Refrigeration increases the lifespan of an egg up to about 5 times. At room temperature eggs will spoil much faster. As eggs age a air pocket forms because eggs are in fact porous. Also in countries like the United States were most people by commercial eggs as opposed to eggs from locally raised chickens that are actually free-ranged the chances of salmonella is greatly increased.
As eggs lose freshness, the quality of the egg can be seen in several stages:
- A fresh egg has a very small air pocket. The yolk is firm and relatively small, and if cracked (say, into a frying pan) it will stand up roundly. Also the white will be firm, compact, and round, with very little "spreading" in the pan.
- As an egg loses freshness, the air pocket enlarges and the yolk loses some of it's firmness. It spreads more in the pan. The white also appears thinner, and spreads farther in the pan, and a very thin, watery sort of white also appears at the edges of the firmer white which still stands up a bit firmly.
- As an egg deteriorates further, the yolk will appear much larger, and almost flat. The white will lose any definition of a firm center, and will appear all watery and spread very far in a frying pan.
- Beyond this stage, an egg really isn't worth eating.
