Structural and techno-functional properties of pH-shifted protein isolates from unhatched hen eggs

This study explores the effects of pH-shift processing (acid and alkaline) on the physicochemical and techno-functional properties of protein isolates (PPI) from unhatched hen eggs—infertile (INF) and dead-in-shell (DIS)—compared to commercial whole hen egg powder (CEP). Structural changes were evident in PPI, with reduced reactive SH content and surface hydrophobicity relative to CEP (p < 0.05). Thermal denaturation temperature (Td) varied by egg type and pH treatment, with DIS alkaline-PPI exhibiting the highest Td (p < 0.05). While CEP demonstrated superior solubility, DIS alkaline-PPI exhibited higher viscosity (p < 0.05). Foamability was highest in CEP (p < 0.05), yet PPI, particularly acid-INF-PPI, showed enhanced foam stability. CEP had the highest emulsifying activity index (EAI), whereas DIS alkaline-PPI displayed superior emulsion stability index (ESI) (p < 0.05). Gel formation was concentration-dependent; CEP formed a soft gel at 20% protein, whereas all samples gelled at 30%. CEP gels demonstrated significantly greater breaking force, deformation, gel strength, and water-holding capacity (WHC) than PPI gels (p < 0.05), which exhibited a more porous and aggregated microstructure. Rheological analysis revealed that INF alkaline-PPI had the highest storage modulus (G'), indicating strong elasticity; however, CEP exhibited superior mechanical strength, suggesting that a compact protein network enhances resistance to external stress. Acid-extracted PPI displayed lower G', indicating weaker gel formation, while DIS alkaline-PPI showed a delayed gelation process. Loss modulus (G'') and loss tangent (tan δ) further highlighted distinct viscoelastic behaviors across samples. These findings illustrate that the pH-shift process significantly influences the structural and functional properties of unhatched egg PPI, with implications for their application in food formulations.