"When 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."