Combining foods can often be like playing matchmaker between two of your friends. You might like them both individually, but sometimes they just don't combine as well as you'd hoped. The idea of marinating a steak in your favorite wine, beer or liquor might appear to be a clever and intuitive way to combine flavors you like. In practice, however, unless it's used judiciously, alcohol in a steak marinade often does more harm than good.
Hot and Raw
Part of the problem with using alcohol in a marinade is that liquor, flavored liqueurs and most wines are simply too strong. Your steak absorbs alcohol from the beverage more readily than water, lending your meat a distinctively hot, raw taste from the concentrated alcohol. Your steak will absorb some of the desirable flavors as well, though they're mostly limited to the surface and won't penetrate into the meat unless you create a pathway for the marinade by poking holes in your steak with a skewer.
The Physical Reaction
Alcohol also causes some physical changes to the steak's muscle proteins. Like a strong acid, it breaks down the molecular bonds inside the muscle tissues and helps the proteins unwind or "denature." Essentially, the alcohol precooks the surface of the steak. Initially this causes a very small degree of tenderization, then as the steak continues to marinate, the proteins begin to contract and toughen. The net effect is a steak with an unpleasant brown color and a chewy surface.
Heat, Browning and Steam
Much of the pleasure in eating a well-grilled steak comes from the browning and searing effect of high heat. The glucose in the muscle caramelizes and its proteins brown through a similar effect called Maillard reactions, creating rich and savory flavors. If the steak's surface is wet from your marinade, it will steam instead until the marinade finishes evaporating. This limits the browning effect, and the tasty charring on your steak. Patting the steak dry aids browning, but removes much of the marinade's flavor.
Doing It Right
The best way to add the flavor of your favorite alcohol to a marinade is by cooking the marinade down first. Simmer it in a pan until the alcohol has largely evaporated and the flavors of your wine or spirit are highly concentrated. They'll make your marinade tasty, without the problems caused by the alcohol. Pricking the steak thoroughly with a skewer or scoring it with shallow cuts will help the flavors penetrate into the meat, where they can't be wiped away. This in turn enables you to pat the steaks dry before grilling them, so you can enjoy both the browning of the grill and the taste of your marinade.