Yolk of Chicken Egg is as Hard and Bouncy as a“Rubber Ball”?


        Some citizens of Macao had suspected that the chicken eggs which they bought were “man-made” since the yolk was hard and bouncy like a “rubber ball” after cooking, and reported the incident to Food Safety Department of the IAM. Upon receipt of the case, the Department sent its staff to sample chicken eggs from local markets for further investigation and testing. The results initially ruled out the possibility of “man-made chicken eggs” and the hardness and bounciness of the questionable eggs were presumably related to the extreme cold weather of earlier months.


       As early before, IAM and Zhuhai Entry-Exit Inspection and Quarantine Bureau had jointly run tests and simulation experiments on chicken eggs suspected to be artificial. After conducting morpho anatomy and tests for protein content, 12 types of food coloring, 5 kinds of metallic element, biuret reaction, and alkali-soluble sodium alginate on the sampled questionable eggs, the analytical results revealed the “suspected man-made chicken eggs” showed no abnormality when compared to their normal counterparts, thus ruling out the possibility that “man-made chicken eggs” existed.


What causes the yolk of cooked chicken eggs to share the properties of a “rubber ball”?

       After ruling out the existence of “man-made chicken eggs”, temperature variation in the production, transport and storage process of normal chicken eggs is the major factor causing their yolk to share the properties of a “rubber ball” after cooking. When the eggs are subject to low-temperature environment, the egg yolk reduces its water content upon exposure to coldness. It becomes more viscous and undergoes irreversible gelation. It turns hard and bouncy after cooking. This is the structural change, due to freeze-dehydration of the bound water in the egg yolk’s lipoprotein, which leads to agglutination among protein molecules, causing the viscosity of egg yolk to be increased and fluidity to be lost, resulting in a suspected “rubber egg” to be produced. This phenomenon is called frozen gelation of eggs. The lower the storage temperature, the more likely the egg yolk will have a “rubber texture” after cooking. Yet, the storage duration has a greater impact on egg yolk. The longer the storage duration, coupled with refrigeration, the more likely the chicken eggs will have a “rubber-textured yolk” after cooking.


How to buy and eat chicken eggs safely?

  • Buy chicken eggs with clean and intact eggshell from reputable shops;
  • Never buy or eat ones that have a bad odor, with cracks or mildew over their shell, whose yolk is disintegrated or stuck to the inside of the shell;
  • After purchase, store them with their blunt end facing upward in the refrigerator;
  • After egg-handling, wash and clean hands thoroughly as well as any utensils and work counter which have come into contact with the eggs;
  • Wash the surface of the eggshell thoroughly before cooking; cook chicken egg immediately after its eggshell is washed;
  • Chicken eggs must be thoroughly cooked until their liquid yolk and whites solidified; never eat raw or undercooked chicken eggs;
  • Be a smart consumer. Do not buy or consume any chicken eggs if doubtful about their quality and safety.