| Natural polymers are considered to be the best alternative to petroleum-based polymers and important biomass for the preparation of degradable packaging materials.The development of new protein-based packaging materials can alleviate major social problems,such as energy scarcity and environmental pollution,etc.Gliadin(Gli)is an ideal substrate for the preparation of natural polymer gel-material because of its easy-available,high-yield,renewable,biodegradable,good film-forming properties,and gelling properties.However,the above materials have the problems of poor mechanical properties,weak water resistance,and single functional characteristics.While the modification with aldehydes crosslinking or polymer blending is hoping to solve the above scientific and technological problems and realize the popularization and application of Gli materials in the food or other industry.In this study,Gli gel-material systems(hydrogel and film)that can be applied to different fields have been constructed through the Schiff base reaction modification by different aldehydes.The effects of Schiff base reaction modification on the mechanical properties,water resistance,functional properties of the systems were explored,and the change rules of the related parameters were also discussed.And the effects of different aldehydes on the microstructures,intermolecular interactions,and related properties of the Gli materials were clarified.Meanwhile,the relevant mechanisms of Schiff base reaction modification in improving its mechanical properties,water resistance,etc.,were revealed.All of these will provide new strategies and theoretical practices for developing degradable packaging materials with good properties and increasing the added value of Gli.The main research contents and conclusions are as follows:(1)Fabrication and evaluation of Gli hydrogel-system with high strength and high toughness crosslinked by glutaraldehyde.Glutaraldehyde(GA),an effective crosslinker,was applied to fabricate the Gli hydrogel-system with high strength and toughness.On the comprehensive study of mechanical properties,swelling characteristics,weight loss,thermal stability,and other properties of the Gli hydrogels,the formation mechanisms of the Gli hydrogels were explored.The results showed that the high strength and toughness of the Gli hydrogels could be fabricated with tailorable mechanical properties by changing the ratio of GA/Gli,the compressive stress of the hydrogel could reach 1.8 MPa under 70% strain;the swelling and degradation behavior of Gli hydrogels could be adjusted by the concentration of Gli,which can be soaked in water at least for 5 d.And the crosslinking effect of GA was helpful to improve the thermal stability of Gli hydrogels.Schiff base bonds(including single-site and multi-site crosslinking)and hydrogen bonds were the main forces in the Gli hydrogels.(2)Screening of dialdehyde polysaccharides(DPs)for constructing and evaluating of its crosslinked Gli films system.Four DPs were successfully synthesized instead of GA and applied to prepare the Gli film-system under alkali conditions.On comparative discussing the optical properties,water resistance,mechanical properties,barrier properties,thermal stability,etc.,of the films,the formation mechanisms of the films were also explored.The results showed that the effect of DPs crosslinking improved the water resistance,barrier properties,and thermal stability of the Gli films,and enhanced its tensile strength in mechanical properties about 8times,obtaining the antibacterial and UV blocking properties.Among the films,the DAD crosslinked Gli film(Gli-DAD)showed the best film properties.Electrostatic and hydrophobic interactions,hydrogen bonds,and Schiff base bonds were the main forces in the films.(3)Construction and performance evaluation of chitosan(CS)-Gli composite film system modified by DAD and reduced glutathione.To improve the tensile strength of DPs crosslinked Gli films,the preparation process of the DAD crosslinked Gli films were optimized under p H= 6.5,and the Gli was modified by reduced glutathione(GSH)to prepare the Schiff base bond and disulfide bond crosslinked Gli films.Meanwhile,the CS-Gli composite films were prepared by adding CS to further improve its tensile strength and other properties.On discussing the changes of various performance parameters of the films,the relevant formation mechanisms were explored.The results showed that the function of double-crosslinking improved the water resistance(the films can keep their integrity in water at least 50 d),mechanical properties,barrier properties,and thermal stability of the Gli films.And CS further improved the tensile strength of the Gli film(reaching to 12.32MPa),but decrease their elongation at the break.The Schiff base bonds,disulfide bonds,and hydrophobic interactions were the main forces in the double-crosslinked CS-Gli composite films.(4)Development and characterization of cinnamaldehyde crosslinked plastic and edible Gli film-system.To improve the plasticity(elongation or EAB)of the Gli films,cinnamaldehyde(CA)was used to prepare the edible Gli film-system under alkali conditions.On discussing the film properties,the formation mechanisms of Gli films were further clarified.The results showed that the CA crosslinked Gli films not only had good plasticity(EAB > 477%with an enhancement about 2 times),water resistance,and thermal stability but also performed functional properties of impeding the UV light and antibacterial activities.The crosslinking of the Schiff base bonds and Michael addition reactions,as well as the intermolecular hydrophobic interactions and hydrogen bonds,were the main forces in the films.And these forces changed the secondary structures of Gli and improved the relevant properties of the Gli films.(5)Study on the loading and release behavior of tea polyphenols(TPs)from the CA crosslinked Gli films system.The plasticity and edible Gli films system crosslinked by CA was applied to load and delivery TPs for active packaging.On studying the influence of TPs on the basic properties of Gli films,their long-term stability in water and simulated gastrointestinal digestion characteristics were also evaluated,and further studies were carried on the release behavior and antioxidant activity of TPs in the different food simulated solutions from the films.The results showed that the mechanical properties of Gli films were not affected by TPs(33g/kg).The films had good anti-gastric digestibility and long-term stability in water(>10 d),which provides a basic guarantee for the release and delivery of TPs from the films.The function of CA crosslinking improved the loading capacity of Gli film by about 10%,and the release of TPs from related Gli films was the fastest in the acidic-simulated food system,followed by the alcoholic-simulated food system,the oil-simulated food system,and the common-simulated food system.And the release of TPs from CA crosslinked Gli films showed a slow-release effect,which endows the films with good antioxidant activity. |
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