Study On Microwave Vacuum Drying Technology Of Yam

Yam?Dioscorea opposita Thunb?is a traditional medicine and food with high nutritional value.It is rich in protein,polysaccharide,amino acids,vitamins,allantoin and other nutritional active ingredients,but it is perishable and can't be stored for a long time.Drying is one of the important processing methods in yam.Microwave-vacuum drying has the advantages of fast drying speed,high efficiency and low nutrient loss.In order to improve the quality of yam products and solve the problem of processing and preservation,in this study,the method of microwave-vacuum drying of yam was used to research the drying characteristics and process,and the quality of dried yam was studied and compared.The main conclusions are as follows:1.The microwave-vacuum drying characteristics of yam under different parameters such as shapes,blanching conditions,microwave power density,vacuum degree,rotating speed,temperature and slice thickness were studied,and the drying kinetic model was established.The results showed that the slicing methods,microwave power density,vacuum degree,temperature and slice thickness had great influence on the drying rate of yam.Nonlinear regression fitting of six drying models under different vacuum were carried out and the model parameters were determined.The results showed that the Page model had higher coefficient of determination?R²?and lower residual sum of squares?RSS?,and P<0.01,which indicated that the model could accurately describe and predict the moisture dynamics of yam during microwave-vacuum drying process.D_eff ranged from 0.4256×10^-9 to6.6377×10^-9 under different conditions.D_eff increased with the increase of vacuum,chip thickness and drying temperature,decreased with the increase of microwave power density,and the value of activation energy?Ea?at different drying temperatures was 15.8621 KJ/mol.2.With the polysaccharide content,whiteness and rehydration rate as evaluation indexes,the microwave vacuum drying process of yam was optimized by single factor and quadratic rotation orthogonal experiment,and the functional active components and antioxidant capacity of dried yam were evaluated.The results showed that the regression model established by quadratic rotation orthogonal experiment can describe the relationship between factors and indexes well.The optimum production conditions are drying temperature 55?,slice thickness 3 mm,vacuum-0.08 MPa,under these conditions,the polysaccharide content,whiteness and rehydration rate of dried yam were 4.12±0.08%,78.04±0.82 and 223.29±1.31%,respectively.The IC₅₀ of dried yam scavenging DPPH radical,hydroxyl radical and superoxide anion radical were 18.61±0.11,19.86±0.15 and21.06±0.14 mg/mL,respectively.The contents of polyphenols,flavones and allantoin were0.71±0.05,1.32±0.04,6.86±0.08 mg/g,respectively.3.The effects of microwave-vacuum drying,hot air drying,far-infrared drying,vacuum drying and microwave drying on the quality of yam were studied.The results showed that microwave-vacuum drying yam had the highest whiteness,the smallest shrinkage and the best overall appearance,while microwave drying had the highest hardness,the smallest bulk density,poor color and luster,high shrinkage,and even a small part of yam appeared burnt.Vacuum drying and microwave-vacuum drying yam have the least loss of nutrients and functional active ingredients.Vacuum drying yam have the strongest antioxidant capacity.Scanning electron microscopy showed that microwave vacuum drying of yam had more uniform honeycomb structure and more complete cell structure.The results of FTIR and XRD showed that the molecular order of the outer region of yam starch granules dried by microwave,hot air and vacuum was lower than that dried by microwave vacuum and far-infrared,and the highest crystallinity of yam powder dried by microwave vacuum was25.16±0.76%.From the gelatinization characteristics,vacuum drying of yam has the highest viscosity,microwave-vacuum drying of yam has good gelatinization stability,and starch granules are not easy to aging.Different drying methods had significant effects on the digestibility of yam in vitro?P<0.05?.Microwave-vacuum drying yam had the highest digestibility?24.13±0.58%?.Cluster analysis showed that microwave,hot air and far-infrared drying could easily cause the loss of nutrients in yam,while vacuum drying and microwave vacuum drying could better retain the nutrients and functional active ingredients in fresh yam.Principal component analysis showed that the comprehensive score of microwave vacuum drying yam was the highest,so its physical and chemical quality was the best.