A water adsorption study on wheat pericarp macrofibrils using atomic force microscopy

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.