Effect Of Ultrasonic Pretreatment On Drying Characteristics And Quality Of Dendrobium Candidum

Ultrasonic is widely used in the field of food and drug processing.Ultrasonic can change the structure of materials,promote the water migration and shorten the drying time in the material drying,which plays an important role in improving the quality of materials.Dendrobium officinale is a precious Chinese herbal medicine,which contain polysaccharides,polyphenols and other effective ingredients.Fresh Dendrobium officinale needs to be dried for convenient transportation and storage.In this paper,the drying characteristics of ultrasonic pretreatment of Dendrobium officinale were studied for the first time.The water transfer and the change of effective components were discussed under the conditions for hot air drying and microwave drying respectively.The effect of ultrasonic pretreatment on effective components and the sensory quality of Dendrobium officinale after drying was studied,which provided the theoretical basis for the application of ultrasonic pretreatment drying of Dendrobium officinale.The results are as follows:(1)Hot air drying characteristics of Dendrobium officinale pretreated by ultrasound.The results showed that the number of micro pores and the pore size increased in Dendrobium officinale treated with ultrasonoc which was beneficial for water transform,reduced moisture ratio of material,increased drying rate and shortened drying time.When the temperature of hot air was 100?,the time of hot air drying treated with ultrosonic for 10 min at 800 w was one hour less than that of hot air drying.The effective diffusion coefficient of water in Dendrobium officinale treated with ultrasonic under hot air drying was 4.5758×10-9 m2/s and it was 1.7095×10-9 m2/s under control.The drying activation energy was about 76.5429 kJ/mol.Boltzmann(100?)was the best model for drying Dendrobium officinale by ultrasonic,and the fitting equation was MR=0.0336+1.0699/(1+exp((t-0.5903)/0.2642)).(2)Microwave drying characteristics of Dendrobium candidum pretreated by ultrasound.Dendrobium officinale was dried by means of ultrasound-assisted microwave drying,the drying rate and effective diffusion coefficient were investigated and a mathematical model was established.The experimental results showed that when the optimal microwave power was 600 W,the rate of ultrasonic assisted microwave drying was faster than that of microwave drying,and the drying time could be shortened by 20 min.The corresponding water diffusion coefficients were 2.6230×10-8 m2/s and 1.6711×10-8 m2/s,respectively.The best model of ultrasonically enhanced microwave drying was Cubic,and the fitting equation is MR=0.9965-0.0955*t+0.0031*t^2-3.2808*10-5*t^3.(3)The effect of ultrasonic assisted different drying methods on the content of active components in Dendrobium officinale.By measuring the changes in the content of polysaccharide,polyphenol and amino acid,the effects of hot air drying and microwave drying without and with ultrasonic pretreatment on the content of active ingredients were explored.The results showed that the maximum contents of polysaccharide,polyphenol and amino acid in the Dendrobium officinale treated with ultrasonic-assisted hot air drying and single hot air drying were 28.75%,23.95%,2.15 mg/g,1.73 mg/g,83.39 mg/g and 75.39 mg/g,respectively.In the Dendrobium officinale after ultrasonic-assisted microwave drying and single microwave drying,the maximum contents of polysaccharides,polyphenol and amino acid are 32.38%,29.38%,2.53 mg/g,2.42 mg/g,53.38 mg/g and 50.26 mg/g,respectively.(4)Effect of ultrasonic assisted drying on sensory quality of Dendrobium officinale.The changes of sensory quality of Dendrobium officinale after hot air drying,microwave drying and ultrasonic pretreatment were observed,and shrinkage rate and rehydration rate were calculated.The results showed that the color,smell and texture of Dendrobium officinale dried by ultrasonic assisted hot air was brown,burnt,and hard,while that of Dendrobium officinale dried by ultrasonic enhanced microwave respectively was golden,burnt and tough.After ultrasonic pretreatment,the values of L*,negative a*and b*will increase.The shrinkage rates of the samples without and with ultrasonic pretreatment were about 60%,and their maximum rehydration rates were 243%,255%,262%and 278%,respectively.The sensory quality of Dendrobium officinale pretreated by ultrasound was better than that of single drying.