Study On Pilot Scale Technology And Quality Control Of Hawthorn Produced By Pulse-spouted Bed Microwave Freeze-drying

Freeze-drying is a high-quality drying technology,which can retain the original nutrients and volume of dried fruits and vegetables to the greatest extent.However,freeze-drying also has the disadvantages of long drying cycle,high energy consumption,and high cost.Microwave freeze-drying(MFD)is a new freeze-drying technology that uses microwave energy(internal heat source)as the heat supply for freeze-drying.MFD can eliminate the heat transfer resistance,achieve the same direction of heat and mass transfer,significantly shorten the freezedrying cycle and reduce energy consumption.However,MFD also has prominent problems such as poor uniformity and low pressure gas discharge,which limit the commercial application of MFD technology.Pulsed spouted-bed microwave freeze-drying(PSBMFD)combines the advantages of pulsed jet-bed drying technology and MFD technology.The results of small experiments have shown that PSBMFD can significantly improve the uniformity of MFD,which has potential advantages in commercial application.In addition,the candy gourd products made of freeze-dried hawthorn are favored by domestic and foreign consumers.However,the pectin content in hawthorn is relatively high(9.94%),which makes hawthorn easy to shrink during freeze-drying process and thus affects its appearance.In this study,through the innovative design of the key components such as freeze-drying chamber,microwave heating chamber,microwave power detection and control,the pilot test equipment of PSBMFD-2450 was developed,and the effect of different pretreatment methods on the process and performance of PSBMFD was studied with hawthorn as raw material.On this basis,the pilot test process,drying time,energy consumption,uniformity,and product quality of PSBMFD were studied,and compared with those of conventional freeze-drying(CFD)to evaluate the feasibility of PSBMFD technology and equipment for commercial application.Firstly,to solve the shrinkage problem of hawthorn during freeze-drying process,the effects of freeze-thawing cycle,high-voltage electrostatic field assisted freezing(HVEF),and high-voltage electrostatic field assisted freeze-thaw pretreatment(H-FT)on the microstructure and quality of hawthorn were studied to optimize the pretreatment process,and the optimization of the process parameters of pulse-spouted bed microwave freeze-drying was also carried out.Through the comparative analysis of refrigerator thawing(RT),room temperature thawing(AT),water thawing(WT),and ultrasonic assisted thawing(UST),it was found that RT had little negative effect on the microstructure,color,and hardness of hawthorn,so it was selected as the thawing method of freeze-thawing.By analyzing the effects of freeze-thawing times on the drying characteristics and quality of hawthorn,it was found that two freeze-thaw cycles could better maintain the quality of freeze-dried hawthorn.Through the study of high voltage electrostatic field assisted freezing,it was found that electric field(0,1,3,5 kV/cm)had an important effect on the microstructure and quality of hawthorn,and could reduce the shrinkage rate of hawthorn.Among them,HVEF at 3 kV/cm(HVEF3)possessed the best effect as indicated by the lowest shrinkage rate(18.12%)and highest of total phenol content(46.31 mg GAE/g)of corresponding dried hawthorn and the shortened drying time(decreased by 11.17%).Thus,3 kV/cm was confirmed as the optimum parameter of HVEF.Then,the results indicated that the high voltage electrostatic field assisted freeze-thaw cycle pretreatment can effectively alleviate the structural damage of hawthorn cells during freeze-thaw process,shorten the drying time,as the high voltage electrostatic field assisted freeze-thaw cycle treatment(H-FT2)of freeze-dried hawthorn shrinkage rate was 17.58%.The H-FT2 treated freeze-dried hawthorn showed shorter drying time(decreased by 22.17%)and higher total anthocyanin content(1.50 mg C-3-G eqiv/G) compared with the untreated freeze-dried hawthorn,while its hardness(16.60 N),rehydration capacity(3.47 g/g),and total phenol content(38.19 mg GAE/g)were also higher than those of freeze-dried hawthorn treated with traditional freeze-thawing of twocycle(CFT2).Through comprehensive analysis,the electric field intensity of 3 kV/cm and high voltage electrostatic field assisted freeze-thaw cycle treatment of two cycles were selected as the pretreatment method of pilot PSBMFD dried hawthorn.On this basis,the impacts of microwave power,spraying time interval,and temperature upper on the quality of PSBMFD hawthorn were explored by PSBMFD small testing platform.By combining with the variation rule of the dielectric properties of hawthorn,the best microwave loading scheme was obtained as follows: the microwave power of first stage(0~0.5 h)was 0 W/g;the microwave power of second stage(0.5 ～ 2.5 h)was 0.9 W/g;the microwave power of third stage(2.5 ～ 4.5 h)was 1.0 W/g;the microwave power of fourth stage(4.5 ～ 6.5 h) was 0.8 W/g,the spray frequency was 1 time/10 min,the spray time was 0.1 s,and the upper temperature was 50℃.Secondly,aiming at the feasibility of commercial application of PSBMFD technology,on the basis of pretreatment technology and optimization of pilot-scale PSBMFD process,the drying characteristics and quality attributes of hawthorn dried by pilot-scale PSBMFD were studied and compared with that dried by CFD.The results showed that,compared with hawthorn dried using pilot-scale PSBMFD without pretreatment,H-FT2 pretreatment technology reduced the color damage and increased the uniformity of hawthorn dried using pilot-scale PSBMFD,decreasing the shrinkage rate of hawthorn dried using pilot-scale PSBMFD by 16.94%.In addition,compared with CFD,pilot-scale PSBMFD not only reduced the unit energy consumption and drying duration by 72.49% and 62.39%,respectively,but also endowed dried samples with porous structure,better texture(hardness 20.83 N),more original flavor,relatively high total phenolic content(37.03 mg GAE/g)and antioxidant capacity.Additionally,the uniformity of the moisture content,color,and shrinkage of hawthorn dried using pilot-scale PSBMFD were all higher than 90%.Finally,to determine the quality changes of freeze-dried products during storage,the changes in moisture content,color,hardness,reducing sugar,total acid,total number of colonies,total phenol content,and sensory score of hawthorn dried using pilot-scale PSBMFD during storage in incubator(25℃,RH75%)for 180 days were studied and compared with those of hawthorn dried by CFD.The results showed that hawthorn prepared by HVEF-assisted pilotscale PSBMFD(H-FT2-PSBMFD)had higher hygroscopic resistance,a* value(11.05),hardness(13.10 N),total phenol and ascorbic acid retention,lower total colony number(886CFU/g)and higher comprehensive score of sensory evaluation(29.75).The results of this pilot study provided an effective theoretical basis and technical support for the commercialization of PSBMFD technology.