Any Southern cook worth her salt knows that the best way to get tender, tangy, juicy fried chicken is to soak the raw pieces in buttermilk overnight. Cooks in India and Greece use yogurt in much the same way. Milk, especially fermented milk products such as buttermilk and yogurt, contains acids and enzymes that help tenderize chicken.
All muscle tissue is held together by a web of protein called collagen, a fibrous substance that is flexible but tough. Cooking meats breaks down the collagen, and so does marinating it, which is why all marinades contain an acid in addition to an oil and seasonings. Acids and enzymes such as those found in lemon juice, lime juice, wine and vinegar help break down the collagen and start the tenderizing process before cooking begins.
Though it seems counterintuitive, given milk’s soft texture in the mouth and mild flavor, milk contains lactic acid as well as calcium. All meats contain proteins that break down over time -- hence, aged beef, which is more tender than fresh beef. It's not quite understood how milk helps tenderize chicken, but it's believed that calcium may awaken the natural enzyme in chicken that makes it more tender over time, speeding up the process. Lactic acid also works to break down proteins. In addition to regular milk, buttermilk and yogurt, which are slightly more acidic than milk, also make good tenderizers for chicken and work in a similar fashion as milk.
Start by seasoning your raw chicken pieces with salt and pepper. Place them in a bowl, a resealable plastic container or plastic zipper bag. If you prefer more intense flavor in your chicken, whisk seasonings into your milk or buttermilk. Italian and Cajun seasonings complement chicken, as does a simple mixture of lemon pepper, rosemary and sage. Pour the milk over the chicken pieces, covering them completely. Cover the bowl or seal the container or bag and let the chicken soak in the refrigerator for 60 minutes to overnight.