Study On The Uniformity And Process Optimization Under Continuous Microwave Drying Of Raspberry Puree

Raspberry berries have high nutritional value and commercial potential,but their moisture content is high(over 80%,w.b.),making them susceptible to mechanical damage during the harvest-storage-transportation,as well as mold and rot.Drying is an effective way to reduce moisture content and avoid damage and rot.Continuous microwave dryer has the advantages of high drying efficiency and strong processing capacity,making it suitable for drying raspberries.However,microwave drying has the problem of uneven drying,which can affect the quality of raspberry products after drying.To address this issue,the following research has been conducted:1)analyzing the effect of process parameters on the drying uniformity of raspberry puree in continuous microwave drying;2)establishing a multi-physical field coupling model of the electromagnetic-temperature-moisture field to simulate the microwave drying of moving raspberry puree in the continuous microwave dryer to analyze the formation cause of the non-uniformity of dried puree;3)introducing an batch process to improve the uniformity of dried puree in a continuous microwave dryer and analyzing the effect of various process parameters on the dried index of dried puree;the comprehensive evaluation indexes were developed by hierarchical analysis with entropy weight method,and then obtaining an optimal process parameter through the neural network with genetic algorithm(ANN-GA).The main contents and conclusions are as follows:(1)Through the central combination test,the effects of microwave intensity,air velocity and drying time on the indexes of dried raspberry puree were studied,including the average temperature,average moisture content,and uniformity indexes,including temperature dispersion value V_T(the uniformity distribution of overall temperature),temperature contrast value CON(the temperature difference degree between"hot and cold spots"in the local area),hot zone distribution value HTD(the aggregation degree of hot zone distribution),and moisture dispersion value V_M(the uniformity distribution of overall moisture).The lower value of uniformity indexes,the better drying uniformity.The results showed that the degree of influence of process parameters on drying indexes was in the order of microwave intensity,drying time,and air velocity.Microwave energy absorption was the main factor affecting the drying indicators of raspberries.As the air velocity ranged from 0.7 to 1.3 m/s,the uniformity index was relatively low(V_T of 33.04～33.64,V_M of 44.98～47.06,and CON of 162.87～169.06),which improved the overall distribution uniformity of temperature and moisture,as well as the temperature difference between"cold and hot spots"in the local area.(2)The continuous microwave drying of raspberry puree was simulated by computer simulation.The results showed that the area with high electric field intensity spread from the edge to the center during the drying process,and the V_T and HTD values increased first and then decreased and tended to be stable,that is,the overall temperature distribution gradually changed from uneven to uniform,and the distribution of hot zone position first concentrated and then dispersed.The spread of area with high electric field intensity during drying can improve the temperature uniformity after drying and ameliorate the corner effects.As the dielectric constant at the edge of the material layer decreased,the area with high electric field intensity spread from the edge to the center,and the degree of spread of area with the high electric field intensity to the center increased with the drying process.The electric field distribution in the later stage of drying determined the temperature uniformity after drying.The more uniform the electric field distribution is,the lower the V_T of raspberry after drying is,indicating that the overall temperature distribution is more uniform.(3)By introducing the batch process to continuous microwave drying,the V_T,CON,HTD,and V_Mof dried puree improved by 20.25%,22.83%,37.01%and 26.38%,respectively,as compared to that without batch process.The central composite experiment was employed to investigate the effects of microwave intensity,air velocity,batch time ratio,and single drying time on the average temperature,average moisture content,the average retention rate of anthocyanin content,V_T,CON,HTD,and V_M.The results showed that the increase of microwave energy absorption could reduce the V_T,CON and HTD of dried puree,resulting in some improvements on temperature uniformity.Suitable air velocity and batch time ratio can reduce the CON,indicating that improvement of temperature difference between"hot and cold"region.Quantitative and comprehensive evaluation of dried puree quality was established by analytic hierarchy process with entropy weight method to establish comprehensive evaluation indexes and process optimization was achieved by comparing ANN-GA with response surface methodology(RSM).The ANN-GA model was more suitable for continuous microwave drying process optimization,and the optimal technology parameters of continuous microwave drying process for raspberry puree were developed as microwave intensity of 4.43 W/g,air velocity of 1.51 m/s,batch time ratio of 4.44,and single drying time of 3.25 min.This study may provide a theoretical and technical basis for drying processing and quality control of dried berry fruits like raspberry puree.