Papain is an effective meat tenderizer because it severs the protein chains that make meat fibrous and difficult to cut. This protein-cleaving and protein-digesting enzyme breaks down the fibrils and connective tissue that contribute to the meat's structural integrity, thereby tenderizing it.
Tough connective tissues manage the mechanical stress to which muscles are subjected. Each muscle cell has fibrils that make the cell structurally sound and help the muscle contract. The fibril's internal structure is a complex network of long protein chains, and the tissues connecting the muscle cells are mostly protein as well. Papain cuts long protein chains into shorter pieces, which causes the meat to become more tender.
Papain is similar to the enzyme bromelain found in pineapples. Both of them are proteolytic enzymes, meaning that they help digest proteins found in food. Most commercial meat tenderizers contain papain, bromelain or both. Papain is similar to the stomach enzyme pepsin, which breaks down meat, egg and dairy proteins during digestion.
Papain played an important role in early scientific studies of plant protein structure. It was applied to proteins to break them into smaller pieces for easier examination. Central American natives utilized the meat tenderizing properties of papain by wrapping it fresh papaya leaves to tenderize it.