Quality and Processing Behavior of Egg White and Yolk from Commercial Free-Range and Barn-Laid Eggs: Physical, Compositional and Rheological Assessment in Raw and Heat-Treated (Grilled) States

This study evaluated how two commercial egg types (free-range and barn-laid) influence the physical, compositional, and rheological properties of egg white and yolk in raw and grilled states. Free-range eggs showed stronger correlations between external dimensions and internal composition, suggesting potential for nondestructive grading, whereas barn eggs exhibited heavier shells but weaker morphometric–composition relationships. Haugh units differentiated production systems, and yolk redness was the only color parameter clearly associated with free-range origin. Mechanical tests revealed that barn eggs had shells capable of absorbing more energy during rupture. Rheological measurements showed matrix-dependent behaviors: in raw samples, egg white behaved as a weakly structured viscoelastic fluid, while yolk exhibited characteristics of a concentrated lipoprotein emulsion. Stress, frequency, and temperature sweeps revealed contrasting behaviors between the two commercial egg types: barn-laid eggs displayed a stronger egg-white protein network, whereas free-range eggs showed a more reinforced yolk lipoprotein matrix under the conditions evaluated. Yolk behavior fitted the weak gel model with excellent accuracy (R2 ≈ 1), while egg white did not. Steady shear and three-step tests confirmed pronounced shear thinning and thixotropic behavior in both matrices, with barn eggs showing higher viscosities but lower structural recovery. Thermal treatment reduced the strong rheological differences between raw egg white and yolk, yet production system effects persisted. All grilled samples behaved as weak gels, with barn egg whites forming stiffer networks and free-range yolks generating more elastic, cohesive, and energy-absorbing gels. A trend toward higher MUFA levels was observed in raw free-range yolks. Microscopy further clarified how production system shapes the structural and functional behavior of egg matrices.