Changes in gelation, aggregation and intermolecular forces in frozen-thawed egg yolks during freezing

The changes in gelation, aggregation and intermolecular forces in frozen-thawed egg yolks during freezing were determined to clarify the mechanism of frozen-induced egg yolk gel formation. The results indicated that the frozen-thawed egg yolks lost their fluidity after 6 h of freezing, as shown in the graph. Back extrusion rheology showed that the yolk viscosity and consistency increased significantly when the egg yolk was frozen for the designated amount of time (1, 2, 3, 4, 5, and 6 h). The gel strength, hardness and adhesiveness of egg yolks frozen for 7–12 h tended to increase. Turbidity analysis results indicated that the turbidity of frozen-thawed egg yolks significantly increased, but SDS-PAGE results showed that the protein patterns of the frozen-thawed egg yolks did not change during freezing. Upon gelation of the frozen-thawed yolks, the free sulfhydryl content in the frozen-thawed egg yolks decreased significantly, and the content of disulfide bonds initially increased and then slightly decreased. There was no significant difference between the surface sulfhydryl content and the surface hydrophobicity of the frozen-thawed egg yolks, but both were higher than those of fresh egg yolks. The results of the analysis of intermolecular forces demonstrated that hydrophobic interactions, hydrogen bonding, and interactions between hydrophobic groups and disulfide bonds could stabilize the protein gel network. Raman spectroscopy further revealed that the secondary structures of egg yolk proteins changed. These results suggested that the characteristic gel of frozen-thawed egg yolks was formed as a result of multiple interactions among protein molecules.