Eggshells are porous because they are mainly composed of calcite crystals, which form a porous structure allowing the necessary gas exchanges for the development of the embryo inside the egg.
If you look closely at an eggshell, you'll quickly notice that it is not as smooth as it appears. Under a microscope, it looks like a sponge made up of a multitude of small holes, called pores. These pores run throughout the shell and allow for gas exchanges between the inside and outside of the egg. The entire shell is primarily composed of calcium carbonate (the same mineral as chalk or limestone), combined with a few proteins that act as a "glue" to hold it all together. This porous structure is organized into several layers, each having a slightly different consistency and density, thus ensuring good strength while allowing it to breathe.
The pores of eggshells mainly come from the way the shell forms. When the egg develops in the hen's body, small crystals of calcium carbonate gradually accumulate around an initial structure called the shell membrane. However, this accumulation does not occur uniformly. It leaves tiny open spaces in the process, the famous pores. This somewhat random formation also depends on genetic and environmental factors, influenced by diet, age, or the health of the hen. These tiny openings play a crucial role in allowing the egg to "breathe," meaning to exchange gases with the outside.
The pores are essential for gas exchange: they allow oxygen to enter the egg and the carbon dioxide produced by the embryo to escape. Without this renewal, the embryo would quickly suffocate, unable to breathe under the shell. They also allow a little water to escape in the form of vapor, which helps regulate internal humidity, crucial for proper development. These tiny holes ensure that the future chick breathes properly while growing peacefully under its shell.
The pores of the shell play an important role in gas exchange between the inside and outside of the egg. Thanks to them, oxygen easily penetrates the shell, allowing the embryo to breathe during its development. At the same time, they let out the carbon dioxide produced by the embryo, preventing it from accumulating dangerously inside the egg. However, because of this same porosity, less pleasant things can also happen: bacteria, microbes, or bad odors can enter. As a result, the egg loses freshness more quickly if it is not stored properly, which reduces its shelf life and increases health risks. That’s why we avoid washing eggs, so as not to remove their precious natural protective cuticle, which helps limit these unwanted invasions and reduces water evaporation from the inside to the outside.
Scientists study fossilized eggshells to learn more about ancient climate and environment, with the pores playing a key role in these analyses as they record clues about past atmospheric conditions.
The eggshell is composed of 95% calcium carbonate, the same chemical substance found in limestone, coral, and chalk.
The color of eggshells depends on the breed of the hens and not on their diet or the nutritional quality of the eggs. A hen with white feathers will generally lay white eggs, while a hen with brown feathers will often lay brown eggs.
A chicken egg shell has an average of between 7,000 and 17,000 microscopic pores, allowing the embryo to breathe while protecting it from bacteria and external contaminants.
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