There's a lot of confusion out there about marinating meat and what effects it has in terms of flavor, moisture, and especially tenderizing. Nevertheless, many people wrongly believe that it does. Maybe you're one of them; that's about to change.
How Acid Changes Meat
The theory is that certain ingredients, particularly acidic ones like lemon juice, vinegar, or wine, do something to the proteins in meat, causing it to become more tender. The theory is partly true. The acid in those ingredients does do something to meat — but it's making it firmer, not more tender.
Look no further than your nearest ceviche for proof. The whole principle behind ceviche is that marinating raw fish in acid, such as lime juice, causes the proteins to coagulate and become firm, almost as if it had been cooked with heat.
More flavor can be beneficial for a cut of meat that's on the leaner side, such as a sirloin steak, or even a tenderloin steak. Lean steaks tend to be less flavorful because it's the intramuscular fat (or marbling) that contributes much of a steak's flavor. This is why you so often see tenderloin steaks prepared with a strip of bacon wrapped around them.
On the other hand, rib eyes, T-bones and strip steaks don't need to be marinated. They are already naturally flavorful and juicy and don't need much more than salt and pepper. With expensive steaks like that, you want to taste the beef itself, not the marinade.
Don't overcook your steaks. Overcooked steaks are tough and dry, no matter how tender they were to start out, or what type of marinade you used.
A Good Marinade
Liquids like wine and fruit juices are good for marinating, not because, but rather, in spite of the fact that they're acidic. Fruit juices contain sugars which caramelize when they hit the grill and wine contains all kinds of interesting flavor compounds, which become deeper and more complex when exposed to high heat.
The key with wine, however, is to cook off the alcohol before using it to marinate. That's because alcohol will also cause the proteins in the meat to coagulate. Even a simple marinade of olive oil chopped garlic and fresh herbs will add flavor to a steak or roast.
Marinating Only Affects the Surface
The truth about marinades is that they really don't penetrate much beyond the surface of the meat, a few millimeters at the most.
The marinade isn't soaking into the meat. It's merely coating the surface with the flavorful ingredients. This is why marinating beef in an acidic liquid doesn't turn it into ceviche. The acid simply doesn't penetrate, largely because of the amount of collagen-based connective tissue in meat. This connective tissue surrounds the muscle fibers, forming a barrier against the marinade. Fish and seafood have a much less connective tissue, which is why ceviche is possible.
It's also why beef carpaccio, which is probably the closest thing to beef ceviche, is made with beef that's sliced paper-thin, which exposes cross sections of the muscle fibers.
Acid won't have any effect on the tenderness of meat one way or another. If you marinate a piece of meat in an acidic liquid for a long time, those acids will cause the surface of the meat to take on a mealy, mushy texture. This undesirable effect should not be confused with tenderizing.
Note that because marinating is mainly about flavor and to a much lesser extent moisture, and because marinades only flavor the surface anyway, dry rubs are just as effective as marinating when it comes to imparting flavor to a steak or roast.