Exploration on manual preparation of the tensile mechanical test samples of wheat bran structural layers

Wheat bran is the outer layer of the wheat kernel, particularly as a source of fiber in the food industry. The mechanical properties of the structural layers generally dominate the quality and efficiency of the superfine grinding of wheat bran. The sample preparation of the wheat bran structural layer can be the prerequisite for the tensile test of mechanical properties. Manual stripping can be widely used to completely separate the intermediate and aleurone layer, in order to prepare the wheat bran structural layer samples. The "two-layer theory" and "three-layer theory" have been formed at present. The sample length can reach 8mm using manual stripping without any microscopic device. However, it is still lacking consistence in the actual manual stripping of wheat bran structural layers. In this study, the feasible evaluation was performed on the manual stripping of the intermediate and aleurone layer, according to the preparation requirements of wheat bran structural layers. Five aspects were considered, such as the microstructure, geometric characteristics, mechanical properties, actual stripping situation, and the mixing law of Young's modulus of wheat bran. The results showed that it was very difficult to obtain the complete, pure, large-size samples of intermediate and aleurone layers described in the recent research literature by manual stripping without any microscopic device. And the constitutive relations of intermediate layer and aleurone layer given by previous researchers did not satisfy the mixing law of laminate composite. The main reasons were as follows: 1) There was basically no gap between the intermediate and the aleurone layer, resulting in a large contact interface between the two layers. At the same time, since the contact interface between the intermediate and the aleurone layer was a rough surface with concave and convex pits, it was difficult to strip them manually. 2) There were a larger length-thickness ratio and the width-thickness ratio of the intermediate and the aleurone layer, reaching 200:1 and 80:1 respectively. There was a greater interlaminar adhesion between the two layers. 3) similar mechanical characteristic curves were found in the intermediate and the aleurone layer. As such, two kinds of structural layer samples were assumed as the same material, leading to hardly stripping them manually. 4) There were relatively complete intermediate layers after manual stripping, whereas, the aleurone layers were basically debris, failing to meet the length requirement of the wheat bran structural layer samples. At the same time, the stripping of the intermediate and the aleurone layer was considered a scraping process rather than a tearing, according to the "three-layer theory". It infers that there was no pure in the prepared samples of the intermediate and the aleurone layer at present, indicating the residues of cell tissue in each other. Only in this way, an explanation was proposed for the relatively close thickness and mechanical properties of the two kinds of samples. The tensile mechanical test of wheat bran structural layers using the "two-layer theory" was also conducted to verify the universal applicability of the mixing law of Young's modulus of laminated composite materials. The inaccuracy of experimental data was further revealed on the mechanical properties of the intermediate and aleurone layer samples using the "three-layer theory" and the defects of the empirical compound formula of Young's modulus. The above research solved the doubts about the difficulties in manually separating the intermediate and aleurone layer. On the premise that the mechanical properties of wheat bran structural layers are known, the functional relationship between the mechanical properties of wheat bran structural layers and the dynamic parameters of the superfine grinding equipment can be constructed by using finite element and discrete element numerical simulation technology, which can provide basic theoretical guidance for the optimization and improvement of the superfine grinding performance of wheat bran.