Wheat is one of the most vital staple crops in the world. Its milling process is one type of physical operation using extrusion-induced fragmentation. This study aims to explore the influence of the kernel morphological structure on its mechanical behavior under compressive loads using X-ray micro-computed tomography (micro-CT) modeling and finite element analysis (FEA). The research subjects were selected as wheat kernels with a moisture content of 16%. Accurate three-dimensional (3D) models were constructed for the simulation. The milling parameters were optimized for highly precise and low-loss wheat processing. A texture analyzer was employed to capture the stress-strain curves of the wheat kernels. Different morphological types were utilized under the ventral and lateral compression modes. These curves were used to determine the relationships among kernel morphology, elastic modulus, compressive strength, and ultimate load. The results show that the minimum elastic modulus and compressive strength were calculated as 35.15 and 5.57 MPa, respectively in the largest kernel type (A1). Large-grained wheat also exhibited a higher limit load during extrusion, indicating a stronger resistance to deformation. In contrast, the small-grained wheat shared the lower limit load more prone to rupture under relatively lower loads. Furthermore, the ultimate load reached 59.62 N under ventral compression, which was significantly higher than the 50.44 N observed under lateral compression. Therefore, the minimum extrusion load of 59.62 N was recommended for the full fragmentation of the wheat kernel in industrial milling. Three-dimensional geometric models of the wheat kernels were reconstructed using micro-CT scan data and reverse engineering techniques. Subsequently, the optimal models were imported into the FEA software. The distribution of the stress and strain fields was then simulated to clarify the total deformation behavior under compression. Simulation results indicated that the ventral groove region exhibited the most significant concentration of stress and strain, indicating the primary structural vulnerability during loading. In contrast, the equatorial plane was identified as the key governing region for the propagation of deformation throughout the kernel. Furthermore, the crack propagation paths in micro-CT images demonstrated that the high degree of spatial consistency with the high-stress regions was predicted by FEA simulation. In the ventral and lateral compression modes, the cracks were consistently extended inward along the longitudinal axis of the ventral groove. The initiation and propagation areas of these cracks closely matched the simulated regions of the maximum stress concentration. The high alignment between experimental observations and simulation validated the reliability and effectiveness of the model with micro-CT imaging. The mechanical behavior of the wheat kernels was also obtained to integrate the micro-CT imaging and FEA simulation. A robust model was also provided to accurately simulate the internal stress. The deformation mechanisms of wheat kernels under compression offered valuable theoretical insights for the milling industry. The grain morphology was highlighted to determine the structural role of the ventral groove and equatorial plane, particularly the mechanical response of the kernels. Such insights were crucial to refine the milling strategies, in order to minimize the structural damage for the high yield and nutritional integrity. Ultimately, the findings can greatly contribute to the theoretical models and engineering design for the optimal parameters of wheat milling. The key areas of the stress concentration were identified as the mechanical response of the kernels under various loading. The finding can also provide the scientific foundation to improve the milling efficiency, energy saving, and nutritional quality of wheat products.

The combination of vacuum freeze drying (FD) and hot air drying (HAD) has been observed to markedly reduce the volumetric shrinkage of hot air dehydrated products. While the FD can be attributed to concurrently optimizing energy consumption. However, the appearance quality of combined drying materials is still unstable at present. This study aims to clarify the shrinkage mechanism of apple products during the FD-HAD. Four points of moisture transition (moisture contents of dry basis were 1.00, 0.76, 0.53, and 0.33 g/g, respectively) were selected for the combined drying of apples. Shrinkage, texture, microstructure and pore distribution were determined in the dehydration products. The water migration and distribution of the samples were analyzed at the HAD stage using low-field nuclear magnetic resonance (LF-NMR). The results showed that the shrinkage of FD-HAD samples was significantly (P<0.05) better than that of HAD ones. The moisture transition point shared a significant (P<0.05) effect on the shrinkage of the samples (6%-45%). There was no outstanding volumetric shrinkage when the moisture content at the transition point was below 0.53 g/g. The samples were more shrinkable, harder and less crisp, as the moisture content increased at the transition point. There was a great variation in the center collapse. The decrease was found in the porosity and average pore size, with the increase of moisture content at the transition point. FD-HAD treatment saved energy more substantially (23.58%-28.95%), compared with the FD. The unit energy consumption decreased gradually with the increase of water content at the conversion point. The shrinkage of the sample in the FD-HAD occurred in the HAD when the moisture content at the conversion point was greater than 0.53 g/g. The sample was divided into two phases of rising and falling in the HAD, where the volumetric shrinkage of the sample mainly occurred in the rising phase. A three-phase transition of water was found in the sample from the ice crystals to the liquid and water vapor, which was then removed from the sample. There was a rapid decrease in the free water, and little change was measured in the content of bound and immobile water. There was a larger humidity difference between the center and the surface of the sample. The water was migrated from the center to the surface. The ice crystals inside the sample were basically removed by sublimation, and then to reduce the melting of ice crystals, when the moisture content at the transition point was less than 0.53 g/g. The small amount and range of free water and its migration resulted in a small shrinkage of the sample. In summary, the controllable transition point of co-drying controls shrinkage can be expected to reduce the amount and extent of ice crystal melting, as well as the free water migration in the HAD phase of the sample. There was less damage to the microscopic pores caused by moisture migration. This finding can provide the basic data and theoretical reference for the precise regulation of the appearance quality of FD-HAD samples and energy saving.