Papain is a protease enzyme that can be added to beer before filtration and packaging. It was the first haze preventative used in the brewing industry. The enzyme is capable of digesting dissolved proteins—with amino acids as a by-product—and thus increasing the clarity of beer. One of the main natural sources of papain is unripe papaya fruit, which contains high levels of the compound. The enzyme can be purified by modern processing techniques and combined with other enzymes to produce specific mixtures suitable for chillproofing in brewing and other applications in the food industry. Among the many traditional uses of papain are as a meat tenderizer, as a medical aid on wounds and stings, and as a digestive aid. Meat wrapped overnight in crushed papaya leaves, for instance, becomes tender and requires less cooking time because of the release of papain in the milk-like papaya juice.

In brewing, papain is used to digest excess proteins after fermentation, because high protein levels can cause hazes, high viscosity, and excess foaming in the finished beer. See chill haze and foam. Using papain, however, is not without risks. The amino acids produced by papain digestion may also serve as nutrients for spoilage microorganisms. Papain is also very resilient and acts under a wide range of conditions including high and low temperatures. This makes it suitable for many applications, particularly in foods with low pH and during beer processing at low temperatures. On the flip side, however, its high temperature resistance may allow it to survive pasteurization and thus continue to be active in packaged beer. If improperly used or allowed to survive into packaged beer, papain can eventually digest foam-positive proteins as well as haze-forming proteins, thereby damaging a beer’s foam stability. Largely for this reason, use of papain has declined in recent decades, and it has been replaced by modern adsorbents that are reliably filtered out of finished beer.