Amir, in basic medical terms, there are three things that govern movement of fluid between a semipermeable membrane: 1. hydrostatic pressure, 2. osmotic pressure, 3. integrity of the membrane.
In an animal,
hydrostatic pressure refers to the pressure generated by the heart (i.e. the blood pressure) + the pressure generated by the column of fluid above it. Both of these are lacking in a piece of meat, but in meat the hydrostatic pressure comes from the contraction of muscles from heat. The more contraction there is, the more fluid will be squeezed out. This is one reason why meat cooked on the bone is juicier - the bone physically resists excessive meat contraction.
The other factor that causes contraction is high heat. High heat is essential to browning the surface of the meat, but a thicker piece of steak will contract less than a thin steak because all that heat has to travel through the muscle first. I have talked in some other threads about the gradient of heat in a steak. You can actually see the gradient of heat by the change in colour of the steak as the heat passes through:
Note the meat closest to the surface is well done, with the colour progressively getting more pink towards the center. In a thin steak, you will rapidly heat up the whole steak before you get adequate colour on the surface. This is why I advocate two stage cooking, because you have two aims when you cook steak - (1) to get the center medium rare, and (2) to develop the Maillard reaction on the surface. See my other threads about two stage cooking.
Osmotic pressure (i.e. pre-salting) is not likely to be the culprit in your case. If you salt your meat before cooking, there is no reason for the osmotic gradient to increase so precipitously
during cooking that it suddenly draws out large quantities of liquid. Besides, as more liquid is drawn out, the osmotic pressure should decrease. Excessive salting
will dry out your steak, but not to the extent that it leaks out pools of liquid.
Most of the explanation lies in the integrity of the
cell membrane. When the membrane is disrupted, fluid readily leaks out from within. Here are a few things that can damage the cell membrane:
- Freezing. Freezing causes ice crystals to grow in the meat. These shear holes in cell walls. When the meat is defrosted, the crystal retreats leaving the holes. The effect is even worse if the meat is re-frozen. When the meat is cooked, contraction of the muscle causes even more fluid to leak out.
- High heat. In addition to causing contraction described above, high heat damages cell walls. Avoid exposing the interior of your meat to too much high heat (i.e. don't overcook your steak).
- Physical treatments, e.g. pounding with a meat tenderizer.
Take home message: use meat which has never been frozen, buy a thick cut, subject it to gentle cooking + high heat for the surface, do not excessively salt. This is what my steak looks like on the carving board. It goes straight off the charcoal grille onto the chopping board, where it sits covered in foil for 10 minutes:
Note there are no juices coming out and the carving board is barely wet.