Study On Quality Control System For Continuous Microwave Drying Of Berry

Berries have unique flavor,rich anthocyanins with high nutritional value.Fresh berries are prone to be mildewed due to high moisture content,resulting in loss of nutritional and commercial value.Continuous microwave drying technology has the advantages in greatly processing capacity and highly drying rate,which can meet the production demand for rapidly and timely drying of fresh berries.However,in continuous microwave drying of berries,the temperature is not easy to be controlled and the drying rate is not synchronize d with the degradation of active substances,which limit s the wide application of microwave technology in food processing industry.To address the above issues,taking blackcurrant berries as the experimental material the bench experiments were carried out in a microwave workstation to investigate the microwave drying characteristics of berries,and the effects of temperature on moisture content and anthocyanin retention during drying were analyzed.Then,a regression model describing the variation of anthocyanin retention with temperature and moisture content was established to develop the critical temperature to ensure quality and efficiency,and a berry quality control system of continuous microwave drying was proposed based on developed model.Finally,according to the central combination experimental method,the effects of the controllable parameters in three-stage temperature and apparent air velocity in quality control system on the average temperature,anthocyanin retention and average moisture content of dried berries were explored to optimize the control parameters.A fuzzy compensation control metho d was proposed to optimize the control system.The main research contents and conclusions are as follows.(1)The drying characteristics and anthocyanin degradation of berries under different microwave intensities(2～10 W/g)were studied.Increasing microwave intensity improves the heating rate of fruit pulp,and the upper limit temperature of finally dried berry becomes higher.The degradation rate of anthocyanins increases with the increase of pulp temperature and the decrease of pulp moisture content.When the microwave intensity at 8～10 W/g,the drying speed is faster,but high temperature makes anthocyanins degrade a lot.At microwave intensity of 2～4 W/g,relatively low temperature of pulp also causes obvious degradation of anthocyanin due to prolong drying time.When the microwave intensity at 6 W/g,the anthocyanin retention rate of dried berries was the highest(40.27%).(2)A model was established to describe the effects of temperature and moisture content on anthocyanin retention of berries.In the other hand,we determine the critical temperature with good drying quality and high drying efficiency.Further,the quality control system of continuous microwave dryer was designed to control the drying process of berries in the range of critical temperature by using the feedback control method.In this case,the anthocyanin retention rate of berries increased from 32.48% to 68.21% as compared to open loop control,which can obtain the high anthocyanin retention rate and drying rate.(3)To ensure the quality of dried berries and improve the drying efficiency,the variable temperature of microwave drying technology for berries was proposed.The central combination method was employed to design experimental procedure with the three-stage temperature(I,II,III)and apparent air velocity as influencing factors,and the average temperature,anthocyanin retention and average moisture co ntent as the response indexes.The optimal parameter was determined as the temperature of stage I of 50 ℃,the temperature of stage II of 48 ℃,the temperature of stage III of 51 ℃,and the apparent air velocity of 2m/s with anthocyanin retention,average temperature and average moisture content of dried berries is 90.87%,60.33 ℃ and 13.89%,respectively.(4)The fuzzy compensation controller,applied to the continuous microwave drying quality control system of berries,is d esigned based on the target temperature and apparent wind speed as the input,and the compensation temperature as the output.According to this,during the microwave drying process,the effects of the response asynchrony of adjacent magnetrons on the drying effect and average temperature after drying is eliminated,thereby improving the control accuracy.The verification results showed that in this case,berries can be dried within the expected temperature range to obtain a better control effect in the last period of drying.