Preparation Of Chitosan Derivatives As Preservative Of Fruits And Vegetables

Fresh fruits and vegetables are rich in nutrition and can be eaten conveniently. But they are highly perishable in normal ambient conditions. The annual yield of fruits and vegetables of our country is very large. But the loss of fruit and vegetable is large too because of the poor storage capability. And the residual organophosphorus pesticides of fruits and vegetables will be a potential risk to consumers. So it is a concerned problem that how to prolong the shelf-life of fruits and vegetables and degrade the organophosphorus pesticides.Chitosan, a kind of natural polysaccharide biological macromolecule, has many outstanding properties and potential application in various areas. Chitosan could be considered as an ideal preservative coating material for fresh fruits and vegetables because of its excellent film-forming and biochemical properties. But when the chitosan is used to coat on fruits and vegetables to prolong their storage life, it has been forced to be associated with some shortcomings, such as low moisture-holding rate, fruits and vegetables dehydration due to its typical molecular structural feature. Moreover, simultaneous maintenance of quality of fruit and vegetable cannot be achieved along with the desired antimicrobial properties. Different groups of chitosan could be introduced for chemical modification to generate various derivatives due to the existence of active amino and hydroxyl groups. Therefore, chitosan was modified using metal ions and gallic acid to enhance the antioxidant and antibacterial activities and degradation ability of organophosphorus pesticides in our research.The main contents and conclusions are described as follows:1. The preparation of complex of chitosan and metal ions (CTS-M). CTS-M was made from chitosan and some metal ions. The effects of deacetylation degree (DD) and viscosity average molecular weight (M.W) of chitosan and concentration of metal ions on film-forming property of CTS-M and the preservation quality of fruits and vegetables were investigated. The obtained results indicated that the CTS-M, which made from 1% chitosan with DD of 84%, M.W of 7.2×10⁵ D, and 0.5 mg/mL Zn²⁺, 0.6 mg/mL Ce⁴⁺, had the best fruit and vegetable fresh-keeping and organophosphorus pesticides degradation ability.2. Pilot production of CTS-M. The CTS-M prepared in pilot production was straw yellow powder. It couldn't dissolve in organic solvent, but prone to dissolve in acidic solution. The CTS-M formation could be described as Zn²⁺ bound to amino group, and Ce⁴⁺ bound to hydroxyl groups of chitosan. Compared with the complex of chitosan and metal ions prepared in lab, the thickness and swelling ratio of CTS-M film didn't differ significantly, tensile strength and water vapor permeability decreased, meanwhile, the dissolution ratio, oxygen resistance and transmittance increased by 10.59%, 67.39% and 5.04%, respectively.3. Effect of CTS-M on the preservation quality of fruits and vegetables and degradation mechanism of organophosphorus pesticides. The CTS-M had quite beneficial effects on the physiological quality and degradation of organophosphorus pesticides compared to chitosan and control. It could prolong the shelf-life and decrease the potential risk of jujube and apple safety. After 5 days storage, the degradation rates of organophosphorus pesticides on jujube and apple treated by CTS-M were above 15% and 20% higher than the control, respectively. Gas chromatography-mass spectrometry (GC-MS), total organic carbon (TOC) and inorganic ions were used to analyze the degradation products and pathway of organophosphorus pesticides. Hydrolysis and oxidation were the main degradation pathway of organophosphorus pesticides by CTS-M, including the oxidation of P=S to P=O and the cleavage of P-S and P-O bonds. Complete mineralization was achieved in all cases and the final degradation products were CO₂,H₂O and some inorganic ions.4. The preparation of chitosan gallate (CTS-GA) and its effect on the preservation of fresh-cut apple. The conjugation of gallic acid (GA) on chitosan can be concluded that the gallate group of GA was successfully introduced onto chitosan via amino and hydroxyl groups. CTS-GA was stable under the conditions of acid, strong light and temperature below 80℃, whereas it was unstable under the conditions of basic, strong oxidation and high temperature. The studies on the free radical scavenging of CTS-GA clarified its significant antioxidant activity on free radicals, such as hydroxyl radical, DPPH radical and superoxide anion radical. CTS-GA showed antioxidant activity on free radicals close to or even higher than pure GA. The antibacterial ability of CTS-GA on Escherichia coli was studied. When the concentration of GTS-GA was 10 mg/mL, the antibacterial zone diameter against Escherichia coli was 11.44 cm. The effect of CTS-GA on preservation quality of fresh-cut apple was investigated. The results showed that CTS-GA had a quite beneficial effect on physicochemical and sensory quality of fresh-cut apple. And the antimicrobial activity of CTS-GA was higher than chitosan and GA. After 4 days storage of fresh-cut apple, the antimicrobial rate of CTS-GA was 30.56% higher than the control.The chitosan derivatives not only have multiple characteristics, such as, safe, non-toxic, antibacterial, edible, degradable, and the source of a wide range of easy to extract, but also control the aging of post-harvest fruits and vegetables and degrade the organophosphorus pesticides. Therefore, they have a bright prospect of application in the fresh-keeping of fruits and vegetables.