SPI/HG composite microgels with tunable structure and rheological properties for semi-solid foods

Excess dietary fat intake is a major public health concern. Using fat replacers to mimic fat's sensory attributes is a key strategy, but reproducing fats' distinctive viscoelasticity remains challenging. Here, composite microgels of soy protein isolate (SPI) and high-acyl gellan gum (HG) were fabricated via heat-induced phase separation and characterized for their structure, interfacial properties, and linear/nonlinear rheology. Tuning HG (0-0.4%, w/w) significantly affected colloidal attributes: at 0.2% HG the particles were smallest (837.87 → 352.63 nm) and most monodisperse, while the magnitude of the negative ζ-potential increased with HG overall; the oil-water-solid contact angle decreased to ≈50°, indicating more hydrophilic wetting and favorable interfacial affinity. Microgel assembly was driven by synergistic hydrogen bonding, electrostatic repulsion, and hydrophobic interactions between SPI and HG. Small-amplitude oscillatory shear (SAOS) tests revealed significantly lower storage (G') and loss (G'') moduli, reflecting a more flexible network structure, consistent with the decrease in the power-law consistency index K from 3149 to 296. Under large-amplitude oscillatory shear (LAOS), the microgel network transitioned from strain-hardening ("hard gel") at low strain to strain-softening ("soft gel") at large strain, accompanied by pronounced shear thinning. Such structural adaptability and shear deformability-evidenced by plastic flow under shear without loss of textural integrity-endow SPI/HG microgels (0.2% HG) with strong potential as fat replacers in semi-solid foods, including margarine and mayonnaise.