Marinades serve two different functions: as a tenderizer and flavor enhancer. You probably already know that some tough cuts of meat benefit from the tenderizing effects of marination, but how does it work?
Marinade Tenderizing Science
The cooking process itself turns connective tissues into gelatin to varying degrees. Depending on the cut and type of meat, it may need a little assistance to bring it to a palatable range of tenderness. Certain plant and fungi enzymes and acids can break down muscle and connective proteins in meats.
As far back as pre-Columbian Mexico, cooks found that wrapping meats in papaya leaves before cooking made for more tender results. The active enzyme in the papaya leaves is papain, now refined from papayas and commercially available. The connective tissue that comes in direct contact with the protein-digesting enzymes gets broken down. These tenderizing enzymes also reduce the capability of the meat to hold its juices, resulting in greater fluid loss and thus drier meat. Enzymes are heat activated at levels between 140 F and 175 F and deactivated at the boiling point, so it serves no purpose other than flavoring to let the meat sit in a marinade at room temperature. Refrigeration is recommended to avoid the growth of harmful bacteria. Let meat come to room temperature before cooking.
Marination Requires Contact
Direct contact is an important point since it is necessary for the chemical reaction to occur. This means that soaking a piece of meat in a marinade will only penetrate just so far into the surface of the meat. If you marinate a large cut of meat in a tenderizing marinade, you end up with a mushy exterior and an unaffected center. Puncturing the meat for the marinade to penetrate gives an uneven result, with the further undesirable side effect of allowing the meat to lose even more juices while cooking. Thus, flat cuts of meat benefit most from tenderizing marinades. Place meat in a heavy zip-top bag with the air squeezed out and turn it often to be sure all surfaces benefit from the marinade.
Some slaughterhouses now inject papain into the animals just before slaughter. The injected papain is carried through the bloodstream to all parts of the animal and is later activated by the cooking process. This sometimes results in a mushy piece of meat due to the enzyme destroying too much of the muscle fiber firmness. The newest method being researched is a machine which immerses tough cuts of meat into a water bath and then sends a shockwave through the meat, breaking down tough fibers.