Encapsulation of Ethylene Gas into V-Starch and Cucurbit[n]urils:Structure and Release Kinetics

Author:Shi Lin Fan

Supervisor:huang qiang

Database:Doctor

Degree Year:2019

Download:29

Pages:121

Size:7182K

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Ethylene is a natural ripener for fruits and vegetables,and is known as a“plant hormone”.It is present in the tissues and organs of plants,and is widely used in the agriculture and food industries.As an active gas,ethylene is often stored in pressurized cylinders for commercial use,which has the major disadvantages of leakage and explosion during transport and storage.In this study,we used V-starch and cucurbit[n]urils as carriers to encapsulate ethylene gas,and investigated the relationship between different molecular structures and ethylene concentration in the inclusion complex(IC).The controlled release characteristics of ICs were also investigated at various temperature and relative humidity conditions.The results provide theoretical guidance for the efficient utilization of ethylene,and have potential application prospects in fruit and vegetable preservation.High amylose maize starch(Hylon V)was used as raw material to prepared a V-starch with different single helix contents(8.5-27.1%)using alcoholic-alkaline treatment followed by ethanol precipitation.The content of single helix,V-type and crystallinity increased with the increase of alkali content.The ethylene gas was entrapped using a solid encapsulation method,and the ethylene concentration in ICs varying from 15.4%to 23.6%(w/w).13C solid-state statenuclear magnetic resonance(NMR)and Raman spectroscopy confirmed the presence of ethylene gas in the V-starch,and the ethylene concentration in IC was positively correlated with the V-type content.In this study,we encapsulated ethylene into V-starches prepared from different botanical origins and the ethylene concentrations in ICs followed the order of Hylon-Ⅶ(HA7)>Hylon-V(HA5)>normal maize starch(NMS)>normal potato starch(NPS)>waxy maize starch(WMS).Avrami’s equation was used to analyze the release kinetics of ethylene gas from ICs,and the results showed that all treatments showed diffusion release mechanisms in temperature experiments,whereas the release mechanism in relative humidity experiments resembled first-order kinetics.The release rate of ethylene gas from ICs accelerated with rising temperature or relative humidity and decreased with increasing DPAM.NPS-IC showed better controlled release characteristics than those of other starches,likely due to its highest DPAM value.The V-starches prepared from different botanical origins were annealed in different ethanol aqueous solution and temperatures,and the ethylene concentrations in ICs increased from 8.031.8%(w/w)to 18.149.6%(w/w).It was found that some amorphous structure in V-starch was transformed into single helices after annealing,and the molecular chains conceivably have ample opportunity to arrange into the most stable structure in a suitable solvent environment.The annealed starch showed three almost unbelievably sharp diffraction rings in X-ray diffraction and the crystallinity increased from 16.0%to 22.5%.The results of release kinetics showed that all treatments showed diffusion release mechanisms in temperature experiments,and the release mechanism in relative humidity experiments resembled first-order kinetics.The release rate of ethylene gas in the ICs increased with the rising temperature or relative humidity,whereas annealing can reduce the release rate of ethylene gas from the ICs significantly.The ICs exhibited the highest ethylene concentration,better thermal stability and controlled release characteristics after annealing in 70℃ and 50%ethanol aqueous solution.Studies on the formation mechanism of V-starch show that amylose can form a IC composed of 6,7 or 8 glucose units per helix under the induction of different molecules.Va-starch could be transformed to Vh-starch by exposure to a humid environment.This helical conformation of V-starch is flexible,and the helical diameter could be transformed to each other under appropriate conditions.In view of the shortcomings of V-starch-IC,such as fast release rate of ethylene gas and difficult storage at room temperature,cucurbit[n]urils(CB[n],n=5-7)were used as materials to encapsulate ethylene gas.Release kinetics results showed that all diffusion release mechanisms except for different relative humidity experiments of V-starch-IC andα-cyclodextrin-IC.With the increase of temperature or relative humidity,the release rate of ethylene gas in CB[n]-ICs accelerated,whereas the moisture content in the environment has no significant effect on the release kinetics of ethylene gas in CB[n]-ICs.Compared with V-starch-IC andα-cyclodextrin-IC,CB[5]-IC showed highest ethylene concentration in IC,better thermal stability and controlled release characteristics,which could released ethylene gas continuously at 4℃ for 700 h.Molecular simulation results showed that the smaller the ring of cucurbit[n]urils molecule is,the stronger the interaction force on ethylene gas of guest molecule is,and the better the stability of the complex is.