Papain is a protease enzyme that comes from the unripe fruit of the papaya tree. Proteins are made up of amino acids linked together to form long chains, and proteases break apart these bonds. Papain is an ingredient in commercial meat tenderizers. Meat consists of strands of muscle tissue held together by a tough protein called collagen. Papain breaks down the bonds in these collagen proteins, making the meat less tough and easier to eat.
Making meat tender isn't the only function of papain. It has a number of applications in the medical field as well. Because it can cleave the bonds that hold protein together, papain has been used in the past as an ingredient in products used to debride wounds. For some wounds to heal, it's necessary to remove dead and damaged tissue. Papain digests" damaged skin tissue without harming healthy skin tissue. Once the healthy tissue is broken down