Grades 3–5 · Everyday Life
Why does a runny egg turn firm when it cooks?
A raw egg white slips and flows. A cooked egg white can hold its shape. The water has not frozen. Instead, heat has changed the proteins mixed through that water. Proteins are long molecules that can fold into compact shapes.
In the diagramHeat changes how egg proteins joinEnlarged protein sketch · water is shown as blue dotsRaw whiteSet whiteFolded proteinsNetwork holds waterCooling a cooked egg does not turn it back into raw egg.
As an egg heats, many of its proteins unfold. Parts that were tucked inside become exposed and can connect with other proteins. More and more connections form a network through the egg white. Water becomes held within this network, making a soft solid called a gel.
This explains why cooking is different from simply warming and cooling water. Cooling a cooked egg does not fold all the proteins neatly back into their original separate shapes. If heating continues too far, the network can tighten and squeeze out water, making the egg tougher. Different egg proteins change at different temperatures, so an egg does not have one single switch that turns every part solid at once.
About this explanation
Starting knowledge: Distinguish a liquid from a soft solid and recognize that materials contain tiny parts.
Sources
- Exploratorium — Science of Eggs: How to Hard Cook an Egg