Towards gliadin nanofoams

Bio-derived, fully biodegradable closed-cell micro- and nanocellular foams were generated from gliadin. Gliadin, an abundantly available wheat storage protein, was extracted with aqueous ethanol from wheat gluten. For the newly developed foaming process, dry gliadin powder was plasticized by equal volumes of water and ethanol resulting in a viscoelastic paste that was then subjected to supercritical carbon dioxide in a high-pressure cell. After tempering, a sudden release of pressure and cooling resulted in closed-cell micro- or nanocellular foams. Depending on the processing parameters such as the foaming temperature, pressure, pressure drop rate, plasticizer concentration, and pre- and past-expansion treatment, foam morphology was tunable. Accordingly, cell size and porosity can be varied as desired. The resulting foams and intermediate states were analyzed by scanning electron microscopy, and a foaming mechanism is suggested. The foams prepared via the proposed procedure exhibit the smallest cell sizes and the highest porosity ever reported for foaming of wheat proteins. For example, we found mean cell size of 6.5 ± 1.7 μm at a porosity of ϕ = 0.97, 1.4 ± 0.5 μm at ϕ = 0.90, 720 ± 340 nm at ϕ = 0.77, and 270 ± 110 nm at ϕ = 0.62. The procedure presented is clean, environment friendly, simple, and low cost, possibly lending itself to a technical realization. The promising new approach may have potential in short purpose packaging or insulation at almost zero environmental cost, because the materials are easily and environmentally friendly recyclable and even edible.