How are chitosans produced commercially?
Chitosans are produced commercially from chitin by partial de-N-acetylation using concentrated caustic soda. This process is typically performed as heterogeneous de-N-acetylation where the sodium hydroxide slowly diffuses into the crystalline chitin flakes, slowly de-N-acetylating the chitin into chitosan. Thus, chitin molecules at the outer surface of the flakes become deacetylated earlier while chitin molecules in the still crystalline core are deacetylated at a later time only, leading to a broad mixture of chitosans of widely varying degree of acetylation. The average degree of acetylation reached at the time when the last chitin crystals dissolve depends on the size and shape of the chitin flakes used and on the process parameters. Prolonged or successive alkaline treatments can then yield chitosans with even lower average degree of acetylation, but the treatment tends to concomitantly decrease the degree of polymerization, too. Alternatively, chitin can be de-N-acetylated in a homogeneous process in which the alkali is first allowed to soak into the chitin crystals at low temperature where it does not lead to de-N-acetylation which is only started by rising the temperature once all the chitin crystals are fully swollen. This process can yield chitosans with higher average degree of acetylation than the heterogeneous process. Such chitosans can also be produced by chemically re-N-acetylating fully de-N-acetylated chitosans, but the process is expensive and requires special equipment so that it is rarely performed at larger scale. All of the so produced chitosans are characterized by a random pattern of acetylation, due to the chemical processes used. Enzymatic production ways using chitin de-N-acetylases have not yet been established on a commercial scale. Low quality chitosan may also be produced from complex sources such as fungal cell walls directly, without first extracting chitin, but these are typically characterized by low degrees of polymerization and low purity.