When you boil an egg, there are some possibilities that the egg will crack.
Egg with crack still can be eaten.
But you may feel unhappy with the imperfect egg.
What contributes to egg cracking while boiling?
There are some factors that contribute to egg cracking while boiling, I will discuss it below.
Why Does An Egg Cracked While Boiling?
Basically egg cracks when boiling is because of gas inside the egg expanding within a very short period of time. This phenomena normally happens when you take a cold egg (long time storage) from the refrigerator and straightaway drop it into hot boiled water.
Extreme temperature change
When you take a cold egg from a fridge and drop it into a pot with hot boiled water, most probably the egg will crack immediately.
Extreme temperature change from fridge cold temperature to 100 celsius will cause the gases in the egg to be heated and expand in a short time within the shell.
When the gas pressure becomes great enough, it will find the weakest point at the egg shell and break it to escape, and the egg cracking.
Air pocket inside the egg
There is an air pocket inside the egg.
After an egg is laid, the content of the egg will cool and shrink, the air pocket will form between the inner and outer membrane of the egg.
The air pocket forms at the larger end of the egg.
As the egg gets older and experiences a longer period of storage, the air pocket inside the egg will get larger as it loses moisture.
Fresh air contains a smaller air pocket compared to a long storage egg with a bigger air pocket.
When you boil an old egg (most probably with a larger air pocket) in hot boiling water, it will crack easily as a larger volume of gas expansion will cause greater pressure towards the egg shell and break it.
Turbulence when boiling
When an egg is inside a pot with hot boiling water, the hot water turbulence will cause the egg to bounce on the bottom of the pot.
The mechanical force that impacts the egg shell during the hot water turbulence may cause cracking and egg white leak out into hot water.
Egg hit with pot bottom
When you drop an egg into a pot with hot water, because the water level is not high enough, the egg crashes with the pot bottom and cracks.
Unnoticed crack on egg shell
During transportation of the eggs, there could be some shaking along the way.
The shaking movement may cause a small crack but not significant on the egg shell.
The crack exists but still the egg content does not leak out.
But this already weakens the egg shell protection level.
When the egg with a small crack is put into the hot boiling water, a slight expansion of the egg will experience the cracking.
Material of egg shell
The egg shell is made of calcium carbonate (95-97%) which is brittle in nature.
Brittle material can’t stand up against mechanical force from internal (gas expansion) and external (shaking or bouncing).
Egg shell quality
Poor egg shell quality also contributes to egg cracking while boiling.
The egg shell quality is determined by nutritional level of the feed to hens, the age of hens, diseases and contamination, feeding time, and many more factors.
To get a perfect boiled egg without any crack, firstly get a fresh egg as a small air pocket inside it will not easily cause crack.
Second, to avoid a huge temperature change when boiling the egg, if the egg is taken out from the refrigerator, please dip it in warm water to increase the egg temperature to reduce the gap.
Thirdly, use a long spoon to put the egg into the bottom of the pot to avoid hitting between them.
Fourth, put the egg into room temperature water and slowly boil the water.