Departamento de Ingeniería Bioquímica, Escuela Nacional de Ciencias Biológicas, Instituto Politécnico Nacional, Unidad Profesional Adolfo López Mateos, Zacatenco, Av. Wilfrido Massieu 399, Col. Nueva Industrial Vallejo, Alcaldía, Gustavo A. Madero C.P. 07738, Ciudad de México, Mexico.
Departamento de Ciencias de la Alimentación, Universidad Autónoma Metropolitana - Unidad Lerma, Av. de las Garzas No. 10, Col. El Panteón Lerma de Villada, Municipio de Lerma C.P. 52005, Estado de México, Mexico.
Micron. 2021 Apr;143:103010. doi: 10.1016/j.micron.2021.103010. Epub 2021 Jan 9.
Wheat pericarp, which is the most external layer of the wheat kernel, is composed of a polysaccharide matrix, where cellulose macrofibrils, hemicellulose, and lignin are their main components. These polysaccharides modified their structure due to the hydric condition to which they are subjected. This effect is considered as an advantage in the wheat milling process. However, no information about micro and nanostructural changes on wheat pericarp macrofibrils due to their hydric condition, studied by the AFM technique and image analysis, has been reported. On the other hand, cellulose macrofibrils have been extensively studied by AFM but performing the study at constant relative humidity (RH) level. Hence, this study aimed to investigate the water adsorption process on wheat pericarp macrofibrils using AFM and control the RH to which samples were subjected during examinations with a lab equipment specially developed for the AFM experiment. The RH was modified from 10 to 90 %, and peak force error images were acquired, from which macrofibrils' diameter, swelling behavior, and water adsorption isotherms were calculated, using image analysis tools. Also, as an application from the water adsorption isotherms, the specific surface area and the hygroscopic swelling coefficients were determined. Results showed that wheat pericarp macrofibrils presented an unusual swelling behavior, with the most notorious changes after reaching a moisture content in equilibrium to 40 % of RH. The average diameter of the macro-fibrils varied from 45 to 48 nm. The water vapor adsorption isotherm obtained from AFM micrographs image analysis did not resemble the sigmoidal IUPAC Type II, generally obtained by applying gravimetric methods. Results suggest that the macrofibrils swelling controls water accessibility to the internal macrofibrils structures. It was proved with this study the feasibility of using AFM and image analysis to build water vapor isotherms and other mass transport parameters based on the macrofibrils swelling.
小麦麸皮是麦粒的最外层，由多糖基质组成，其中纤维素大纤维、半纤维素和木质素是其主要成分。由于所处的水合条件，这些多糖改变了它们的结构。这种效果被认为是小麦碾磨过程中的一个优势。然而，尚未有报道研究小麦麸皮大纤维由于水合条件而导致的微观和纳米结构变化，这些变化是通过原子力显微镜（AFM）技术和图像分析来研究的。另一方面，纤维素大纤维已经通过 AFM 进行了广泛的研究，但都是在恒 RH 水平下进行的。因此，本研究旨在使用 AFM 研究小麦麸皮大纤维的水分吸附过程，并使用专门为 AFM 实验开发的实验室设备控制样品在检测过程中所受的 RH。RH 从 10%调整到 90%，并获取峰值力误差图像，使用图像分析工具从这些图像中计算大纤维的直径、溶胀行为和水分吸附等温线。此外，作为水分吸附等温线的应用，还确定了比表面积和吸湿溶胀系数。结果表明，小麦麸皮大纤维表现出异常的溶胀行为，在达到平衡 RH 为 40%的水分含量后，变化最为显著。大纤维的平均直径在 45 到 48nm 之间。从 AFM 显微图像分析获得的水蒸气吸附等温线与通常通过重量法获得的 IUPAC 类型 II 并不相似。结果表明，大纤维的溶胀控制着水进入内部大纤维结构的可及性。本研究证明了使用 AFM 和图像分析来构建基于大纤维溶胀的水蒸气等温线和其他质量传递参数的可行性。
