For a long time, people have paid close attention to the expiration dates of foods like milk, bread, and canned goods, yet the shelf life of a small egg is often overlooked.
When buying eggs, people only check if any are broken, but never ask about their expiration date, and most people cannot say how long an egg's shelf life is. Especially when the weather is cold, many people feel that eggs simply don't spoil.
Experts from the Food and Drug Administration point out that, just like vegetables and meat, eggs also have a shelf life. Experiments show that at a temperature of 2°C—5°C, the shelf life of an egg is 40 days. In late winter and early spring at room temperature indoors, it is 15 days, and in summer at room temperature indoors, it is 10 days.
Once an egg exceeds its shelf life, its freshness and nutritional value will be affected. If stored for too long, the egg will spoil due to bacterial invasion, exhibiting phenomena like sticking to the shell and a runny yolk.
There have long been many folk sayings about the relationship between egg color and nutrition. Some say "white-shelled eggs have no nutritional value"; others say "the darker the yolk, the better." Are these sayings right or wrong? With these questions, a reporter from the "Life Times" interviewed Chen Jilan, a researcher at the Poultry Research Office of the Beijing Institute of Animal Husbandry and Veterinary Medicine, Chinese Academy of Agricultural Sciences.
The nutritional value of eggs is more or less the same
People often choose eggs based on the color of their shells. Regarding this, Chen Jilan said: "In fact, the nutritional value of an egg mainly depends on the nutritional structure of the feed and the chicken's feeding situation, and is not necessarily related to the color of the shell." In addition, the depth of the shell color is related to the egg production rate; generally, the shell color is darkest at the beginning of the laying period and then gradually lightens; the depth of the shell color can also be changed through selective breeding; as for the claim by many vendors that pink-shelled eggs are the real "free-range eggs," there is no basis for this either. Cross-breeding white-shelled and brown-shelled chickens results in pink-shelled eggs, which is very easy to achieve. The nutritional value is no different from other eggs. "Free-range eggs" are pink-shelled, but pink-shelled eggs are not necessarily "free-range eggs."
Darker yolk, more vitamin A content
Many people also believe that when buying eggs, a golden yellow or reddish-brown yolk means it's a "free-range egg" and has better nutritional value. In fact, this standard of judgment is not scientific. Chen Jilan explained that the natural golden color of the yolk may be due to a high content of carotene or vitamin A, but not all "red-yolk" eggs are "free-range eggs"; it could be that pigments are at play. If a chicken lays many eggs, the yolk color is usually lighter, and some vendors, in order to make eggs "red-yolk" and pass them off as "free-range eggs," often add pigments to the feed. Therefore, yolk color cannot be used as a criterion for judging whether an egg is a "free-range egg," let alone to assume it has higher nutritional value.
"When selecting eggs, there's no need to overthink the color of the shell or the yolk," Chen Jilan said. "As long as the eggs are fresh and hygienic, their nutritional value is more or less the same."
Three tips for buying fresh eggs
Old eggs spoil easily, so how to buy fresh eggs is extremely important. Chen Jilan told us about a three-step method for selecting eggs:
One look: Fresh eggs have a layer of white powder on their shells, the shell color is bright, and the pores are obvious. Conversely, these are old eggs.
Two shake: Gently shake the egg in your hand. If there is no sound, it is fresh; if there is a sloshing sound, it is an old egg.
Three test: Place the egg in cold water. If it sinks, it is fresh; if it floats, it is an old egg.
In addition, pay attention to indicators such as whether the shell color is uniform and whether the shell is smooth. If the shell color is uneven or the shell is relatively rough, it may be an egg from an unhealthy chicken, so do not buy it.