Chitin is a tough, fibrous material that gives fungal cell walls their strength. It sits alongside glucans as one of the main building blocks of the fungal cell wall (Garcia-Rubio et al., 2020), and it is the same material that forms the shells of crabs and the outer skeletons of insects.
Why It Matters for Mushroom Products
Chitin is hard to digest. People do make chitin-breaking enzymes called chitinases (Rathore & Gupta, 2015), but the compounds a mushroom is valued for sit inside walls built from it. That is the reasoning behind extracting mushrooms with hot water and alcohol rather than simply grinding them into powder: extraction pulls the compounds out of the cell wall before you take them. The practice is widespread, but studies directly comparing extracts with plain powder in people are few.
References
Garcia-Rubio, R., de Oliveira, H. C., Rivera, J., & Trevijano-Contador, N. (2020). The fungal cell wall: Candida, Cryptococcus, and Aspergillus species. Frontiers in Microbiology, 10, Article 2993. https://doi.org/10.3389/fmicb.2019.02993
Rathore, A. S., & Gupta, R. D. (2015). Chitinases from bacteria to human: Properties, applications, and future perspectives. Enzyme Research, 2015, Article 791907. https://doi.org/10.1155/2015/791907