Study On Mechanism Of Chitosan Degradation With Hydrodynamic Cavitation

As a kind of natural polymer polysaccharide,chitosan is widely used in food,chemical industry and pharmaceutical industry because of its characteristics of good antibacterial,moisturizing and film-forming properties.However,as the physiological activity of chitosan is closely related to its molecular weight,oligosaccharides with different molecular weight have different functional characteristics,so it is an important research content to prepare oligosaccharides with specific molecular weight on the premise of not changing the natural structure of chitosan.The previous study of our group found that hydrodynamic cavitation can degrade chitosan effectively without changing the structure of chitosan,and it has certain advantages over chemical degradation,enzymatic hydrolysis or ultrasonic degradation.Therefore,it is necessary to further study the reaction mechanism and molecular weight distribution of products of chitosan degradation by hydrodynamic cavitation,which is of great significance for improving the reaction efficiency of hydrodynamic cavitation degradation of chitosan and regulating the molecular weight distribution to obtain oligochitosan with specific molecular weight.The main research contents and results of this paper are as follows:(1)In this paper,based on the mechanism of cavitation effect,it was assumed that chitosan degradation by hydrodynamic cavitation was carried out by chemical(random cut)and mechanical(central cut)effects.Then the Monte Carlo software for the analysis of the mechanism of chitosan degradation by hydrodynamic cavitation was prepared according to the hypothesis of fracture modes.The difference analysis showed that there was insignificant difference between the experimental and simulation results,which indicated that the hypothesis of the degradation mechanism and simulation algorithm established in this paper were valid for the hydrodynamic cavitation degradation of chitosan.The results also indicated that the analysis results were reliable and simulation algorithm established in this paper can accurately simulate the molecular weight distribution of the products of chitosan degradation by hydrodynamic cavitation.At the same time,the existence of mechanical(central cut)and chemical(random cut)effects in the process of chitosan degradation by hydrodynamic cavitation was proved by adding tert-butanol to the experiment.(2)The results showed that the fracture modes of chitosan degradation by hydrodynamic cavitation were mainly random and central cuts,that is,the degradation was mainly caused by the combination of chemical and mechanical effects.The degradation mechanism was affected by the cavitation types,experimental operating conditions and structural parameters of the cavitator.Under the experimental conditions in this paper,the proportion of random cut was 91-100%,while the proportion of central cut was 0%-9%.Among the three cavitation types,the degradation efficiency(degree of hydrolysis)and the ratio of fracture modes were slightly different,but the random cut dominated the degradation process.With the extension of degradation time,the proportion of central cut in the process of chitosan degradation by hydrodynamic cavitation decreased continuously,and the limit molecular weight of products from chitosan degradation caused by mechanical effect was about 40,000.With the increase of the initial molecular weight of chitosan and upstream pressure of hydrodynamic cavitation,the proportion of central cut in the process of hydrodynamic cavitation increased.With the increase of temperature,the proportion of central cut decreased.By changing the structural parameters of the cavitator,it was found that the inlet and outlet angles had a great influence on the degradation mode and the throat length has a small influence.With the increase of the inlet and outlet angles of the cavitator,the ratio of random cut increassd continuously,and the degree of hydrolysis had an optimal value.(3)The effects of different degradation mechanisms on molecular weight distribution of degradation products were studied.In order to predict the molecular weight distribution of chitosan degradation by hydrodynamic cavitation,a predicting software was designed by using Monte Carlo method combined with the ratio of random cut under different average molecular weight conditions.The prediction results of molecular weight distributions proved that in the process of the chitosan degradation with hydrodynamic cavitation,a higher proportion of random cut not only indicated a stronger chemical effect,but also indicated a wider distribution of products.In comparison,a higher proportion of central cut not only indicated a stronger mechanical effect,but also indicated a narrower distribution of products.With more central cuts,the slide-shaped curve of molecular weight distribution of degradation products was gradually transferred into a bell-shaped curve,and the main peak moved from low molecular weight to high molecular weight.The results showed that the regulation and control of the molecular weight distribution of chitosan degradation by hydrodynamic cavitation can be realized by adjusting the ratio of different fracture modes in the reaction process,which provided guidance for the preparation of oligosaccharides with specific molecular weight.