Stress Analysis and Crack Prediction of Microwave Vacuum Drying of Wheat

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.