Many proteins are strongly effected by heat, causing denaturation.
Meat consist to about 20% of proteins, like e.g. myosin and collagen.
They undergo substantial structural changes at temperatures between 57°C
to 75°C, which impacts the texture, water holding capacity, and
shrinkage of meat. On the one hand these changes make meat more easily
digestible, on the other hand the water-holding capacity is reduced.
Therefore, resting a steak after cooking allows some of the water to
find new interactions with proteins, resulting in less juice leaking out
when the steak is cut.
While heat usually causes a change in the 3D structure of proteins, there are some exceptions to this:
Casein, a protein in milk is exceptionally stable to heat, which
makes it possible to boil and even pasteurize milk without causing
coagulation. The protein beta-lactoglobulin, however coagulates at
boiling temperatures and forms the characteristic protein skin on top of
boiled milk.
