Study On The Drying Characteristics Of Ultrasound-Heat Pump And The Isotherm Of Pea Seeds

Grain security is an important problem for agricultural development.The development of drying technology can effectively reduce grain losses,improve seed quality and promote sustainable development of agriculture.In order to improve the drying efficiency and ensure the quality of seeds after drying,the ultrasound assisted heat pump drying method is studied,which could provides a new way for the innovation of drying technology.Meanwhile,the studies on sorption isotherms of seeds could provide an important theoretical basis for the optimization of seed storage conditions.The main contents of this paper are summarized as follows:(1)Sorption isotherms of pea seeds were determined by using a static gravimetric at temperature of 30-50? and within the range of 0.11-0.92 water activity.The characteristics of pea seed conformed to III type isotherm and hysteresis existed in the adsorption processes.GAB model was the best model for predicting the isotherm data of pea seeds.The thermodynamic properties involving in net isosteric heat,differential entropy,enthalpy-entropy compensation,spreading pressure,net integral enthalpy and net integral entropy were studied.The results showed that net integral entropy has the lowest value under certain condition.The ranges of water activity and equilibrium moisture content corresponding to the lowest values are 0.017~0.064 and 2.2%~2.7% in the temperature studied.These conditions can be considered as the maximum stability storage conditions of pea seed.(2)On the basis of the closed air-borne heat pump drying system,ultrasonic equipment is added.In this combined drying system,pea seeds were dried at different temperature(30?,35? and 40?),different ultrasonic power(0W,60 W and 100W)and frequencies(0kHz,28 kHz and 40kHz).The results showed that Midilli model could describe well the drying processes of pea seeds.According to the change rule of different drying parameters,high temperature or application of ultrasound has a positive effect on drying rate and moisture diffusivity increased with the increase in temperature,ultrasonic power and frequency.(3)The physical and chemical characteristics and germination characteristics of pea seed after ultrasonic drying assisted heat pump were measured for studying the effect of different temperature(30?,35? and 40?),different ultrasonic power(0W,60 W and 100W)and frequencies(0kHz,28 kHz and 40kHz)on SOD,POD and CAT activity,MDA content,germination,mean germination time and germination index of pea seeds.The results showed that application of ultrasound could improve the activity of antioxidant enzymes,reduce MDA content and increase the germination percentage and germination rate of pea seeds.However,the increase of temperature could decrease SOD activity and accelerate the accumulation of MDA,which was unfavorable to the germination of seeds.