When boiling an egg in a microwave, there are a couple of factors that can lead to explosions if not done properly:
Steam pressure buildup: As the egg heats up, the liquid inside turns into steam. Unlike a pot on a stove where steam can escape through the lid's small gaps, a microwave's sealed environment can lead to significant steam pressure buildup. When the pressure inside the egg exceeds the strength of its shell, it can explode.
Superheating: In a microwave, it's possible to overheat water beyond its boiling point without it visibly boiling. This phenomenon is called superheating, and it happens when there are no nucleation sites (bubbles) for boiling to start. When an object is introduced into the superheated water or it's disturbed (like picking up the cup), it can cause a sudden and explosive release of steam, leading to an eruption.
When using cold water, the microwave has to heat the egg from a lower starting temperature, leading to more time for steam pressure to build up. Additionally, superheating is more likely to occur when heating water from cold, as there are fewer impurities or nucleation sites for bubbles to form.
In a pot of hot water on the stove, the water is already at a high temperature, so the egg will require less time to cook. This reduces the chances of excessive steam pressure buildup. Furthermore, in a pot of hot water, any potential superheating is less likely to occur since the water has already been heated and some nucleation sites might be present.
To minimize the risk of explosions when boiling eggs in the microwave, you can take certain precautions:
Prick the egg's yolk before microwaving: This allows steam to escape and reduces the pressure buildup inside the egg.
Use a microwave-safe container with a loose lid or cover: Allowing some steam to escape while still preventing splatters.
Microwave in short intervals: Heat the egg in short bursts with breaks in between to prevent superheating.
Add a pinch of salt or vinegar to the water: This provides some nucleation sites for bubbles to form and reduces the likelihood of superheating.
It's essential to exercise caution and use proper techniques when cooking eggs or any food in the microwave to avoid accidents and ensure safe cooking.