Study On Wheat Gluten-Based Edible Films Improving And Film-Forming Mechanism

With the high-speed development of social economy and people's increasingawareness of environmental protection, serious ecological problems caused by synthesizedchemical packaging materials has aroused great concerns around the globe. Vegetableprotein films has become the focus of current research interest in edible film filed due to itsuniformity, transparency, excellent POV, good mechanical properties, rich nutrition,biodegradability and good digestibility. Wheat gluten (WG) is the byproduct of wheatstarch production, mainly consisting of gliadin and gliadin; WG can form athree-dimensional network structure with water, after dried, it can form protein films withgood toughness, high POV, good heat sealability and excellent oil resistance. However,because of poor moisture resistance and mechanical property, the commercial applicationof WG films is directly limited. Therefore, enhancement and improvement of moistureresistance and mechanical property is of current urgency for applications of WG films.In this paper, WG was chosen as substrate for film preparation. Eight kinds of WGwith different qualities were purchased from seven large-scale domestic representativecompanies who produce WG protein. We investigated the film-forming ability andproperties of the aforementioned eight WG films with different qualities; based on this, theeffects of film-forming conditions on the WG film properties were studied, and the optimalfilm-forming preparation technology was obtained; Then, we used gliadin or Zein tomodify the WG films and optimized the film-forming preparation technology of gliadinimproved films and Zein composite films; finally, we investigated the working mechanismof gliadin and Zein on the improved films on the molecular level. The main results were:(1) The film-forming ability of eight WG films and the effect of quality on the WGfilm properties were studied. The results suggested that:The resolvability, film liquid uniformity, drying rate, difficulty of film-uncovering,film color and surface smooth degree of the chosen eight kinds of films were significantlydifferent. The tensile strength (TS), fracture elongation rate (E), water vapor permeability(WVP) and transmission of the eight WG films prepared from the eight WG protein werealso obviously different. It was found that the Lotus1WG protein produced by HenanLotus Flour Co. Ltd. was the most suitable one for the edible WG film preparation due toits advantages of high protein content, low fat content, and good resolvability, uniform filmliquid, easy film-uncovering and smooth surface etc.(2) The effects of film-forming factors on the WG film properties were investigatedvia single factor test and orthogonal test, the optimal film-forming conditions wereoptimized. Results indicated that: when the solid-to-liquid ratio was1:15, WG filmsshowed the best comprehensive properties; glycerin can reduce the film brittleness andhence enhance the film flexibility, but affected the moisture resistance relatively little.Hence, it can be a best plasticizer for WG films. The sequence of factors influencing the WG film TS was as follows: glycerinconcentration> pH value> concentration of alcohol> temperature of treatment, with thetreatment combinations of pH12, the concentration of alcohol of45%, glycerinconcentration of15%, temperature of treatment of55℃, the maximum of TS reached3.78MPa. The sequence of factors influencing the E (elongation at break) of WG films:glycerin concentration> pH value> concentration of alcohol> temperature of treatment,with the treatment combinations of pH11, the concentration of alcohol of45%, glycerinconcentration of30%, temperature of treatment of60℃, the maximum E (elongation atbreak) of WG films reached the maximum of189.2%. The sequence of factors influencingthe WG films WVP: glycerin concentration> pH value> concentration of alcohol>temperature of treatment, with the treatment combinations of pH12, the concentration ofalcohol of45%, glycerin concentration of15%, temperature of treatment of60℃, the WVPof WG films can reach the minimum of6.98g·mm/m~2·d·KPa.Considering the three main indexes of TS, E (elongation at break), and WVP, theoptimal film-forming conditions was determined as follows: pH12, the concentration ofalcohol of45%, glycerin concentration of20%, temperature of treatment of60℃. Underthese conditions, the TS, E, WVP and oil permeability of the WG films were3.67MPa,157.9%%,7.03g mm/m~2d KPa and0.(3) Zein was used to modify WG films, Zein composite films were prepared, and theeffect of Zein on the WG films properties was investigated. The results showed that:Appropriate additive amount of Zein can effectively reduce the film brittleness,improve the film toughness, water resistance, mechanical properties and POV. When theadditive amount of Zein was25%, the concentration of alcohol was55%, the glycerinconcentration was10%, the pH value was12, the temperature of treatment was60℃, theTS, E and POV ability for the WG composite film were4.89MPa,179.1%and21.7mmol/Kg, increasing by33.2%,17.2%and28.1%respectively when compared with WGfilms (as the control group); the WVP and the transmission were5.20g·mm/m~2·d·KPa and35.6%, decreasing by26%and75.3%respectively when compared with WG films.(4) The gliadin was used to modify WG films, gliadin improved film was prepared,and the effect of gliadin on the WG film properties was investigated. The results showedthat:Appropriate additive amount of gliadin can effectively reduce the film brittleness,improve the film toughness, water resistance, mechanical property, transmission and POV.When the additive amount of gliadin was33.3%, the concentration of alcohol was65%,the glycerin concentration was10%, the pH value was11, the temperature of treatmentwas60℃, the TS of the improved film reached the maximum of5.77MPa, increasing by52.7%and18.0%compared with WG films and composite films respectively; the WVPwas minimum of4.48g·mm/m~2·d·KPa, increasing by35.8%and16.1%compared withWG films and composite films respectively; the POV is18.6mmol/Kg, increasing by 39.0%and14.3%compared with WG films and composite films respectively; and thetransmission rate was60.3%, increasing by139.3%and238.8%respectively, comparedwith WG films and composite films;(5) By analysis of infrared absorption spectroscopy, differential scanning calorimetryspectrum, thermal gravimetric spectrum, mercapto-group and disulfide bond contents,SEM micrographs and hydrophobicity, the modification mechanism of gliadin improvedfilms and Zein composite films were explored. The results showed that:After the addition of gliadin or Zein into WG, the surface of gliadin improved filmsand Zein composite films became smooth, the content of insoluble matter was significantlyreduced, the surface structure of improved films were obviously better than the compositefilms; The TG curve of WG was divided into two stages as water evaporation and WGdecomposition; the TG curves of WG films, composite films and improved films weredivided into four stages of air escape, water evaporation, glycerin evaporation and WGfilm decomposition; in the process of heating treatment, the maximum temperatures oftreatment of gliadin improved films and Zein composite films can be decreased by15.01℃and14.8℃compared with WG films.From a molecular perspective, it was proposed that the reasons why the addition ofZein and gliadin can modify WG film properties possibly were the comprehensive result ofthe weakening hydrogen bond interaction between the protein molecular, the decreasingcontent of mercapto-group, the increase of disulfide bond contents and hydrophobicity inthe gliadin improved films and Zein composite films.