Marinating a steak with a fruit juice concoction not only adds flavor, but it also tenderizes the meat; however, not all juices work equally well for tenderizing meat. Juices with tenderizing enzymes and those high in acid work best for tenderizing. For best results, choose a juice to complement the other flavors of your steak's marinade while tenderizing it.
Papaya
Papaya juice contains an enzyme called papain. This natural enzyme is used in commercial meat tenderizing products, but those do not add a tropical touch to the steak when used in a marinade. Add papaya juice or mashed papaya to marinades to take advantage of the enzyme to tenderize your meat. The natural sweetness of the papaya juice will also foil the savoriness of the steak.
Pineapple
Pineapple juice contains both acids and enzymes to tenderize steak. Fully ripened pineapples contain about 2 percent acid, based on the variety. This acid helps the marinade to penetrate the meat and get deeper into the steak tissue. Bromelain is an enzyme in pineapple that breaks down protein. Add a splash of pineapple juice to a steak marinade for a tropical flavor to make use of the enzyme and the acid.
Figs
The enzyme in figs that breaks down protein and tenderizes steak is called ficin. For this to tenderize meat, the juice must directly touch the portion of the meat it will tenderize. No matter how long the meat is soaked in a marinade, it will not tenderize the piece all the way to the center, only the surface. Fresh fig juice is not easy to find, but puree some fresh figs or dried figs that have been soaked with their soaking juices and strain off the solids for a homemade fig juice to marinate steak.
Citrus Juices
Acids do not tenderize as well as enzymes, but they do begin to unravel proteins in the meat in a process called denaturing. This softens the texture of the meat and changes the proteins in the meat to resemble cooked meat. Some thin, lean cuts in ceviche recipes, such as fish, are cooked using acid alone and not heat. The high acidity of citrus juices makes them ideal for use in steak marinades. Lemon juice, lime juice and orange juice all appear in recipes for steak marinades to tenderize the meat and add flavor.