脉冲式连续微波干燥树莓果浆的控制系统和品质优化

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

为改善微波加热均匀性,提高连续式微波干燥树莓果浆干后品质,在自行研制脉冲式微波输出可编程控制器(Programmable Logic Controller,PLC)控制系统上,采用中心组合试验设计方法,分析微波占空比、脉冲周期、风速和干燥时间四个实验因素对树莓果浆的平均温度、温度离散值、含水率、水分含量离散值、花青素保留率和花青素保留率离散值指标的影响规律,获得脉冲式微波干燥树莓果浆的最佳工艺。对比脉冲式微波干燥与常用的反馈控温微波干燥、间歇微波干燥和连续式微波干燥工艺的果浆干后品质及均匀性指标,进而研制连续式微波干燥树莓果浆品质优化控制系统。结果表明,脉冲式微波干燥通过PLC控制系统,使磁控管以脉冲形式输出矩形波形式功率,间歇性地开启磁控管比连续微波加热更有效,并且采用脉冲式微波干燥树莓果浆具有更优产品品质和加热效率,对比反馈控温系统成本更低可靠性更好。脉冲式微波干燥树莓果浆的最佳工艺为:微波占空比0.6、脉冲周期17 s、风速1.9 m/s、干燥时间12.5 min。相比于连续式微波干燥,脉冲式微波干燥果浆品质最优,脉冲式干后果浆花青素保留率提高35.31%±0.55%,反馈控温下提高33.59%±0.23%,间歇微波下提高16.42%±0.18%。脉冲式微波干燥通过控制功率降低果浆干后温度,改善干后品质均匀性,为树莓干制品工业化生产提供依据。

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.