孙铜生,樊金生,王凯,等. 小麦微波真空干燥应力解析及裂纹预测[J]. 农业工程学报,2024,40(21):1-9. DOI: 10.11975/j.issn.1002-6819.202403066
    引用本文: 孙铜生,樊金生,王凯,等. 小麦微波真空干燥应力解析及裂纹预测[J]. 农业工程学报,2024,40(21):1-9. DOI: 10.11975/j.issn.1002-6819.202403066
    SUN Tonsheng, FAN Jinsheng, WANG Kai, et al. Stress Analysis and Crack Prediction of Microwave Vacuum Drying of Wheat[J]. Transactions of the Chinese Society of Agricultural Engineering (Transactions of the CSAE), 2024, 40(21): 1-9. DOI: 10.11975/j.issn.1002-6819.202403066
    Citation: SUN Tonsheng, FAN Jinsheng, WANG Kai, et al. Stress Analysis and Crack Prediction of Microwave Vacuum Drying of Wheat[J]. Transactions of the Chinese Society of Agricultural Engineering (Transactions of the CSAE), 2024, 40(21): 1-9. DOI: 10.11975/j.issn.1002-6819.202403066

    小麦微波真空干燥应力解析及裂纹预测

    Stress Analysis and Crack Prediction of Microwave Vacuum Drying of Wheat

    • 摘要: 针对微波真空干燥过程中小麦易产生应力裂纹从而影响品质的问题,该研究采用质构分析仪进行了小麦的压缩试验,结合广义Maxwell模型建立了小麦的应力模型,确定了小麦的应力松弛及干燥后小麦承载能力随变形量的变化规律,获得了有效水分扩散系数并研究了活化能对其影响,分析了干燥过程中小麦的含水率、温度、爆腰率和应力之间的关系。结果表明:干燥温度在50~70 ℃之间时,干燥后小麦颗粒的裂纹数量显著增加,承载能力大幅下降,容易在储藏过程发生破裂。此外,温度的升高加速了小麦内部水分传输,但同时也破坏了小麦的内部结构,导致干燥裂纹的产生。裂纹的产生总体上加快了水分的传输速率,但也存在传输速率降低的可能。活化能反映水分传输的难易程度,结合有效水分扩散系数可知,它们之间的关系反映了水分扩散过程中的能量需求和效率之间的平衡。温度和湿基含水率对小麦所受应力的影响程度不同,当湿基含水率低于14%时,应力随温度的升高呈现先增加后减少的趋势,而在湿基含水率高于14%的范围内,随着温度的增加应力呈现先减少后增加的趋势。当应力超过小麦颗粒的强度极限时,会导致裂纹形成,并进一步降低小麦颗粒的强度极限。为了控制小麦的爆腰率和减少裂纹数量,提出了最佳干燥工艺为:将常温下的小麦以2.86 ℃/min的温度上升速率干燥至40 ℃,温度到达40 ℃后恒温干燥7 min,然后将温度上升速率调整到1.63 ℃/min,温度到达61.4 ℃后结束干燥,此工艺不仅降低了干燥小麦的爆腰率,还可以预测爆腰小麦的裂纹数量,为微波真空干燥小麦工艺提供了理论和试验基础。

       

      Abstract: Drying is a crucial step in the post-harvest processing of crops, aimed at reducing the moisture content and extending the shelf life of agricultural products by lowering the water activity to levels that inhibit microbial growth, enzyme reactions, and other deterioration reactions. Wheat drying research aims to shorten processing time, reduce costs, improve drying product quality, and enhance drying efficiency. Commonly used wheat drying techniques include hot air drying, microwave drying, natural drying, freeze drying, and vacuum drying. Microwave vacuum drying combines the rapid and efficient drying of microwave drying with the low-temperature characteristics of vacuum drying, effectively addressing the trade-off between wheat quality and economic benefits. During the microwave vacuum drying process, wheat experiences drying stress due to temperature and moisture gradients. When the drying stress exceeds the wheat's strength limit, cracks and kernel bursting occur, resulting in a decrease in the grade of dried wheat. To address the issue of stress cracking in wheat during microwave vacuum drying, a digital image measurement system-based texture analyzer was used to conduct 30 repeated compression tests on wheat samples. A stress model for wheat was established based on the generalized Maxwell model. The stress relaxation behavior, stress variation with deformation, and effective moisture diffusivity were determined. The influence of activation energy on effective moisture diffusion was also investigated. Aiming at the problem that wheat is prone to stress cracks during microwave vacuum drying and thus affects the quality, the study conducted a compression test of grain by using a texture analyzer, established a stress model of wheat by combining it with the generalized Maxwell model, determined the stress relaxation of grain and the change rule of the load-bearing capacity of wheat after drying with the deformation amount, obtained the effective moisture diffusion coefficient, and investigated the effect of the activation energy on it. The relationship between moisture content, temperature, bursting waist rate, and stress of wheat in the drying process was analyzed. The results showed that when the drying temperature was between 50 and 70 ℃, the number of cracks in the dried wheat grains increased significantly, the carrying capacity decreased substantially, and rupture occurred easily in the storage process. Moreover, higher temperatures accelerated internal moisture transfer in wheat but also disrupted the internal structure, resulting in the formation of drying cracks. The formation of cracks generally speeds up the water transfer rate, but there is also the possibility of decreasing the transfer rate. Activation energy reflects the difficulty of water transport, and combined with the effective water diffusion coefficient, the relationship between them reflects the balance between energy demand and efficiency in the water diffusion process. The impact of temperature and moisture content on the stress experienced by wheat varied. When the moisture content was below 14% (wet basis), the stress initially increased and then decreased with temperature. However, within the moisture content range above 14%, the stress exhibited a decreasing trend followed by an increasing trend as temperature increased. Exceeding the strength limit of wheat particles resulted in crack formation, further reducing the strength limit of wheat grains. To control the kernel bursting rate and minimize the number of cracks in wheat, an optimized drying process was proposed. The process involved drying wheat from ambient temperature to 40 ℃ at a heating rate of 2.86 ℃/min, maintaining a constant temperature of 40 ℃ for 7 min, and then adjusting the heating rate to 1.63 ℃/min until reaching 61.4 ℃, at which point the drying process was concluded. This optimized process not only reduced the kernel bursting rate during wheat drying but also allowed for the prediction of the number of cracks in burst wheat, providing a theoretical and experimental foundation for microwave vacuum drying of wheat.

       

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