4. Wheat Gluten-Based Film; Preparation and Characterization
Wheat Gluten (WG) is the primary protein in wheat grains
[74]. Films that are made from wheat gluten have potential to develop an edible film, adhesives, binders, and biomedical substances. The main advantages of wheat gluten films include being insoluble in water, elastic in nature, and non-toxic. Gluten matrix is biodegradable and glassy, with characteristics similar to epoxy resin
[75][76][77].
4.1. Production of Wheat Gluten-Based Film
Wheat-gluten based films can be produced via two common methods:
Wet-type mechanical milling is a common approach for producing nanoparticles for a variety of bio-materials, including starch and gluten
[78]. For gluten, a milling process is used to obtain gluten powder. The wheat gluten suspension solution is made by mixing the gluten powder with ethanol (70% aqueous ethanol). Then fibers are immersed in gluten suspension-solution. After the mixture is homogenized, the composite is dried in a vacuum air oven to allow the solvents (water and ethanol) to evaporate more quickly
[79].
This method can be performed by either; (1) spreading dry powder with dry fibers in the mold, where the gluten powder will be first distributed in the mold. Next, the dry fiber preforms will be placed into the mold. Subsequently, another gluten powder layer would be added through a sieve. These steps will be repeated until the desired thickness is achieved (2), by spreading dry powder on wet fiber in the mold. In this method, fiber must be wetted again (after combing and drying), as the water will be a processing aid, after casting the gluten powder and wet fiber on the mold, the drying process needs to be conducted in dryer oven
[80].
4.2. Properties Characterization of Films Based Wheat Gluten
Due to the fact that polar amino acids such as glutamic acid, aspartic acid, lysine, arginine, serine, threonine, and tyrosine are present in proteins, the addition of protein in biocomposite films improves the mechanical properties. Amino acids contain reactive groups that can be useful in cross-linking and creating covalent connections, improving the mechanical characteristics of biocomposites
[81]. It has been found that proteins rich in sulfur amino acids, particularly rapeseed proteins when combined with rubber, cause a substantial enhancement of the cross-linking process. Protein-rich composites have a higher thermal resistance due to the high number of nitrogen atoms in a single polypeptide molecule
[82].
Wheat-gluten films revealed lower water absorption (settled on 80% after 4000 min), this amount of water absorption is a response for (C=O, C=C) bonds existence in gluten film
[83]. While the starch-based films revealed higher water absorption, which reached approximately 520% after 210 min on cassava-starch-based films
[42] and 295% after 240 min on corn-starch-based films
[84]. All starch-based films showed very strong water absorption capacity. However, the amount of absorbed water is different from one starch to another. This behavior is attributed to the size of starch particles, the smaller the particles the earlier and higher water absorption. Also FTIR analysis shows hydrogen bonded hydroxyl group peak more intensely with small-particle content compared to the larger particles, this explains the increase in water absorption capacity
[85]. Wheat-gluten-based films, plasticized with glycerol show elongation at break in the range from 320.5–474.5%, 6.33 MPa tensile strength, while the moisture content was just about 5%
[86]; the addition of a plasticizer reduces hydrogen bonding, which allows molecules to move and increase the elongation, while the high tensile appears when starch-starch hydrogen bonds overcomes starch-plasticizer bonds in a low amount of plasticizer
[87]. Reinforcing wheat-gluten with flax fiber improves the tensile strength and the elastic modulus, because of the hydrogen bonding between the fiber and the protein
[80][88][89][90][91]. Heat treatment of wheat gluten at temperature higher than 100 °C reduces the effect of the reinforcing filler which reflected as reduction in the Young’s modulus. This explains the reduction of wheat gluten adhesion when it is heat treated
[92]. However, treating the filler with alkaline and/or silane improves adhesion between wheat gluten and filler. This surface treatment increases the mechanical properties by reducing the fiber pullout length
[93] As confirmed by FTIR results, fiber chemical treatment removes lignin and hemicellulose and reduces the hydrophilic nature of the fiber and, hence, improves the interfacial adhesion between fiber and matrix
[94][95]. Natural structures of bio-polymers have relatively low degradation temperatures
[96]. This refers to the low energy level required to break the weak interactions between the polymer chains. To avoid undesirable decomposition of wheat-gluten-based bioplastics, hydrophobic liquids, e.g., castor or silicone oil are used
[97][98]. Blending gluten with hydrophobic polymers, such as polyvinylalcohol improves the degradation temperature
[25][99]. The addition of hydrophobic polymers widens the gap between the energy required to break bond interactions and the energy required to cause chains breakdown. Although wheat-gluten-based films also prepared with solution cast method, compression molding have given better properties
[100]. The wheat-gluten films reinforced with fiber filler can be prepared either by wet or dry method:
Tensile strength increased when drying temperature increased at 35% RH, while it decreased when temperature increased at 70% RH
[101]. N. Vo Hong et al.
