Microstructural Change in Hen Egg Yolk Spheres With Storage and Boiling.

Abstract

The size and shape of fresh hen egg yolk spheres and dense granules in the yolk sphere were changed by storage (20°C for 20 days) and boiling (15min). These changes were analyzed by the image processing technique. Fresh, stored, and boiled yolk samples were each cut from the outer, middle and inner layers, and from the latebra, and the contents of water and protein were measured. Pieces of each were immersed in a fixation solution and, after sample preparation, were observed by transmission electron microscopy. Binary images of the yolk spheres and dense granules were input to an Excel image processor.
The moisture and protein contents of fresh yolk were higher in the outer layer than in the inner layer. After storing, the moisture content of the yolk was increased, and the protein content was almost unchanged in each layer.
The outer and inner layers of the yolk spheres after boiling had expanded by 1.19 to 1.30 times those of the fresh yolk. The ratio of the long to short dimensions of the yolk spheres after boiling in the middle and inner layer was smaller than that of the fresh yolk.
The proportion of dense granules in the yolk spheres tended to decrease toward the inner layer of the yolk as a result of storage and boiling; therefore, the proportion of dense granules in the yolk spheres was lower than that of the fresh yolk. The transverse sectional area of the dense granules was also greates larger in each region than that of the fresh yolk. The dense granules in the inner layer and latebra were larger and more circular than those of the fresh egg because of granule fusion. Many spots appeared in the internal parts of the dense granules after boiling and storage. Furthermore, the dense particles were dissolved by boiling and their form disappeared.