Multi-scale structural regulation of Rushan cheese gels by glucono-δ-lactone toward enhanced stretchability

The stretchability of stretched cheese is predominantly governed by the structural plasticity of casein gels. While glucono-δ-lactone (GDL)-induced milk gelation is a well-established process, its targeted application to modulate the structural plasticity of Rushan cheese and to circumvent the inherent limitations of conventional processing methods remains largely unexplored. This study systematically examined the influence of different GDL concentrations (0-0.060%, wt/wt) on the stretching behavior, microstructure, and intermolecular interactions of Rushan cheese gels. After being heated in whey maintained at approximately 70°C for 50 s, the curd was removed and manually stretched into thin sheets, during which the curd containing 0.030% GDL showed the most favorable stretching behavior. Texture profile analysis indicated that GDL significantly increased gel adhesiveness and hardness, with hardness increasing from 1,072.85 g to 2,298.02 g. Rheological measurements showed that the storage modulus (G'), loss modulus (G"), and viscosity of the gels reached their maxima when 0.030% GDL was used. CLSM and SEM revealed that moderate GDL promoted a homogeneous, fibrous protein matrix with uniform lipid embedding, whereas excessive GDL (0.060%) led to microstructural coarsening and lipid phase separation. FTIR analysis showed that the relative β-sheet content increased from 15.7% to 21.3% at 0.030% GDL, accompanied by a decrease in β-turn content, indicating a redistribution of protein secondary-structure components within the cheese matrix. Fluorescence spectroscopy revealed GDL-dependent changes in the microenvironment of aromatic residues, consistent with reorganization of the protein matrix. Additionally, DSC analysis showed GDL-dependent changes in the thermal transition behavior of the cheese matrix, while XRD patterns indicated more compact molecular packing at 0.030% GDL. At GDL concentrations above 0.030%, the samples exhibited network coarsening, greater phase heterogeneity, and reduced stretchability, suggesting that excessive GDL addition disrupted the balance between network cohesion and structural mobility. Overall, this work provides multiscale structural evidence that moderate GDL addition can improve the stretchability of Rushan cheese and offers a plausible framework for interpreting the associated changes in the protein matrix.