Why do milk and eggs congeal when heat is applied?

[sanjuro]

We all know that heat energy can change the form of a substance. For example, ice into water. However, not all substances follow this simple rule; egg white, for example, congeals when heat is applied. Milk is another example. What is the reason for this?

 

[Njorl]

Egg white congealing, and milk curdling are not just phase changes, they are changes of compounds on the molecular level. When water freezes, the molecules stay the same, they are just arranged in a 3D lattice, rather than a 1D chain.

Also important, egg white and milk are not pure chemically, they are mixtures of many different chemicals, while water is pure.

Njorl

 

[Chemicalsuperfreak]

Both milk and eggs contain protein. You can kind of think of protein as spagetti rolled up into little balls. When you heat proteins they unravel, this process is called denaturing. When they unravel all the little strings get mixed up with each other and tangled, and whatnot; so when they cool the proteins are a big mess instead of renaturing back to the original little balls.