Fabrication Of Zein/Gelation Films And Their Applications In Controlled Release Of Active Compounds

With the improvement of living standards in our country,people's demand for food is transformed from quantity to quality,which also puts forward new requirements for the improvement of food quality and preservation technology.The development of biobased edible films with the advantages of environmental friendliness,safety and active functions has become a research hotspot in the field of food packaging.To ensure the quality of food during shelf life,one of the key factors is the efficient water barrier property of food packaging materials.Ideal edible packaging film should have permselective barrier property against water vapor on both sides.That is,while preventing the penetration of external moisture,the relative humidity required for food preservation inside the packaging is maintained.In addition,the development of active packaging that can efficiently load natural functional ingredients and control their release has been considered as a powerful measure to reduce or prevent the quality and safety issues such as oxidation,browning or microbial contamination of foods during transportation and storage.Addressing the problems that the traditional protein-based edible films do not have the selective barrier or and adsorption to water vapor molecules and the difficulty of controlled release of active compounds,this essay aims to take complementary advantages of zein and gelatin to construct multilayer film system(ZZG-G)and composite film system(ZG),and to further establish novel active packaging materials with one-way moisture barrier and long-term controlled release properties.We systematically study the effects of microstructure of active films on physicochemical properties and controlled release of active ingredients,and apply the film to the field of fresh-cut fruit preservation.The main contents are as followed:(1)Based on the Multilayer films with the stacking order of hydrophobic zein outer layer,hybrid zein/gelatin middle layer and the hydrophilic gelatin inner layer(Z-ZG-G)were fabricated by layer-by-layer solvent casting method.When the ZG ratio of the middle layer is 1:2,the Z-ZG1:2-G multilayer film exhibited tri-layers structure with different characteristics,which had well-defined borders but was tightly coupled.The film displayed good transparency and mechanical properties while keeping UV protection property.The results showed that when the moisture transferred from the zein side to gelatin side,the WVP value was significantly lower than that from the gelatin side,indicating that the multilayer film can achieving one-way moisture barrier property.The physical model of the structural response(Zein hydrophobic selfaggregation,gelatin swelled by water absorption)on both sides of the multilayer film during water vapor infiltration was constructed to reveal the formation mechanism of the one-way moisture barrier of the multilayer film.(2)Based on amphiphilic characteristics and self-assembly behavior of Zein,Zein/Gelatin composite films(ZG)were constructed,and the microstructure and characteristics of the composite films at different mass ratio of zein to gelatin were studied.When the Zein content was high,the macroscopic phase separation took place between Zein and Gelatin,and a layered structure with identified phase boundary was obtained.When the gelatin content was high(ZG ratio is 1: 1,1: 2,1: 3),microscopic phase separation occurs,and the compartmentalized structure that Zein spherical aggregates distributed in the gelatin continuous matrix can be obtained.The ZG1:2 composite film exhibited a dense gradient structure,in which Zein aggregate with moderate particle size tended to be distributed on the air side of the gelatin matrix.The ZG1:2 film also showed good comprehensive performance and the lowest WVP value,and the WVP value on the air side was lower than the bottom side,realizing one-way moisture barrier property.The results indicated that precise control of phase separation structure can be achieved by simply changing the ZG mass ratio.(3)Based on the different affinity between the film matrix and the active ingredients,the compartmentalized structure of the ZG1:2 composite film was utilized to achieve co-loading.The hydrophilic antioxidant tea polyphenol(TP)was loaded in gelatin matrix,and the hydrophobic antibacterial oregano essential oil(OEO)was loaded in Zein aggregates to develop bifunctional composite film with controlled release of various active ingredients.The incorporation of TP affected the formation of the regular network structure of gelatin,making the surface morphology of gelatin continuous matrix rough,while the encapsulation of OEO resulted in a graininess cross-section morphology of zein aggregates.FTIR results confirmed the hydrogen bond and hydrophobic interactions between TP,OEO and film matrix,respectively.The simultaneous loading led to stronger water barrier property with higher retention rate of TP and OEO in ZG1:2 film.Kinetic analysis suggested the release mechanisms of TP and OEO from ZOGTP film were non-Fick diffusion and were close to the firstorder release kinetic.(4)Based on the different retention mechanisms of the active ingredients in the layered structure,the optimized Z-ZG1:2-G multilayer film was used to load tea polyphenol(TP)with concentration gradient.The multilayer functional film with prolonged release property was fabricated,in which the inner G layer was loaded with a lower concentration of TP to play an initial antioxidant role,and the middle ZG layer as antioxidant reserve layer was loaded with a higher concentration of TP.TP released from the multilayer film in a prolonged manner and the release kinetic was closely related to the structure responses of each film layer.The release experiments of TP from multilayer film into distilled water suggested the combination mechanism of diffusion and first-order kinetics(0–175 min),followed by the concentration-driven first-order kinetic mechanism(175-300 min),and finally diffusion-dominated kinetic mechanism(> 300 min).When applied on freshly cut fruits,the TP loaded multilayer film exhibited preservation effects in terms of controlling weight loss,preventing rapid browning and inhibiting bacterial deterioration.