Marinating tough beef in acid makes it more tender, but there are alternatives to acid that work just as well. Harsh acids like those in lemon juice and vinegar can actually dry out the meat and leach flavor from it. Milk and buttermilk are traditional marinades in Southern cuisine, and Indian cooking often starts with a yogurt marinade. Milk-soaked beef softens and becomes tender without drying out or getting mushy. Making milk marinades is not difficult once you understand the basics.
Beef is basically muscle fibers held together with protein webs called collagen. The more the muscle is used by the animal, the leaner the muscle tissue is, stronger the muscle fibers are and the tougher the meat is. Breaking down the muscle fibers and softening the collagen into gelatin adds moisture to the meat as well as tenderizing it.
There are several methods to tenderize meat. Breaking the muscle fibers by scoring or pounding the meat is excellent for thick cuts of meat. Powdered marinades contain papain and bromelain – extracts from the papaya and pineapple, respectively – and will soften the surface of the meat but they don't penetrate. This can leave you with a mushy outside and tough inside. Marinating meat also breaks down tough muscle fiber.
Milk contains calcium, which, according to Shirley Comer at "Fine Cooking," may have an effect on an enzyme in the meat itself that breaks down the proteins. This process is similar to the way that aging tenderizes meat. Milk also contains lactic acid, which helps to break down the proteins and soften the collagen.
An effective marinade contains seasonings for flavor, an acid to break down the muscle fibers and collagen in the meat and an oil to moisturize the meat and act as an emulsifier. Milk is not a harsh acid, so you can season milk or buttermilk and leave out the oil. It doesn't matter whether you use a plastic zipper bag, a plastic container with an airtight lid or a baking dish covered in aluminum foil or plastic cling wrap to soak the meat. The main point is to cover the meat in milk and seal it in a closed container.