| A large amount of waste is generated during the processing and manufacturing of aquatic products,which easily causes waste of resources and environmental pollution,and how to recycle these wastes has become a major research hotspot.Chitin is widely found in crustacean marine organisms,such as shrimps and crabs,and has anti-bacterial,anti-cancer and moisturizing effects,and is widely used in agriculture,industry,medicine and other industries.Chitosan is the product of deacetylation of chitin,and chitin with more than 50%deacetylation is usually called chitosan.At present,most of the industrial methods to obtain chitin and chitosan are chemical methods,i.e.using acid and alkali to demineralise and deproteinise the waste and then decolourise it to obtain chitosan,which is deacetylated by strong alkali to obtain chitosan.The chemical method is simple and easy to implement but consumes a large amount of acid and alkali,consumes high energy and is prone to environmental pollution.The enzymatic method using enzymatic deproteinisation and demineralisation to obtain chitin is documented in the literature and can reduce the use of acid and alkali.The ionic liquid method can reduce the use of acid and alkali but has high cost and low extraction rate.The ohmic heating uses the material as resistance for heating,which facilitates the adequate reaction between chitin and lye.This paper investigates the effects of ionic liquids and enzymes on the extraction of chitin from the shell of the East China Sea aquatic product Portunus trituberculatus crab and the effects of ohmic heating and high pressure on the preparation of chitosan.Commercial chitin and chitosan were used as controls,and secondary structure(degree of deacetylation),surface morphology,thermal stability and yield were used as the main evaluation indicators to explore the effects of different methods on the properties of chitin and chitosan.The main research results are as follows:(1)Properties of binary ionic liquid mixtures with different ratios(1:0,1:1,2:3,1:4,0:1)and their effects on chitin extraction The results of solvent extraction with different ratios showed that the highest chitin extraction rate(19.54%)was achieved when[Amim]Br:[Emim]Ac was 2:3,and the quality was closest to that of commercial chitin.The viscosity,conductivity and dielectric loss of the ionic liquid mixtures decreased with decreasing[Amim]Br ratio,and the dielectric constant and radius of gyration increased with decreasing[Amim]Br ratio(p<0.05).The penetration depth at 915 MHz was greater than 2450 MHz for the same ratio of ionic liquid mixtures,and increased with decreasing[Amim]Br ratio at the same frequency(p<0.05).The chitin extracted from the ionic liquid mixture in a ratio of 2:3 met commercial requirements in terms of secondary structure,colour difference,surface morphology and a similar particle size distribution to that of the 0:1 ratio.The high extraction rate and excellent properties of chitin demonstrate the potential of binary ionic liquid mixtures for chitin extraction applications.(2)Effect of recycling ohmic heating on the preparetion of chitosan The enzymatic pretreatment of chitin extraction reduced the use of strong bases,and the highest chitosan deacetylation(94.38±0.21%)(p<0.05)was obtained after ohmic heating three times with the best quality.Pepsin and acidic protease were selected as complex enzymes for the extraction of chitin and their reaction conditions were optimised,and the secondary structure and surface morphology of the chitin obtained after optimisation met commercial requirements.The energy required for water bath heating was 4.8×10~6J and for ohmic heating was 2.8×10~3J.The chitosan prepared by water bath heating and ohmic heating has a similar secondary structure to commercial chitosan and the product prepared by ohmic heating has a higher degree of deacetylation.The correlation between the degree of deacetylation of chitosan and the dielectric loss of chitosan solution was 0.9297(p<0.05),which was positive,and the dielectric loss of chitosan obtained by ohmic heating was most similar to that of commercial chitosan.The thermal stability of chitosan was not destroyed by ohmic heating and the rheology and surface morphology of chitosan after three cycles of ohmic heating were most similar to that of commercial chitosan.The chitosan prepared by recycling ohmic heating not only has a higher degree of deacetylation than the conventional water bath method,but also consumes less time and energy,which has a very good development prospect.(3)Response surface test to optimise the preparation of chitosan by high pressure deacetylation The chitosan deacetylation degree after response surface optimisation was 89.88%,which was similar to the model prediction(89.70%),but the deacetylation effect was weaker than ohmic heating.The reaction time,pressure and feed solution all had an effect on the chitosan deacetylation degree,where there was an interaction between chitosan deacetylation degree,viscosity-averaged molecular weight and solution p H.The response surface results showed that the degree of influence on chitosan deacetylation was from time>feed to liquid ratio>pressure,and the interaction effect was time-feed to liquid ratio>pressure-feed to liquid ratio>time-pressure.The chitosan prepared under optimal conditions had a relative molecular mass of 6765 Da and a solution p H of 3.38±0.02.It had a secondary structure similar to that of commercial chitosan,was more crystalline,less thermally stable and had a denser fibrous structure.The high pressure can reduce the molecular weight of chitosan and ensure its quality,which is an important guideline for the preparation of low molecular weight chitosan.Binary ionic liquid extraction of chitin,ohmic heating and high pressure treatment for deacetylation to prepare chitosan not only meet the needs of green development,but also can guarantee the quality of the product.It provides a reference for the production of high yield and high quality chitin and chitosan. |
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