Study On The Microwave Freeze-drying Process And Quality Control Of Strawberry

Strawberry is a characteristic horticultural agricultural product with great nutritional value and agricultural value.Its seasonality is strong,the fruit is delicate,perishable,and easily damaged by machinery,which is not conducive to transportation and storage.Using freeze-drying(FD)for this kind of high-nutrition,short shelf-life fruit can well retain the nutritional value of it and realize the increase in the economic added value.How to solve the problem of long time-consuming and high energy consumption of traditional FD,while ensuring product quality,is a problem that needs to be considered.This article adopted a new type of microwave efficient freeze-drying technology(MFD),by replacing the traditional heating plate with a high-frequency(2450 MHz)microwave heating source,and using a pulsed spouted bed system(PS)to improve drying uniformity,to achieve the coordination of cost and product quality.In addition,the effects of different pretreatment methods on the freeze-drying process and product quality were discussed,and the mechanism of action was analyzed.The purpose was to produce freeze-dried strawberry snack foods with better nutrition and sensory quality.Firstly,the pulse spouted bed microwave freeze drying process(PSMFD)optimization was carried out.Based on the high frequency(2450 MHz)microwave source and the pulsed spouted bed,the effects of microwave power,spouted interval time and heating upper temperature were studied.The results showed that the PSMFD process was mainly affected by microwave power and heating upper temperature.With the increase of microwave power and heating upper temperature,drying time would decrease.The optimal parameters were:microwave power density 1 W/g-2 W/g,spouting interval 5 min,heating upper temperature 55?.Whether the pulse spouted bed was turned on,MFD could shorten the drying time by 45%compared with FD,and saved more than 42%of the drying energy consumption,while the pulse spouted bed greatly improved the drying uniformity of the MFD,the moisture RSD was reduced by17.72%,and the temperature RSD was reduced by 18.87%.The nutrient retention rate was also improved.In order to use microwave energy more rationally,this paper designed a four-stage variable power microwave input scheme based on the dielectric properties of strawberries.The nutrition(TPC,TFC)and color of the product were better than those of PSMFD samples with constant microwave input.The model Midilli could well describe the MFD and PSMFD process.Secondly,the feasibility of pretreatment of frozen strawberries was explored,and the effects of ultrasonic(US),freeze-thaw(FT)and frozen ultrasonic melting(FUS)pretreatment on the drying process of PSMFD and the quality of dried products were studied.It was found that to achieve effective cavitation effect requires sufficient intensity of ultrasonic action,and the optimal ultrasonic parameters were 15 min and240 W.Freeze-thaw samples had lower WL and higher SL.US,FT,and FUS reduced the eutectic point temperature by 3.16°C,7.67°C,7.83°C,and increased the eutectic point temperature by 0.67°C,0.92°C,2.08°C,respectively.The three pretreatments had no significant effect on the dielectric properties,but had a differential effect on the T₂₂peak in the water distribution state.In summary,FUS had the greatest impact on cell structure and material properties.Its drying rate was the fastest,which was 15.38%less than the control group.The drying uniformity was the best,and the RSD decreased by1.30%(temperature)and 3.79%(moisture)compared with the control group.Its dry products had the best rehydration capacity and the lowest hygroscopicity.Finally,in order to improve the sugar-acid ratio of strawberries and enhance the taste of freeze-dried strawberry products,sucrose,trehalose,mannitol,and isomalt oligosaccharides were selected to perform USOD treatment on frozen strawberries.It was found that USOD increased the WL and SG of strawberries,and increased the?".The T₂ curve shifted to the right to varying degrees,and the T₂₂ peak area decreased.The drying time was further shortened,which could reduce 8.33%(sucrose)to 16.67%(mannitol)compared with the control group.In addition,USOD had increased the a*value and hardness value of the product,and obtained good TPC and TFC retention rates,and the sensory and nutritional quality was better than before treatment.However,the product's anthocyanin content,bulk density and rehydration ratio declined.The difference in the hygroscopicity of the product indicated that the use of a penetrant with low hygroscopicity to partially replace the hygroscopic sugars in the material could reduce the moisture absorption of the product to a certain extent and improve the storage stability of dry products.In general,trehalose,mannitol and isomalt oligosaccharides could all replace sucrose to increase the drying rate and produced more high-quality and healthy freeze-dried strawberry snack foods.