Theoretical And Experimental Study On The Dehydration Characteristics Of Seed Under Ice-temperature And Vacuum

Seeds are typical life materials.The purpose of dehydration of fresh seeds is mainly to extend the shelf life.During the dehydration process,it could lead to too large thermal stress and wet stress inside the seed if the instantaneous temperature or moisture gradient changes greatly,which will cause damage to the seed.In the traditional drying methods,the hot-air drying and vacuum freeze-drying are widely used in many materials.However,it usually damages the structure of living materials to varying degrees.As a new dehydration technology,ice-temperature vacuum drying can make up for the shortcomings of traditional drying methods: effectively preserve the tissue structure and the thermally unstable substances of living materials;ice crystal damage could be avoided because the dehydration temperature is higher than the freezing point.In this work,the ice-temperature vacuum drying was applied to seed dehydration,in order to provide new ideas for improving the quality of life materials after drying.Although this method has obvious advantages,shrinkage will inevitably occur during the dehydration process.It can lead to changes in the microstructure and affect the heat and mass transfer in the dehydration process.Excessive wet stress or thermal stress will cause the tissue rupture of living materials,affecting the activity of living materials.Mathematical models can analyze or guide the ice-temperature vacuum dehydration process of living materials,but the mass transfer model for ice-temperature vacuum dehydration is still rare.Therefore,in this paper,a theoretical model suitable for ice-temperature vacuum dehydration is established.To improve the accuracy of the established model,it considers the impacts of shrinkage on moisture transfer.Firstly,in order to obtain the dehydration characteristics and shrinkage characteristics of seeds during the ice-temperature vacuum drying,an ice-temperature vacuum dehydration test bench was built.Broad bean seeds were selected as the research object,and 9 groups of experiments(0 °C,4 °C,8 °C and 95 kPa,97kPa,99kPa)were conducted at different temperatures and different vacuum degrees.The results show that both temperature and vacuum can enhance moisture transfer,and the effect of temperature on moisture diffusion is more prominent;but temperature has no obvious effect on shrinkage;with the increase of vacuum,the degree of shrinkage of seeds increases.Moreover,during the dehydration,the length,width,and thickness(x-y-z)of broad beans is shrinking differently.The change in thickness direction is significantly greater than the length and width directions.When the vacuum is 99 kPa at different temperatures,the average shrinkage degree in the thickness direction can reach 0.565.The variations of shrinkage and porosity increased significantly when the moisture content is lower.Combining the macro-shrinkage characteristics and the microstructure changes of broad bean,the water content should eventually be higher than 0.05 ~ 0.11 during dehydration.Secondly,based on the dehydration and shrinkage characteristics of broad bean seeds obtained from the experiment,and the Luikov theory and the Fick diffusion law,comprehensively considering the coupling effect of moisture transfer and shrinkage,a theoretical model suitable for ice-temperature vacuum drying was established.And the effective moisture diffusivity coefficient and characteristic length in the model are very important.The traditional model simply assumes that the effective moisture diffusivity coefficient is constant and ignores the impacts of shrinkage.During dehydration,the effective moisture diffusivity coefficient will be affected by temperature,vacuum,moisture content,and characteristic length.The characteristic le ngth is closely related to shrinkage.These assumptions will cause decreasing for the prediction accuracy of the moisture diffusion model.In this paper,considering the dynamic effective moisture diffusion coefficient and characteristic length affected by moisture content,temperature and vacuum degree,a moisture diffusion model suitable for broad bean ice-temperature vacuum drying is established.Finally,according to the established moisture diffusion model,the variation curves of the moisture content of broad bean at different temperatures and vacuum degrees were simulated.The results show that the prediction results of present model are in good agreement with the experimental results.In addition,two traditional models(1.Considering the dynamic effective water diffusion coefficient,but ignoring the effect of shrinkage,2.Even assuming that the effective water diffusion coefficient is constant,and also ignoring the effect of shrinkage),were used to simulate the moisture curves of the broad bean seed.Compare the prediction results with the present the model,it shows that the moisture diffusion model established in this paper is more suitable for predicting the moisture content curves of broad bean seeds during ice-temperature vacuum dehydration.The broad bean sees is also assumed to be a three-dimensional cube to simulate the moisture content curves.The results show that the one-dimensional plate model is more suitable than the simple three-dimensional cube for predicting the moisture content curves of broad bean seeds under ice-temperature vacuum drying.