| Walnut,the second largest nut in the world,has very important development and utilization value.Walnut initial and deep processing is an important way to improve its comprehensive utilization rate and extend the industrial chain and value chain.Among them,the loss of walnut shelling and kernel removal is the main factor that significantly reduces the quality of nut processing and severely restricts the development of walnut deep processing.Therefore,reducing the loss of kernels during the process of breaking the shell and removing the kernels is an important problem that needs to be solved urgently in the current research on walnut processing and deep processing.When the walnut shell is broken,the kernel must have sufficient strength and hardness to prevent it from being broken.The strength and hardness of the shell should be as small as possible to ensure that the shell is broken as much as possible.The moisture content has an important impact on its strength and hardness,which in turn affects the effect of walnut shell breaking.Therefore,the impact of walnut moisture content on its shell breaking is studied,and the critical moisture content of shell breaking is determined,and then a set of drying process parameters that improve quality,reduce damage,increase efficiency and energy saving optimized.It is of great significance to reduce the loss of shell and kernel separation and the process of crushing the shell.It also provides a theoretical basis for formulating the best shelling process and the design of a new type of walnut shelling machine.In order to clarify the impact of moisture content on walnut shell breaking,and then determine the critical moisture content of shell breaking,on this basis,optimize a set of drying process parameters for quality improvement,loss reduction,efficiency increase and energy saving.This paper uses the main production of Xinjiang Wen 185 walnut as an example.The research object is to first analyze the drying characteristics of walnut heterogeneous structure,establish a drying kinetic model and effective water diffusion coefficient model;secondly,analyze the impact of moisture content on the effect of walnut shell breaking,quantify the degree of shell breakage,and use the whole shell to break The shell effect calibrates the critical moisture content of shell breaking;finally,on this basis,a combined hot air and microwave vacuum drying method is proposed,and the process parameters are optimized with the goal of walnut shell breaking.It is expected to provide theoretical basis and production practice guidance for walnut drying processing and equipment design.The specific research conclusions are as follows:（1）Prove the hot air drying characteristics of walnuts.The drying characteristics of husks,cores and walnuts are roughly the same.The hot-air drying process mainly occurs in the reduced-speed drying stage,and there is no obvious constant-speed drying stage;the walnuts show significant instability and heterogeneity during the drying process.The effective water diffusion coefficient ranges of shell,core and walnut are 1.22×10-8～1.23×10-7m2/s,1.07×10-8～8.16×10-8m2/s,1.36×10-8～1.2×10-7m2/s,and it is in a third-order-polynomial relationship with the moisture content;the Two term model is suitable for predicting the change law of the moisture ratio of the husks,cores and walnuts during the hot-air drying process at 43°C.The initial moisture content and crevice structure of walnut have significant effects on drying.During the drying process,the effective water diffusion coefficient of the walnuts with gaps is higher than that of the sealed walnuts,which lays the foundation for the analysis of walnut hot-air drying characteristics and the proposal of the drying process.（2）It is clear that the grain size distribution of broken walnut shell conforms to lognormal distribution,and the relationship model of characteristic parameter value and quantity fractal dimension of water content and grain size distribution is obtained,and the correlation model of quantity fractal dimension and characteristic parameter value is established.It is concluded that the fractal dimension DNcan be used as a quantitative index of the degree of fragmentation of walnut shell particles,so as to quantify the degree of fragmentation of walnut shell,and it is proposed that the whole state of broken shell and kernel can significantly reduce the loss of kernel.With the decrease of water content,the breaking of walnut shell decreases firstly and then increases.The optimal water content of walnut shell is 7.72%-11.6%,and the critical water content is 9.41%.The whole kernel percentage increased first and then decreased with the decrease of moisture content.The optimal range of moisture content was 8.27%～16.98%,and the critical moisture content was 11.2%.（3）Optimized a set of drying process parameters for improving quality,reducing damage,increasing efficiency and energy saving for the purpose of shell breaking.The optimal process parameters are:the hot air temperature is 63℃,the moisture content at the conversion point is 35%,the microwave power is 600W,and the microwave temperature control temperature is 50℃.Tests have verified that the water loss rate per unit time is 0.323%/min,the drying energy consumption per unit mass is 7.65k W·h/kg,the target moisture content error value is 0.13%,and the nut color difference value is 2.46%.The results of dry-shell cracking test showed that the fractal dimension of particle distribution of broken fruit shell was 0.524,and the kernel consolidation rate was 73.88%.The effect of shell cracking met the requirements of kernel consolidation.It can lay a solid foundation for the improvement of walnut drying and shell breaking effect.The research results can provide theoretical basis and technological guidance for the development of walnut industrialization,and will also promote the development of walnut drying and shell-breaking continuous processing industry. |
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