There is a commonly held belief that soaking a tough cut of meat in a marinade will make it tender. Sadly, this just isn't true much of the time. While some marinades are very successful at adding flavor to meat, chicken, and fish, they are, with one exception, a disaster at tenderizing.
The two most popular types of marinades are acidic (made with citrus, vinegar, or wine) and enzymatic (made with ingredients such as pineapple and papaya). Although both types work primarily on the surface of the food, they lead to different results: highly acidic marinades can actually toughen food, while enzymatic marinades can turn the surface of the food to mush. For true tenderizing, the most effective marinades are those that contain dairy products.
Acidic marinades add flavor but may toughen
One marinade family relies on mildly acidic ingredients, like citrus juice, vinegar, or wine.
Acidic marinades "denature" proteins. Imagine the protein in raw meat, chicken, or fish as individual units of coiled ribbon, with bonds holding each coil in a tight bundle. When these proteins are exposed to an acidic marinade, the bonds break and the proteins unwind. Almost immediately, one unwound protein runs into another unwound protein and they bond together into a loose mesh. (This is the same thing that happens when proteins are exposed to heat.)
At first, water molecules are attached to and trapped within this protein mesh, so the tissue remains juicy and tender. But after a short time, if the protein is in a very acidic marinade, the protein bonds tighten, water is squeezed out, and the tissue becomes tough. If you've ever tried marinating shrimp in highly acidic ingredients, it's likely that you're familiar with this result.
In limited cases, mildly acidic marinades can add wonderful flavor to fish and meat, especially if you enhance the mixture with fresh herbs, spices, or perhaps another liquid like Worcestershire sauce. The key is to use the correct strength acid for the food you're marinating. For shrimp, I use a low-acid marinade (perhaps one part mild acid to four parts oil) to avoid toughness. For example, I might use two tablespoons each of vinegar and caper juice and one cup of oil.
A fairly tight-textured cut of meat like flank steak can survive a more acidic marinade. Since the marinade only penetrates a fraction of an inch, it won't toughen the meat.
Enzymes make meat mushy
Another approach is to use enzymatic marinades, which work by breaking down muscle fiber and collagen (connective tissue). Raw pineapple, figs, papaya, honeydew melon, ginger, and kiwi all contain such enzymes, known collectively as proteases (protein enzymes). Unfortunately, these enzymes work almost too well, turning tough meat muscle into mush without passing through any intermediate stage of tenderness. The longer the meat marinates, the greater the breakdown of proteins and the mushier the texture.
My experience with tenderizing enzymes mirrors that of Dr. Nicholas Kurti, a famous Oxford physicist who tried tenderizing a pork roast by injecting half with pineapple juice, leaving the other half untouched. A noted chef, Michel Roux, was to judge on television which side was better. After cooking, the half treated with pineapple was total mush and looked like a pile of stuffing. Not surprisingly, Chef Roux preferred the untreated half. (He did try to find something nice to say about the mushy half. Noticing its crisp skin, Chef Roux announced, "But the crackling is superb!" Dr. Kurti used the comment as the title for his book on his experiments with tenderizing enzymes.)
Most commercial meat tenderizers rely on enzymes to do their "tenderizing" (a papaya enzyme, papain, is a common ingredient in these products), so I stay away from them.
For true tenderizing, use buttermilk or yogurt
Dairy products are, in my opinion, the only marinades that truly tenderize. Hunters have long known to marinate tough game in milk, Indian recipes use yogurt marinades for lamb and tough goat meat, and some southern cooks soak chicken in buttermilk before frying. Buttermilk and yogurt are only mildly acidic, so they don't toughen the way strongly acidic marinades do. It's not quite clear how the tenderizing occurs, but it seems that calcium in dairy products activates enzymes in meat that break down proteins, a process similar to the way that aging tenderizes meat.
In deciding how long to marinate, consider the texture of the meat or fish. In general, open-textured flesh like fish fillets needs only a few minutes of soaking. I love making "fish fingers" by briefly immersing strips of fish fillets in buttermilk seasoned with cayenne, dusting them with seasoned flour, and then frying them. Food with a tighter texture, such as chicken or lamb, can tolerate several hours in a marinade, even one that's mildly acidic.