What are the quality criteria for chitosans?
Clearly, the quality of all chitosans depends on the quality of the chitin used for their production, and the batch-to-batch reproducibility of chitosans depends on the availability of a constant high quality source of chitin. Firstly, the same quality criteria apply to chitosan as to chitin, namely ash and heavy metal content, protein and endotoxin content. The quality (and price) of commercially available chitosans greatly differ depending on their purity. The purest (and most expensive) chitosans are typically labeled as 'pharmaceutical grade'. Increasingly, chitosan producers adopt the rules of ISO certifications for the quality control of their production processes. Further factors critically influencing the performance of chitosans are their degree of polymerization and their degree of acetylation, and many different specifications of chitosans, varying in these parameters, are available on the market. Of course, the degree of polymerization cannot exceed that of the chitin used as a raw material and in fact, the de-N-acetylation procedures used to convert chitin into chitosan tend to decrease the degree of polymerization. Chitosans are often categorized as 'high molecular weight chitosan' or 'low molecular weight chitosan', but there is no generally accepted definition of the terms. The former typically should have a range of degrees of polymerization between 500 and 5,000 (i.e., molecular weights between 100 and 1,000 kD), the latter between 50 and 250 (equaling 10 to 50 kD). The size range in between might be called 'medium molecular weight chitosans', and the range below 10 kD (but above the range of chitosan oligomers) might be called 'very low molecular weight chitosans'. The higher the quality of the chitosan, the lower should be its polydispersity index, high quality chitosans should have a value below 2, ideally below 1.5. Most chitosans commercially available have degrees of acetylation in the range of 10-30% so that the terms 'low DA chitosans' or 'high DA chitosans' should be reserved for chitosans with degrees of acetylation below 10% and above 30%, respectively, but this is not a generally accepted rule. All chitosans commercially available today have random patterns of acetylation.