[77] used water as a processing aid together with the use of unidirectional flax fibers to obtain the strongest properties in the fiber direction. Pakanita Muensri et al.
[102] found that lignin content in the fibers does not affect the fiber/matrix adhesion. The type of wheat proteins and compression molding conditions controls the properties of wheat-protein films
[103]. To make edible films out of wheat gluten, Francisco Zubeldía et al.
[104] employed the dry process. They observed that molding temperature has a greater impact on the films’ ultimate mechanical and physical properties than mixing time. This was due to increased disulfide bonding during heating, resulting in a more cross-linked polymeric network, according to the study. Further work needs to be undertaken to understand the mechanism of cross-linking wheat gluten with fillers
[105].
5. Antioxidant Properties of Wheat Based Film
The inhibition of oxidation improves the stability of polymers to be effective in more applications
[106]. The addition of antioxidant into films can change the structure of the film
[107], where the reduction in the antioxidant impairs the resistance to degradation
[108]. Antioxidant materials are added to prolong the useful life of the constituents of polymers
[74][109], the polymer type and the compound formulation and the end use application are governing the selection of the correct combination of antioxidants
[110]. Wheat starch–chitosan films show the highest antioxidant (α-tocopherol) capacity. However, the addition of α-tocopherol led to more heterogeneous film structure
[111]. Feruloylated arabinoxylans extracted from wheat bran show high antioxidant activity in the presence of bound ferulic acid
[112].
6. Antimicrobial Properties of Wheat Based Film
Antimicrobial property has received more attention recently, especially in the bio-packaging food industry
[113]. It has been found that composite wheat gluten-chitosan-based films can prevent microbial growth in intermediate-moisture conditions
[114], where gluten is thought to act as an antimicrobial agents carrier
[115][116][117]. Organic acids, enzymes, various plant extracts, bacteriocins, and essential oils have been integrated into biopolymers as antimicrobial agents
[118][119][120]. Essential oils (EOs) used in food packaging films to inhibit the growth of bacteria and fungi
[121][122][123]. Essential oils are natural, volatile, complex compounds with a strong odor extracted from plants
[124]. They have health benefits, antimicrobial and antioxidant properties
[125][126]. (EOs) used to reinforce bio-matrix composites
[127], such as reinforcing corn wheat starch matrix with lemon oil, and the addition of lemon oil, significantly increased antimicrobial activity
[128]. However, the addition of (EOs) concentration reduced the tensile strength, while the elongation at break does not change
[129]. Potassium Sorbate (PS) has been used as an antimicrobial agent for wheat gluten films. (PS) shows antimicrobial activity, but it has been found that when the film is exposed to an absorbing medium, most of the PS is released
[130]. Thymol has been added as an antimicrobial to hydroxyethyl cellulose wheat-starch-based films and the results show the film kept the same chemical properties, whereas mechanical properties improved
[131].