Chitin is a naturally-occurring polymer (complex molecule) found in shells, either of crustaceans such as crabs, lobsters or shrimp, or of insects. It is also found in mushrooms.
Chitin is indigestible, but it is still used a great deal in things we drink and eat. It is used for purifying water, as a “fining” for clarifying alcohol beverages such as wine or beer, or juice, as a stabilizer in processed foods, and to add fibre in baked goods.