Kiwi Fruit Freeze Drying-vacuum Microwave Combined Drying And Probiotic Dipping Process Optimization

The kiwifruit is rich in organic acids,vitamins,carotenoids,flavonoids and various mineral elements.It is rich in nutrients and is widely used as medicine and food raw materials.The nutritional characteristics of kiwifruit are gradually recognized.Because of high water content(more than 80%(wb)),fresh kiwifruit is prone to microbial spoilage.Improper handling will quickly deteriorate in a short period of time,making it difficult to store it for a long time.The period is greatly shortened.If fresh kiwi can be processed into dried products,its shelf life can be extended well.Freeze drying(FD)-vacuum microwave drying(VMD)combined with dried fruits and vegetables can be close to FD products,Can not only meet the current domestic and foreign market demand for high-end fruit and vegetable products,but also reduce production costs to enhance the market competitiveness of the product.FD and VMD combination drying,which reduces the drying time in the FD analysis section,and delivers semi-dry materials with low moisture content to VMD processing.The overall drying speed can save more than half of the time compared with the conventional FD process,and energy consumption It also fell sharply.In this dissertation,kiwifruit is used as the research object,and the kiwifruit slices were FD-VMD,and their quality characteristics and water distribution were studied.The effects of different moisture conversions on the drying characteristics of FD-VMD kiwi fruit and the water distribution characteristics of kiwi slices at different moisture conversion points were discussed.The optimal moisture conversion point and the optimal microwave power were determined during the combined drying process.Aiming at the quality characteristics and shrinkage of kiwi slices during the combined FD-VMD,a shrinkage model of kiwi row slices under different microwave power was simulated.In order to enrich the variety of probiotic products and extend the shelf life of probiotic products,FD-VMD probiotic-rich kiwi fruit tablets were prepared by vacuum dipping.Before drying,the process of vacuum dipping kiwi fruit was optimized to determine the optimal dipping parameters.Based on the optimal dipping parameters,the process of FD-VMD probiotic-rich kiwifruit tablets was optimized with the total number of viable bacteria as indicators,to determine the optimal combined drying process parameters,and to provide a theoretical basis for the later research of probiotic-rich dried products.The main contents of the study are as follows:1.Study on drying characteristics and moisture distribution of FD-VMD kiwifruit Slices:study different moisture conversion points(FD 4 h,FD 6 h,FD 8 h,FD 10 h,FD 12 h),different processing states(cold still,non-cold still)and different microwave Variation of moisture with drying time under power(0.25 W/g,0.30 W/g,0.39 W/g),and using drying time,appearance quality,color difference,bulk density,microstructure porosity,etc.as indicators,Optimal process parameters,and study the moisture distribution in different locations of the material during the drying process.The results show that FD 8 h is the optimal moisture conversion point after considering the total drying time and quality(sensory,microstructure).Microwave power is an important parameter during VMD.When the microwave power is high,the energy provided by the microwave is greater than the energy required for water evaporation.The cell structure is then destroyed by too much energy.In addition,when the microwave power is low,the energy provided by the microwave is lower than the energy required to evaporate the water.Then,because the drying time is longer,the drying speed becomes slower and the quality is also lowered.Therefore,0.30 W/g is the optimal microwave power.The better the microstructure of the material,the smaller the shrinkage and the smaller the bulk density.The structure of the kiwifruit itself is also not uniform.There are significant differences in cell sizes in the central and peripheral parts of kiwi,resulting in different migration of water in the central and peripheral parts.Terahertz imaging is used for the first time to study the quality of the surface structure of fruits.Studies have shown that the terahertz signal strength is related to the apparent structure of the material,providing a reference for the qualitative analysis of food structure.2.Study on quality characteristics and shrinkage model of FD-VMD Kiwi Fruit Slices:freeze-dry kiwi slices and put them into a vacuum microwave dryer to study different microwave powers(0.25 W/g,0.30 W/g,0.39 W/g)sensory quality,color difference,rehydration ratio,microstructure and shrinkage ratio of lower kiwifruit slices,and a shrinkage model was established.The results show that during the FD-VMD,the microwave power has a great influence on the drying characteristics of kiwi slices.Under different microwave powers,the drying characteristics of kiwi slices are very different.The higher the score,the smaller the color difference,the larger the rehydration ratio,and the larger the shrinkage ratio,that is,the larger the volume to reach the end of drying,and the larger the porosity of the kiwi slices.As the drying progresses,the lower the moisture content of the kiwi slices and the smaller the shrinkage ratio,it is possible that the volume shrinkage of the kiwi slices is related to the heat and mass transfer of the materials during the drying process,which will be further studied later.The data analysis results of four common shrinkage models show that the R² results of the Quadratic model under different microwaves are all above 0.99,the RSS is relatively small,and the expression is relatively simple,which can well predict the change of water ratio and shrinkage ratio of kiwi fruit in FD-VMD process.So choose Quadratic model for FD-VMD kiwi fruit slices of the optimal contract model.3.Optimization of FD-VMD probiotic kiwifruit slices technology:kiwifruit tablets are used as carriers for enriching probiotic bacteria,and probiotic bacteria enrichment treatment is carried out by means of vacuum immersion.Variables,taking the total number of viable bacteria as indicators,optimize the vacuum impregnation enrichment process through orthogonal design on the basis of single-factor experiment,and the moisture conversion point of FD-VMD based on the optimal impregnation process.The microwave power is a variable,and the total number of viable bacteria is used as an indicator.The process of combined drying is optimized through orthogonal design on the basis of single-factor experiment,with the aim of optimizing the preparation process of FD-VMD of probiotic kiwifruit tablets.In the optimization of the vacuum impregnation process,the impregnation temperature has a greater impact on the total number of probiotic viable bacteria,followed by the vacuum time,and finally the recompression time.The results from K value show that the optimal combination of vacuum impregnation is A₂B₂C₃,that is,the optimal condition is that the impregnation temperature is 35?,the vacuum time is 15 min,and the recompression time is 20 min.The main factors of the probiotic kiwifruit tablets in the FD-VMD stage are microwave power>vacuum degree>moisture conversion point,that is,microwave power has the largest impact on the total number of viable bacteria in the probiotic kiwifruit tablets,followed by vacuum degree and moisture conversion.Point impact is minimal.According to K value,the optimal process parameter of FD-VMD is A₃B₂C₁,that is,when the moisture conversion point is FD 10 h,the microwave power is 0.30 W/g,and the vacuum is 3 k Pa,the probiotic kiwifruit tablets contain the most total live bacteria.