Multiscale structural and mechanical characterization of acid milk gels with low-concentration fish gelatin during gelation and cold storage

ABSTRACT This study examined the physical and structural properties of acid milk gels prepared with and without low concentrations (0.5%, w/w) of fish gelatin during acid-induced gelation using glucono-δ-lactone (2%, w/w) and subsequent cooling, annealing, and cold storage (up to 7 days at 4°C). A multiscale investigation was conducted using small- and large-oscillatory shear rheology and texture analysis at the macroscopic level, particle tracking at the microscopic level, and small-angle X-ray scattering (SAXS) at the nanoscopic level. The addition of gelatin resulted in a temperature-dependent, two-phase effect on gelation at the macroscopic level, as well as higher gel strength and delayed yield upon large deformation. Stress relaxation analysis showed that gelatin promoted slower and more evenly distributed stress relaxation over time, whereas the sample without gelatin displayed faster relaxation and a broader distribution of relaxation time. SAXS analysis revealed that gelatin promoted a more relatively homogeneous structure, whereas control samples showed the fusion and rearrangement of more heterogeneous protein clusters into larger aggregates during storage. Particle tracking further revealed increased spatial heterogeneity and porosity in the gel without gelatin at later storage, in contrast to the more stabilized microstructure in gelatin-containing samples. These findings demonstrate that even at low sub-gelling concentrations, gelatin can modulate the time-course development of milk protein gel networks across multiple length scales, leading to enhanced structural and mechanical stability and reduced heterogeneity.