ac.th (J. Varith). Contents lists available at ScienceDirect Journal of Food Engineering journal homepage: http://www.elsevier.com/locate/jfoodeng https://doi.org/10.1016/j.jfoodeng.2021.110559 Received 27 November 2020; Received in revised form 23 February 2021; Accepted 24 February 2021