The Effect of Brining on Meat

Meat is muscle tissue that is that contains bundles of long proteins
that are coiled or wadded up molecules with bonds that hold the proteins in 
single units. When cooked, either by heat, acid or drying, these bonds 
breakdown and the protein bundles straighten out. This is called denaturing.
The proteins can then bond together with other unwound molecules. This is 
called coagulation.

You can watch this process as an egg cooks. The white is initially clear 
because there is a lot of room for light to go between the molecules. As the
coagulation progresses, there is no longer enough room for light to 
penetrate and the white becomes opaque. There is a lot of moisture trapped 
between these barely cooked proteins. This is when they are juiciest and 
most tender.

Below 120F, the bundles shrink in size and moisture loss is minimal.
Moisture loss begins above 120F. This initial sweat is from the space
between the individual cells. The next sweat is when the cells
themselves begin to breakdown above 140F releasing their moisture. The
key to perfectly cooked meat is temperature control.

Brining or salting is a way to increase the moisture holding capacity of 
meat resulting in a moister product. Salt and water can move through cell 
walls. Salt will move to areas of lower concentration. When meat is brined 
there is some uptake of salt but it is only in the intracellular spaces 
because the cells themselves contain high concentrations of sodium already. 
Meat will gain about 20% in weight which will be all lost on the first 
sweat. Even very concentrated solutions will be less concentrated than the 
intercellular fluids.

What this means is that water inside the cells move out when brining.
That sounds like the opposite effect of what you are trying to
accomplish. But what happens is that the increased sodium concentration in 
the cells increases the bonding between the protein molecules. This raises 
the temperature at which they breakdown releasing their bound moisture...the
second sweat.

Does brining tenderize the meat? Not directly in the way a marinade
does. It allows a longer time for collagen to breakdown without the meat 
drying out. Also the increased water retention lubricates the individual 
fibers.

The resulting flavor is regarded by many cooks to be mandatory for all
poultry. The flavor change in red meats, however, gives a ham-like
flavor. While desirable in smoked pork chops, ham, bacon, corned beef,
prosciutto, and pastrami, it is not appropriate for traditional forms of BBQ
like ribs, pork shoulder and briskets.

Water is trapped in cells because of the proteins. When salt is applied, the
proteins denature and cross link. Cross linking means the cells break up and
lose their moisture at higher temperature than unbrined meat. That last 
sentence explains the process and its effect.

No tenderizartion from brining. Only moisture retention and texture
impovement. Marinating (involving an acid and oil) only tenderizes the
outer 1/4". That tenderization is mushy if left too long unless the
marinade involves buttermilk, but that is a WHOLE 'nother story.