Effect of Low-Density Lipoprotein (LDL) and High-Density Lipoprotein (HDL) on Frozen Gelation of Egg Yolk

This study aimed to investigate the roles of low-density lipoprotein (LDL) and high-density lipoprotein (HDL) in the gelatinization behavior of egg yolk, as well as the underlying mechanisms of action. This research examined the rheological properties, moisture distribution, and structural characteristics of a system containing reconstituted egg yolk components during the freezing process. The results indicated that increasing the concentration of LDL and HDL in the egg yolk system enhanced the apparent viscosity of egg yolk following a freeze–thaw treatment. Specifically, as the LDL and HDL content increased, G’ and G” values increased significantly, whereas tanδ values decreased significantly and l* values declined. These findings suggest that both LDL and HDL are critical contributors to the gelatinization process of egg yolk. Furthermore, as the concentrations of LDL and HDL in the system increased, the amount of fixed water also rose, while the bound and free water content decreased. This observation implies that LDL and HDL facilitate water migration during the freezing of egg yolk. The increase in fluorescence intensity observed in the fluorescence spectra indicates a greater exposure of tyrosine residues on the protein surface, an enhancement of surface hydrophobicity, and a modification of protein conformation. Fluorescence inverted microscopy revealed that elevated levels of LDL and HDL in the system led to increased structural damage to the protein due to freezing, which subsequently promoted the aggregation of yolk proteins. This suggests that both LDL and HDL undergo aggregation during gelation. In egg yolk, LDL and HDL are essential for gel formation during the freezing of liquid egg yolk and play critical roles in both protein structure and water migration. Of the two lipoproteins, HDL has a more pronounced effect on gel formation during liquid egg yolk freezing. This study investigates the key substances involved in the gelatinization of egg yolk, providing a reference for further improvements in egg yolk gelatinization during freezing.