On the pseudoelastic-viscoelastic behavior of starch hydrogels at various degrees of gelatinization and retrogradation

Starch, a naturally abundant and widely consumed energy source, is biocompatible and extensively used in the food and pharmaceutical industries. Digestion of starch involves both the mechanical breakdown through mastication and peristaltic waves of the gastrointestinal wall as well as chemical breakdown through diffusion and reaction with saliva and gastrointestinal fluids. The mechanical properties of starch hydrogels significantly influence their breakdown and digestion speed, making it essential to investigate these properties. However, the effects of the degree of gelatinization (DOG) and degree of retrogradation (DOR) on the loading-unloading mechanical properties, which are essential for material model development and in silico digestion simulations, remain inadequately investigated. This study performed mechanical experiments to investigate the mechanical properties of starch hydrogels across different degrees of gelatinization and retrogradation, influenced by heating and storage conditions. Gelatinization was observed under optical microscopy by noting the loss of the spherical shape of the starch granules. The degree of retrogradation was measured using Differential Scanning Calorimetry (DSC). Starch hydrogels exhibit stress-softening behavior and viscoelasticity, varying with different degrees of gelatinization and retrogradation due to the crystallite melting and the recrystallization of the starch molecules. A pseudoelastic-viscoelastic constitutive law was developed to describe the mechanical behavior of the starch hydrogel, and the material parameters were calibrated based on experimental results. Such models are required for the simulation of the digestion of starch hydrogels. This study assists in understanding the influence of cooking and storage conditions on the mechanical behavior of starch hydrogels.