STUDIES ON THE GELATION OF EGG YOLK AND PLASMA UPON FREEZING AND THAWING

Abstract

ABSTRACTWith the aid of viscometry, turbidimetry, paper electrophoresis, and electron microscopy, more insight into the gelation of yolk upon freezing and thawing has been gained. The pseudoplastic nature of yolk and plasma was found to be enhanced by freezing for a few hr at −14°C. With a rise in temperature from 10 to 50°C, the apparent viscosities of frozen‐thawed yolk and plasma decreased considerably, whereas little viscosity change was obtained with unfrozen samples. During the freezing process, low‐density lipoproteins (LDL) of the yolk presumably interacted through at least some heat‐labile bonds (perhaps H bonds) to produce a more viscous, pseudoplastic mass. The results of paper electrophoresis of unfrozen and frozen (−10 and −14°C)‐thawed yolk and plasma suggested that LDL aggregates were formed during frozen storage. Turbidity experiments supported this supposition. As in the case of yolk, the rate of viscosity change for plasma increased with a drop in the frozen storage temperature from −10 to −14°C. The loss of LDL mobility during paper electrophoresis was also related to the storage temperature. The higher LDL micelle content and soluble salt concentration may account for this temperature effect. The gelation rate for plasma increased progressively as the granule content was raised to 20% of the total solids of the dispersion. Possibly the granule's of yolk are disrupted during freezing by a high concentration of soluble salts in the unfrozen phase and, thereafter, LDL micelles are liberated. In electron micrographs of frozen‐thawed yolk, a large lipid mass and some fluffy electron‐dense masses, smaller than the granules in native yolk were observed. These fluffy masses are presumably residues of completely disrupted granules.