The fruit that contains the enzyme papain is papaya.
Papain, also called papaya proteinase I, is a cysteine protease found in papaya fruit, especially in the milky latex of the papaya tree, Carica papaya. As a protease, it breaks down proteins. That protein-digesting action explains why papain has been used in cosmetic products and exfoliating masks: it can help loosen some proteins in dead surface skin, although cosmetic formulations vary in strength and effectiveness.
Papain has a much older history outside skincare. Indigenous peoples of tropical America used papaya and its latex to tenderize meat before European contact. Today, purified or crude papain is used in commercial meat tenderizers and has applications in food processing, leather, textiles, detergents, pharmaceuticals, and laboratory cell preparation.
Papaya is sometimes confused with pineapple because pineapple contains bromelain, another protein-digesting enzyme used in food and cosmetic contexts. Kiwi also contains proteolytic enzymes, but papain specifically comes from papaya. People with latex sensitivity may also need caution because papaya and natural rubber latex can share allergenic proteins.