Here's a quote I found regarding brining and what it does to meats:
The most commonly offered explanation is that the flavor brine solution contains a higher concentration of water and salt than the meat, so the solution passes into the meat cells through their semi-permeable membranes, adding water and flavor to the inside of the meat cells. This explanation is offered by authorities including Cook's Illustrated magazine and Robert L. Wolke, author of What Einstein Told His Cook: Kitchen Science Explained.
Other experts state the opposite situation, but with the same end result: That meat cells contain a higher concentration of water and dissolved solids than the flavor brine solution, so the solution passes into the meat cells through their semi-permeable membranes, again adding water and flavor to the inside of the meat cells. Shirley O. Corriher, author of CookWise, provides this explanation in her book.
Yet another explanation is that the flavor brine solution does not actually penetrate the meat cells at all. Instead, it just flows into the spaces between cells, where it draws out some moisture through the semi-permeable membrane of meat cells, increasing the concentration of naturally occurring sodium inside the cells. Some of the flavor brine solution remains between meat cells where it flavors the meat. The California BBQ Association website provided this explanation in an article written by Joe O'Connell (article no longer online).
Regardless of the explanation, all sources seem to agree that a higher concentration of salt inside meat cells causes protein strands to denature. The tightly wound proteins unwind and get tangled together, and when heated, the proteins form a matrix that traps water molecules and holds onto them tightly during cooking. In the case of the first two explanations, the denatured proteins hold on to some of the water, salt, and flavorings that flowed into the meat cells; in the case of the third explanation, the denatured proteins are holding on to free water that was already inside the meat cells and would have been lost had the meat not been brined.
Which of these explanations is correct? I'm not sure, but in the end, it doesn't really matter. The bottom line is that flavor brining results in meat that is more moist and flavorful than unbrined meat, regardless of which explanation you choose to believe.
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