Thermodynamic Analysis And Damage Mechanism Of Rice Seed During Drying Process

Drying process is very essential for rice seed storage.Drying of seeds can bring the moisture content down and minimize the spoilage due to microorganisms,avoid injury associated with freezing or chilling and extend the longevity of seeds.However,the inappropriate drying methods may damage the cell membranes,inactivate enzymes,degrade storage protein and split the seed coat,all of which could lead to the loss of viability and vigor of seeds.The research of grain drying was much more than that of seed drying in China.Therefore,it is very important to study the physiological and biochemical changes in rice seeds during the drying process for the seed drying industry.(1)The equilibrium water content of rice seeds with different temperatures and water activities was studied in this paper.The GAB,Halsey,Smith,Oswin,Herderson and modified Chung-Post models were used to fit the seed desorption data.The results showed that the GAB model had the highest R2 and minimum errors,which was most suitable for describing the desorption process of rice seeds.Through the thermodynamic analysis,the minimum integral entropy was used to determine the optimum water content for the rice seed storage and the applicability of the enthalpy-entropy compensation theory to desorption phenomenon for rice seed was evaluated.(2)The thin layer drying properties with different temperatures of rice seed were studied using empirical and semi-theoretical models.The total drying occurred in falling rate period,indicating the influence of liquid diffusion during drying.The drying behavior was simulated based on the sphere,finite cylinder and slab.Results showed that the effective moisture diffusivity was improved when the drying temperature increased and the temperature dependence of the effective diffusivity followed an Arrhenius relationship.The results also indicated that the simulation of drying behavior based on sphere was in better agreement with the experimental data as compared with other geometries.(3)Stress-relaxation and the dynamic viscoelastic behaviors of the single rice seed were studied using a dynamic mechanical analyzer(DMA)in compression mode.The relaxation modulus was found to decrease as the temperature and moisture increased.A master curve of relaxation modulus as a function of temperature and moisture content was generated using the time-moisture-temperature superposition principle.Results showed that the generalized Maxwell model satisfactorily fitted the experimental data of the stress-relaxation behavior and the master curve of relaxation modulus(R2>0.997).The dynamic viscoelasticity of the rice seed was affected by the temperature,moisture content and frequency.The storage modulus and loss modulus decreased when the temperature and moisture content increased and the higher values of storage and loss modulus were observed as the frequency increased.The changes of loss modulus had the similar trend with the tan delta when the temperature increased.They increased at lower temperature,reaching to the maximum eventually and the declined as the temperature increased further.The temperature corresponding to the maximum value of the loss modulus and tan delta decreased as the moisture content of the rice seed increased.(4)The alterations of the physiological properties associated with the ultrastructure changes of rice seed after different drying conditions were studied.The germination percentage of the rice seeds was reduced as the increasing of the drying temperature and the highest value of vigor index was recorded at 45℃ due to the dormancy breakage.There were no changes in the color of the raw rice seed dried at different temperatures;however,the yellowness of the brown rice and the electrical conductivity increased with the increasing drying temperature.The structure of the starch and the embryo cells were severely impaired as the temperature increased.It was found that the physiological quality of rice seed was closely associated with the alterations of cell structure during drying process.(5)The effect of seed initial moisture content,drying temperature and drying time on the seed germination was analyzed in this work.The results demonstrated that the modified Giner 1and 2 model could simulate the changes of seed germination satisfactorily during the drying process.The parameters of the models were also obtained and the models could be used to predict the alterations of the seed germination after drying.(6)The method of rice seed drying with high temperature followed by tempering at ambient temperature was investigated to determine whether this method could preserve the seed viability and vigor.The results showed that the intermittent drying followed by tempering could improve the drying efficiency and reduce the drying time.It was verified that the appropriate drying temperature,drying time and tempering time could keep the seed quality and increase the seed germination rate due to the dormancy break.