Shell
The shell accounts for about 9 to 12% of its total weight depending on egg size. It comprises about 94% calcium carbonate with small amounts of magnesium carbonate, calcium phosphate and other organic matter including protein.
Shell strength is influenced by two factors. Firstly, the hen's diet, particularly its calcium, phosphorus, manganese and vitamin D intake. Secondly, egg size, which increases as the hen ages while the mass of shell material that covers cover it stays fixed. Hence the shell is thinner on larger eggs.
Between seven- and seventeen-thousand tiny pores are distributed over the shell surface. As the egg ages, moisture and carbon dioxide diffuses out, and air diffuses in through these causing the air cell to grow and the net mass to decrease. The shell is covered with a protective coating called the cuticle. By blocking the pores, the cuticle helps to preserve freshness and prevent microbial contamination of the contents. This is why good quality eggs (e.g. EC class A) should not be washed, which removes the cuticle, until immediately before they are to be used.
The colour of the shell is determined the breed of hen. Brown-shelled eggs tend to be more expensive because they come from larger birds, and these are more costly to feed.
In the U.K. mass-market eggs seem to crack more easily these days than they used to; this is apparently because their shells are rather thinner than they were twenty or thirty years ago. I believe that this is mainly because the proportion of 'grit' (a calcium supplement) in chickens' diet has been reduced (it's expensive to transport). Eggs need careful handling to avoid damage, which can be invisible to the naked eye.
Pricking the blunt end of an egg does not, as is commonly thought, prevent the shell from cracking; it relieves pressure and reduces the extent to which the egg is forced out through any crack that does develop. Adding vinegar and/or salt to the water also helps by causing the emerging liquid to coagulate, and thereby plug the gap, more rapidly than it would in do in simple hot water.
ShellThe shell accounts for about 9 to 12% of its total weight depending on egg size. It comprises about 94% calcium carbonate with small amounts of magnesium carbonate, calcium phosphate and other organic matter including protein.Shell strength is influenced by two factors. Firstly, the hen's diet, particularly its calcium, phosphorus, manganese and vitamin D intake. Secondly, egg size, which increases as the hen ages while the mass of shell material that covers cover it stays fixed. Hence the shell is thinner on larger eggs.Between seven- and seventeen-thousand tiny pores are distributed over the shell surface. As the egg ages, moisture and carbon dioxide diffuses out, and air diffuses in through these causing the air cell to grow and the net mass to decrease. The shell is covered with a protective coating called the cuticle. By blocking the pores, the cuticle helps to preserve freshness and prevent microbial contamination of the contents. This is why good quality eggs (e.g. EC class A) should not be washed, which removes the cuticle, until immediately before they are to be used.The colour of the shell is determined the breed of hen. Brown-shelled eggs tend to be more expensive because they come from larger birds, and these are more costly to feed.In the U.K. mass-market eggs seem to crack more easily these days than they used to; this is apparently because their shells are rather thinner than they were twenty or thirty years ago. I believe that this is mainly because the proportion of 'grit' (a calcium supplement) in chickens' diet has been reduced (it's expensive to transport). Eggs need careful handling to avoid damage, which can be invisible to the naked eye.Pricking the blunt end of an egg does not, as is commonly thought, prevent the shell from cracking; it relieves pressure and reduces the extent to which the egg is forced out through any crack that does develop. Adding vinegar and/or salt to the water also helps by causing the emerging liquid to coagulate, and thereby plug the gap, more rapidly than it would in do in simple hot water.
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