Effects of Heat Treatment and Freeze-Drying on the Protein and Lipoprotein Structures of Egg Yolk as Determined Using Differential Scanning Calorimetry (DSC)

The unfolding of the structures of proteins and lipoproteins in the yolk, plasma and granules of eggs that was induced by heat treatment (at 67, 72, 77, 82, 87 °C for 5 min) and freeze-drying (durations of 1, 3, 15 h) was examined using differential scanning calorimetry (DSC). The structural unfolding of livetins, low-density and high-density lipoproteins (LDL and HDL) depended on their resistance to heating and dehydration. At 67 °C, representing a high pasteurization temperature, only partial structural changes of the LDL and a distinct unfolding of γ-livetin occurred. Heat treatment at 72 and 77 °C led to measurable unfolding of livetins, LDL and HDL with the completed unfolding of LDL and αlivetin at 77 °C. Heating to 82 °C caused a high degree of structural unfolding of the proteins and lipoproteins which was completed at 87 °C. Therefore, at 82 and 87 °C in the DSC-thermograms no endothermic reaction was measurable. Freeze-drying to dry matter levels that exceeded 96 % stabilized the proteins and lipoproteins by immobilizing the matrix in the vitrified state. The structure of granules, composed of HDL-phosvitin-complexes, was restorable by rehydration at every dry matter level. Below a critical moisture level of between 0.11 - 0.16 gg -1 water/solids the core-shell structure of the LDL is destabilized. Therefore, the unfolding of the structure of the LDL that was induced by freeze-drying was partially irreversible. Because of the high content in LDL, plasma exhibited higher susceptibility to heating and freeze-drying than egg yolk and granules.