Posted by analeis on October 30, 2004

Q: I have some more questions on egg shells. How does the calcium carbonate get into the eggshell? Is it different between organic eggs, free range eggs and commercial eggs? which one would have more calcium carbonate in the egg shell? During the making of the egg, in which process is calcium carbonate absorbed into the eggshell?

thanks so much!

A: Let's take your questions in order:

1.) How does the calcium carbonate get into the eggshell?

The calcium in the shell comes partly from the feed, which for a laying hen

should be 3 grams per day and partly from the bones, which act as a

reservoir. Adult female birds store calcium in the spongy calcified

material at the center of the bones. The transporting mechanism is the

blood.

The carbonate in the shell comes from bicarbonate ions in the blood or, more

likely, from carbon dioxide in the ovarian tissue.

A captive chicken gets the calcium from the packaged feed, a free range

chicken gets it from the soil, seed, insects, etc. that it eats.

2.) Is it different between organic eggs, free range eggs and commercial

eggs?

No, it is not.

3.) Which one would have more calcium carbonate in the egg shell?

Neither, it would be the same. Shell strength depends partly on genetic

factors. In a thorough review of genetic studies, Hunton (1982) concluded

that 50% or less of the variation in egg shell quality is genetic in origin.

It is therefore possible to control egg shell quality to some extent by

breeding.

There is evidence that insecticides or fungicides in the diets of laying

birds can lead to the thinning of their egg shells.

4.). During the making of the egg, in which process is calcium carbonate

absorbed into the eggshell?

The shell membranes are deposited on the outside of the albumen in the

isthmus region of the oviduct. Calcification begins at the junction of the

isthmus and the shell gland in the tubular shell gland region of the

isthmus. Hen's eggs spend about 15 to 20 minutes in the region during

formation.

In birds eggs, 3.9 percent of the dry weight of an egg is made up of calcium

carbonate. Of just the egg shell itself, 97 percent of it is calcium

carbonate. This calcium comes from the mineral calcite that the bird

absorbs through its diet.

Jim Berry ( November 2, 2004)