## 7 Biological Properties
For instance, the growth of tested pathogens such as _Salmonella typhimurium, Escherichia coli, Staphylococcus aureus_ and _Bacillus cereus_ were inhibited by CNP-starch films about 15-20%. * Adsorption. Chitosan has forceful adsorption function and can selectively adsorb heavy metal ions, cholic acid, cholesterol, triglyceride and grease [72; 73; 74; 75]. It cannot be digested and absorbed in the gastrointestinal tract in a short time, on the contrary, it can absorb the fat that can significantly prevent the digestive system from absorbing cholesterol and triglycerides, avoiding the excessive accumulation of cholesterol and fatty acids. Eating 2 g of chitosan every day can effectively absorb the fat in the food, which is beneficial to losing weight [76]. Researchers try to find out the relationship between the structure of chitosan and the lowering of blood lipids and blood glucose, which is mainly due to the amino group of chitosan itself, which makes chitosan a poly ionomer. One possible way is that the chitosan catabolic compound can be adsorbed with negatively charged fatty acids and cholesterol. The simple chitosan can adsorb many times of its weight of oil, which can effectively prevent the digestive system from absorbing cholesterol and triglycerides, prevent the accumulation of cholesterol and fatty acids in the body and promote its excretion from the body. By reducing intestinal lipid absorption, the levels of cholesterol and triglycerides in plasma were reduced. Another possibility is that chitosan combines with negative bile acids to reduce the amount of bile in the liver and empty the gallbladder. There must be a certain amount of bile acid reserve in the gallbladder, so that the cholesterol in the plasma or liver can be converted into bile acid to maintain the bile acid reserve, thus reducing the cholesterol concentration in the plasma or liver [77]. * Moisture retention. The molecular chains of chitosan and its derivatives contain numerous active hydrophilic polar groups, such as -OH, -NH2, -COOH, etc. [78]. The content of the carboxyl group in carboxylated chitosan is far more than other derivatives, and the repulsion of the negative charge on the carboxyl group makes the polymer chain space to be extended especially large, even at a lower concentration, there is a strong interaction between molecules. The force on the water molecule is strengthened due to the hydrophilicity of the carboxyl group and the large extension of the molecular chain so that it has better moisture absorption and retention performance. * Film forming. Chitosan has stable physicochemical properties and outstanding film-forming performance, its film-forming ability is closely related to the internal structure. The higher the deacetylation degree of chitosan, the lower the swelling and tensile strength of the membrane and the more difficult the degradation of the corresponding chitosan membrane in vitro and in vivo [79]. Since there are more crystal structures in chitosan with a high degree of deacetylation, the molecular rigidity is stronger and the water absorption is lower. The film formation and its characteristics are greatly regulated by the relative molecular weight of chitosan. The lower the molecular weight is the lower the tensile strength and the stronger the permeability of the membrane. The larger the molecular weight is, the more the crystal structure is, and the higher the molecular entanglement is. Therefore, the flexibility of the molecule is poor, the tensile strength is high and the permeability of the membrane is poor [80]. Since chitosan is degradable, the changes in the molecular weight might affect the properties of the membrane; the chitosan crosslinking degree also affects the properties of the membrane. With the increase of the crosslinking degree, the spatial network structure formed between the molecules increases, the tensile strength of the membrane increases and the water permeability decrease.