Many people believe than an acidic marinade - one containing wine, vinegar, or citrus juice - will make meat tender. In fact, the opposite is often the case. Acid interacts with the proteins in the meat, causing the protein molecules to squeeze more closely together and thus forcing liquid out of the meat. What you end up with is tough and dry steak, chicken, or wildebeest.
What's more, extended exposure to acid can cause the surface of the meat to become mushy because the proteins start to break down. The rule, then, would be to keep acidic marinade periods short, but then of course the flavor won't get into the meat very well. My approach is to rely on non-acidic marinades. The fact is, marinades in general don't have nearly the effect that many people think because the flavors just cannot penetrate beyond a millimeter or two at the surface. Salt and water in a marinade can penetrate deeper, and the benefit of most marinades is the result of this factor.
