We investigated a composite hydrogel composed of bovine serum albumin (BSA) and xanthan gum (XG). Using small-amplitude oscillatory shear (SAOS) and large-amplitude oscillatory shear (LAOS) tests, we explored the effects of both BSA and XG concentrations on the BSA-XG composite hydrogel across linear and nonlinear deformation length scales. In the SAOS tests, an increase in XG concentration, at a constant BSA content, resulted in a linear increase in the storage modulus ( \({G}^{\prime}\) ) and loss modulus ( \({G}^{{\prime}{\prime}}\) ) of the hydrogel. The power-law viscoelastic frequency dependence of the BSA-XG hydrogel increased with higher XG concentrations, indicating a denser crosslinking network. The LAOS behavior, analyzed through storage modulus ( \({G}^{\prime}\) ) and loss modulus ( \({G}^{{\prime}{\prime}}\) ) via Fourier transform rheology, revealed that the critical yielding strain amplitudes decreased as the XG concentration increased. Additionally, intra- and intercycle analyses were performed using Lissajous plots in conjunction with the stress decomposition method, stiffening and thickening ratios, and the sequence of physical processes technique. Strong elastic strain-stiffening was identified as the dominant nonlinear behavior at large strain amplitudes. Moreover, at higher XG concentrations, the strain-stiffening response was better preserved beyond the first critical yield strain amplitude. Fourier transform infrared spectra confirmed that the unfolding and crosslinking intensity of BSA was proportional to the XG concentration.
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