Control System and Quality Optimization of Pulsed Continuous Microwave Drying of Raspberry Pulp

Abstract

To improve the uniformity of microwave heating and enhance the quality of continuous microwave drying for raspberry pulp, a self-developed programmable logic controller (PLC) control system with pulsed microwave output was implemented. The central composite experimental design method was employed to analyze the effects of four experimental factors, namely microwave duty cycle, pulse period, air velocity and drying time on the average temperature, temperature dispersion value, moisture content, moisture content dispersion value, anthocyanin retention rate, and anthocyanin retention rate dispersion value of raspberry pulp. The optimal process for pulsed microwave drying of raspberry pulp was obtained. Compared with dried pulp quality and uniformity indexes from pulsed microwave drying, commonly used feedback temperature control microwave drying, intermittent microwave drying, and continuous microwave drying processes to develop a continuous microwave drying quality optimization control system for raspberry pulp. The results showed that the magnetron from pulsed microwave drying in the form of pulse output rectangular wave form power through the PLC control system. Turning on the magnetron intermittently was more effective than continuous microwave heating. Pulsed microwave drying of raspberry pulp has better product quality and heating efficiency, lower cost and better reliability than feedback temperature control systems. The optimal process parameters for pulsed microwave drying were microwave duty cycle of 0.6, pulse period of 17 seconds, air velocity of 1.9 m/s, and drying time of 12.5 minutes. Compared with continuous microwave drying, pulsed microwave drying resulted in the highest quality of dried pulp as a 35.31%±0.55% increase in anthocyanin retention, a 33.59%±0.23% increase under feedback temperature control, a 16.42%±0.18% increase under intermittent microwave drying. Pulsed microwave drying can control the power to reduce the dried fruit pulp temperature, improve the uniformity of post-drying quality and provide a basis for the industrial production of dried raspberry products.

Keywords

• pulse
• microwave drying
• raspberry pulp
• programmable logic controller
• anthocyanin