Structural evolution of alginate/calcium β-hydroxy-β-methylbutyrate hydrogel based on nonlinear rheology

In this study, we analyzed the structural evolution of alginate/calcium β-hydroxy-β-methylbutyrate (ALG/CaHMB) hydrogels with three M/G ratios (2:1, 1:1, and 1:2) from the perspective of nonlinear rheology. The microstructure of the ALG/CaHMB hydrogels was strongly influenced by the M/G ratio, resulting in the various nonlinear viscoelastic behaviors of the hydrogels. A disordered coil structure was observed at M/G = 2:1 because the few egg-box crosslinks formed; however, smaller-radius egg-box bundles and a more orderly gel network were observed at a lower M/G ratio of 1:2. Small-amplitude oscillatory shear (SAOS) tests revealed that the structure of egg-box bundles significantly enhances the viscoelasticity of the ALG/CaHMB hydrogel. In medium-amplitude oscillatory shear (MAOS), the weak crosslinks formed by ALG (M/G = 1:1) polysaccharide chains are more easily destroyed, making the G" overshoot more readily observable. During large-amplitude oscillatory shear (LAOS), the ALG/CaHMB hydrogel (M/G = 1:2) had the highest resistance to deformation due to the slower unzipping rate of the egg-box bundle structure, resulting in a delayed transition to strain-softening behavior from 67% to 262%. These findings offer important insights into the structural evolution of ALG/CaHMB hydrogels under significant deformation, which is essential for both fundamental understanding and practical applications, including those in the food industry.