Bottom-up model for understanding the effects of wheat endosperm microstructure on its mechanical strength

Abstract

Wheat flours are essential ingredients of daily food products like bread, cookies or pastries. Their quality depends on the milling process and mechanical strength of wheat grains. Although it is well known that the strength and rupture of grains are strongly controlled by the endosperm microstructure, the respective roles of the starch and polymer volume fractions and their adhesion are not yet fully understood. This typical biological microstructure can be modeled as a cemented granular material, where the two size populations of starch granules (large:A-type, small:B-type) are the particles, and the protein matrix, which partially fills the space between granules, plays the role of a cement. This structural model of wheat endosperm is used, together with mechanical characteristics of starch and proteins obtained by means of Atomic Force Microscopy (AFM) measurements, to simulate the mechanical behavior and breakage of wheat endosperm in milling process. We find that the porosity outweighs the effect of other parameters for the elastic modulus, which declines as a nearly linear function of porosity. We also show that the tensile strength is an increasing function of the amount and connectivity of starch granules with increasing concentration of stresses along chains of granules. This effect is more significant at low porosity where stress distribution is mainly controlled by the contact network between starch granules. This effect explains why the protein content is not fully correlated to vitreousness, and samples of similar protein content can be different in vitreosity. Finally, we find that the starch-granule adhesion strongly affects the tensile strength whereas the effect of starch volume fraction appears mainly at high interface adhesion, which is the case of hard type wheat grains

Investigation of food microstructure and texture using atomic force microscopy: A review

“…Transition force maps of the section of the nanotubes (C and E) using contact mode and AtomicJ software (Maldonado et al, 2019) behavior due to starch and protein in wheat endosperm has been analyzed. The degree of porosity of the system was found to be linearly correlated with the Young's modulus (Chichti et al, 2016). A counter flow ultrasound (CFU) system has been utilized to pretreat wheat gluten.…”

Numerical modeling of the tensile strength of a biological granular aggregate: Effect of the particle size distribution

“…The peridynamic approach is an alternative to continuum mechanics. It is based on integral equations and is more accurate than classical LEM [6,7]. Peridynamics allows taking into account heterogeneous distributions of mechanical properties using bond-based schemes with brittle linear springs.…”

“…In Series 2 the toughness of the interface K i was decreased by 50%. The parameters were chosen similar as was reported in [7], taking into account nano-mechanical measurements of starch and protein inside wheat endosperm by atomic force microscopy [16]. Samples were submitted to quasistatic uniaxial tension in the y-direction, with the lower boundary being fixed, until integrity failure occurred.…”

“…The environmental conditions impact the porosity of the protein matrix, commonly measured as endosperm vitreousness [3]. Both components have been studied by experimental and numerical approaches [4][5][6][7], while other possible sources for differences in milling behavior, such as the starch granule size distribution (SGSD), are still less well understood. Experimental studies on the connection of SGSD and endosperm texture are not numerous and even not convergent [8,9].…”

“…Therefore, the study of SGSD using numerical modeling appears essential to understand its specific effect on the fragmentation behavior and they also e-mail: kmheinze@um2.fr can help the selection of wheat varieties for breeding. The fragmentation behavior of wheat endosperm has already been the focus of DEM or LEM studies [6,7,11], but with a focus on starch-protein adhesion and the volume fractions of the different material phases. We use a peridynamic approach [12,13] to study how the variation of the starch granules polydispersity affects the yield stress and failure properties.…”