Study On Drying Characteristics And Quality Of Sequential Freeze-infrared Drying Jujube Slices

Jujube slices have high sugar content and strong heat sensitivity of nutrients.The existing single drying method has the problems of long drying time,serious loss of nutrients,collapse of internal structure and poor appearance and taste.Combined drying is to complement the advantages of two or more single drying,which has the characteristics of saving drying time and retaining the original quality of materials to the greatest extent.Based on the analysis of the research status of single drying and freeze-infrared combined drying（FD-IRD）of fruits and vegetables at home and abroad,this thesis compares the effects of four single drying methods of fruits and vegetables on the drying characteristics and quality of jujube slices,selects the segmented combination of freeze-drying and infrared drying to dry jujube slices,and studies the drying characteristics and drying kinetics of FD-IRD of jujube slices under different drying conditions,The FD-IRD process parameters were optimized by response surface method,and the drying time and quality of FD-IRD products,single FD and single IRD products were compared and analyzed,so as to provide theoretical basis and technical reference for the development of a combined drying process of jujube slices with improved quality and efficiency.The main research content and test results of this thesis are as follows:（1）The effects of freeze drying（FD）,infrared drying（IRD）,hot air drying（HAD）and microwave vacuum drying（MVD）on the drying characteristics,color,rehydration ratio,texture and microstructure of jujube slices were compared.It was found that the drying time of FD and had was the longest,that of MVD was the shortest,and that of IRD was the second;FD products have good quality,but their crispness and brittleness are general.IRD products are better than HAD and MVD products in color,texture（Hardness/Brittleness）and microstructure,and their crispness and brittleness are better.The sequential combination of FD and IRD,combined with their advantages,is expected to shorten the drying time while ensuring the drying quality.（2）The effects of different conversion moisture content,infrared temperature and slice thickness on the drying characteristics and drying kinetics of FD-IRD were studied.The results show that the lower the conversion moisture content is,the shorter the IRD drying time is,but the lower the conversion moisture content is,the longer the FD drying time will be;Properly increasing the drying temperature of IRD is beneficial to improve the effective diffusion coefficient of water;The thinner the jujube slice,the greater the drying rate.Reducing the slice thickness can improve the effective diffusion coefficient of water and shorten the drying time;The first and second stages are both in the slow drying process,and the water effective diffusion coefficients of FD stage and IRD stage under different drying conditions change in the range of 3.39×10^–9～9.47×10^–9 m²·s^-1and 3.34×10^–9～2.01×10^–8 m²·s^-1 respectively by solving Fick’s second law;The drying activation energy of IRD stage is 59.03 k J/mol according to anius formula.（3）Based on the single factor experiment,the conversion moisture content,infrared temperature and slice thickness were selected as the influencing factors,the drying time and vitamin C retention were taken as the evaluation indexes,and the FD-IRD process parameters of jujube slices were optimized by ternary quadratic general rotation combination design.The regression models of drying time,VC retention and conversion moisture content,infrared temperature and slice thickness were established by response surface method.R² were 0.993 8 and 0.994 3 respectively.The influence order of each factor on drying time is conversion moisture content>infrared temperature>slice thickness,and the influence order of VC retention is infrared temperature>conversion moisture content>slice thickness.The better FD-IRD process parameters are as follows:the conversion moisture content is 34%,the infrared temperature is64℃,the slice thickness is 5 mm,the drying time is 3.62 h,and the VC retention is 68.92%.（4）The drying effects of FD-IRD products with better parameters were compared with those of single FD and IRD products.The results showed that the drying time of FD-IRD was 57.6%shorter than that of FD,and the retention rate of VC was 34.6%higher than that of IRD;Compared with IRD,the color difference is reduced,and the texture characteristics of low hardness and high brittleness are maintained.At the same time,the deep and uniform micro pore structure and high porosity are retained.