Effects of multi-functional additives during foam extrusion of wheat gluten materials

The environmental impact of fossil-based polymer foams necessitates the development of sustainable bio-based alternatives. Wheat gluten (WG), a readily available and inexpensive byproduct of bioethanol and starch production, presents a promising option. WG's known foaming properties, stemming from its ability to aggregate, crosslink, and polymerize via disulfide bonds, allow processing with conventional plastic methods including extrusion. Previous research demonstrated successful extrusion foaming of WG using ammonium bicarbonate (ABC) as a foaming agent, yielding more uniform and porous materials than with sodium bicarbonate. This study builds upon previous work, focusing on the impact of three non-toxic, multifunctional additives – genipin (GNP), gallic acid (GA), and citric acid (CA) – on the overall properties of glycerol-plasticized WG foams produced via extrusion with ABC. The aim was to determine how these additives affect the foam's mechanical properties (cushioning and sealing) and liquid absorption, and to identify potential applications based on the resulting properties. The multi-functionality of the additives, with potential roles as crosslinkers, plasticizers, and radical scavengers, introduces an element of uncertainty as to their dominant effect within the WG foam matrix.

Existing literature extensively documents the use of wheat gluten in various applications, including food processing and biomaterial development. Studies show that the properties of WG-based materials are significantly influenced by processing conditions and the incorporation of additives. The potential of WG for foam extrusion has been explored, highlighting the impact of foaming agents and processing parameters on the resulting foam structure. Previous research emphasizes the use of crosslinking agents to enhance the mechanical strength, rigidity, and water resistance of WG materials. The use of genipin, gallic acid, and citric acid as crosslinkers and modifiers in various biopolymer systems has also been reported, but their individual effects on WG foam properties remained unexplored. The literature review established the basis for understanding the potential of WG as a sustainable foam material and highlighted the need for investigating the effects of the chosen natural additives.

Wheat gluten powder was mixed with glycerol (70:30 weight ratio) and 5 wt% ammonium bicarbonate (relative to the total weight of gluten and glycerol). Different formulations included 1 wt% and 5 wt% of gallic acid, citric acid, or genipin. The mixtures were extruded using a single screw extruder with controlled temperature zones (50-60-70 °C) and screw speed (120 rpm). The extrudates were dried and stored before testing. A series of analyses were performed to characterize the resulting foams: 1. **Foam Structure and Visual Appearance:** Visual observation and scanning electron microscopy (SEM) were used to assess the foam structure (open vs closed cells, pore size, density). Expansion ratio was calculated to understand the degree of foam expansion. 2. **Gas Generation Analysis:** The decomposition rate and total volume of gas produced from ammonium bicarbonate were determined experimentally and compared with theoretical calculations to assess the foaming agent efficiency. 3. **Density and Porosity:** Density and porosity (open and closed) were determined using a modified Archimedes principle, employing limonene or n-heptane as immersion liquids. 4. **Swelling Capacity (SC):** The saline uptake capacity of the foams was determined by immersing samples in saline solution for different durations (1 s to 24h). 5. **Mechanical Testing:** A uniaxial compression testing machine was used to measure yield strength, compression strength, elastic modulus, and hysteresis loss rate at various compression strains (10%, 30%, 50%). Compression set testing assessed the degree of permanent deformation after compression. 6. **Molecular Structure Analysis:** Fourier-transform infrared spectroscopy (FTIR) and proton nuclear magnetic resonance spectroscopy (¹H NMR) were employed to analyze the molecular interactions and reactions between additives, glycerol, and the wheat gluten matrix. SE-HPLC was used to determine protein solubility and assess the degree of crosslinking. 7. **Crosslinking Degree:** Lowry's method was used to quantify the degree of crosslinking, normalized to the reference sample.

The study revealed significant variations in foam properties based on the additive used: * **Citric Acid:** Produced foams with the lowest density (641-650 kg/m³), highest porosity (~50%), and predominantly closed cells. The density and porosity were similar at both 1 wt% and 5 wt% concentrations, suggesting similar crosslinking extents. * **Gallic Acid:** Resulted in foams with higher density (820-840 kg/m³) compared to citric acid foams, but lower than genipin foams. The 5 wt% gallic acid foam exhibited the highest saline uptake (~130% after 24 hours). FTIR and SE-HPLC data indicated that gallic acid acted primarily as a radical scavenger, reducing the extent of protein aggregation and crosslinking. * **Genipin:** Produced the densest (804-950 kg/m³) and stiffest foams with a high proportion of open cells, leading to rapid saline uptake (50% within one second). Genipin acted as a crosslinking agent, with the 1 wt% concentration resulting in the highest density and lowest porosity. **Mechanical Properties:** All foams exhibited high energy loss rates (84–92% at 50% strain), suggesting promising cushioning behavior. Compression set tests showed low permanent deformation, indicating potential for sealing applications. Comparison with a commercial nitrile butadiene rubber (NBR) foam indicated comparable mechanical behavior, especially the 5 wt% citric acid foam, although the NBR showed lower energy dissipation and greater resistance to permanent deformation. **Molecular Analysis:** FTIR and ¹H NMR data showed interactions between additives and glycerol/ABC but were inconclusive concerning the extent of specific reactions. SE-HPLC data showed 5 wt% gallic acid produced the least aggregated/crosslinked foams while 5 wt% genipin and 5 wt% citric acid produced the most aggregated/crosslinked foams. The degree of crosslinking measured by Lowry's method confirmed the least crosslinking in the 5 wt% gallic acid foam.

The results demonstrate that the carefully selected natural additives significantly alter the structure and properties of wheat gluten foams, allowing for tuning of their properties for specific applications. The different functionalities of the additives (crosslinking, plasticization, radical scavenging) lead to distinct foam morphologies and performance. The high energy dissipation and low compression set values indicate promising applications in cushioning and sealing. The significant saline uptake of certain foams highlights their potential for absorbent applications like sanitary products. The comparison with NBR foam confirms the competitiveness of the bio-based foams regarding mechanical behavior. The work underscores the feasibility of designing wheat gluten foams with tailored properties for diverse applications.

This research successfully demonstrated the ability to tune the properties of wheat gluten foams by incorporating naturally occurring multifunctional additives. The resulting foams exhibit properties suitable for applications requiring absorption (sanitary products), cushioning, and sealing. Further research could explore the optimization of additive concentrations and processing parameters to enhance specific properties, while broader life cycle assessments are needed to fully quantify the environmental sustainability advantages.

The study focused on a limited set of additives and concentrations. While the chosen additives are non-toxic, a comprehensive toxicity assessment for specific applications would be necessary. The sample preparation and testing methods may have introduced some variability. Further investigation is needed into the long-term stability and durability of the foams in diverse environmental conditions.