การพัฒนาระบบควบคุมวงปิดสำหรับการอบแห้งชิ้นแครอทด้วยการอบแห้งแบบแช่เยือกแข็งร่วมกับคลื่นไมโครเวฟ

Abstract

Microwave freeze-drying (MFD) is the process that applies the microwave as the heating source to freeze-dry the food product. It is a complex process due to the combination of microwave power, vacuum pressure, drying time, material temperature, and moisture content that affects the drying rate, product quality, and energy consumption. The proper control of MFD process conditions to achieve the optimum drying performance and quality of products is a challenge for researcher. Therefore, the objective of this research was to design the MFD process strategy called dynamic microwave logic control (DMLC) and integrated it into the closed-loop control (CLC) system for improving the MFD performance. In the first part of this research, the uncertainty of the measurement system and moisture content calculation were evaluated following the Guide to the expression of uncertainty in measurement or GUM (ISO, 2008) to verify the accuracy of the measurement systems and the calculation of the moisture content. The results show that the uncertainty of temperature measurement is ±1.5oC or 3.9% from a full-scale range of 30 - 50oC, the uncertainty of weight measurement is ±0.347 grams or 0.8% from a full-scale range of 0 - 150 grams, and the uncertainty of real-time moisture content calculation is ±1% (wet basis). As the results, the measurement systems were acceptable for use in the CLC system with no affected in terms of process control and product quality which confirm by the same later results in repetition of each treatment in this research. In the second part, the temperature variations during MFD of carrot slices were studied by applying the closed-loop temperature (CLT) control system to control the carrot slice's temperature in the final stage of the MFD process. The MFD with CLT can improve the temperature control efficiency up to 60% compared to the MFD without CLT while providing the product quality similar to freeze-drying (FD). Therefore, the CLT was applied with the MFD process for developing the CLC system in the final part of this research. In the final part, the CLC system was developed to improve the MFD process and to examine the effects of a DMLC on the drying characteristics of MFD. The development consists of two sections. In the first section, 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 -15oC to 40oC, and the final moisture content of 6% (wet basis). The DMLC was strategically developed in the second section 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 developed the DMLC, it was integrated into the CLC system. The results showed that by applying the DMLC into the control logic, the CLC could work properly in the MFD process by shortening the drying time by 62.4% and 23.4% compared with those of FD and MFD with no DMLC, respectively. The MFD-DMLC provided the final product with a quality equivalent to that of the FD process but achieved 44.3% better SMER. The findings from this research suggested that the DMLC based on the moisture content and temperature of the samples could be combined with the MFD process to enhance its efficiency while maintaining the superior quality of the FD process.