How to strengthen egg quality
The vitelline membrane forms a diffusion barrier between the albumen and the yolk2. The role of this mechanically strong membrane is to protect the future chick against harm from the highly alkaline albumen, until the embryo gets inside of what is called the yolk sac. This process occurs between the first 72 to 96 hours of incubation. The contact between the embryo and the albumen before the formation of the yolk sac is fatal for the future chick. A strong VM is therefore required to protect the embryo for the first days of incubation when embryo mortality could be relatively high.
According to the International Egg Commission, in 2012 around 30 % of the eggs in the USA were further processed into either liquid or dry forms for use in the food service industry. A significant portion of these products are based on egg whites only (also used for cosmetics and non-food applications). Yolk contamination is a real concern for egg processors worried about the quality of the egg albumen. If the VM is weak, the yolk will broke and the content will mix with the albumen. The structural integrity of the VM is essential to allow accurate separation of egg yolk and white.
Egg yolks are used as an emulsifier or thickener in culinary applications, such as mayonnaise, custard, hollandaise sauce and crème brûlée. Egg albumen, on the other hand, is a particularly effective foaming agent. Used in baking and during the preparation of various other recipes, the foaming ability of the egg white is dependent upon the quality of the albumen proteins. Any slight contamination with egg yolk can alter protein functionality and reduce the foaming properties as a result. Once again, the strength of the vitelline membrane is crucial to get the desired products. Additionally, a weak vitelline membrane will be reflected as a flat yolk, resulting on the lack of perceived freshness in the mind of the final consumer. A round and firm yolk is highly appreciated by chefs and bakers alike.
Considerations to preserve the strength of the vitelline membrane:
The VM is formed almost exclusively by proteins. These proteins are linked together by a series of (disulfide) bonds highly susceptible to oxidation. When the bonds get oxidized, the membrane becomes weaker as a consequence. Storage temperature and antioxidants are the main factors to consider when preserving the integrity of the VM.
- Storage temperature: Refrigeration will have a major impact on the quality of the VM. Eggs exposed to high temperatures will suffer a higher degree of oxidation. Egg albumen will become weaker as well (flat eggs).
- With no vitamin C present in the yolk (naturally eggs have no vitamin C activity whatsoever); feed carotenoids (especially canthaxanthin) play an important role in preserving the integrity of the vitelline membrane thanks to their physical antioxidant properties. Birds cannot synthesize their own carotenoids.
- A strong vitelline membrane is essential for good quality eggs for reproduction, processing and bakery.
- The antioxidant properties of carotenoids (specially canthaxanthin) play an important role in preserving the integrity of the vitelline membrane
1 Mann, K. 2008. Proteomic analysis of the chicken egg vitelline membrane. Proteomics 2008, 8: 2322–2332.
2 Bellairs, R., Harkness, M. & Harkness, R.D. 1963. The Vitelline Membrane of the Hen's Egg: a Chemical and Electron Microscopical Study. J. Ultrastructure Research 8: 339—359.