Amaranth protein films reinforced with starch nanocrystals

M. CECILIA CONDES; ADRIANA N. MAURI; ALAIN DUFRESNE

Washington

Conferencia; 2011 TAPPI International Conference on Nanotechnology for Renewable Materials. Washington, D.C., USA; 2011

The aim of this work was to enhance the physicochemical properties of amaranth protein (Amaranthus hypochondriacus) films through their reinforcement with waxy maize and wheat starch nanocrystals. Since the amylose- amylopectine ratio and the granules’ size depends on the starch origin, the addition of different starches nanocrystals to protein formulations would lead to films with distinctive characteristics. Amaranth protein isolates (API, used as the source of protein) were prepared by protein solubilization at pH 11, isoelectric precipitation, suspension at pH 8, and lyophilization. Nanocrystas were obtained by controlled hydrolysis of native waxy maize or wheat starch. The starch granules were mixed with 3.16 M H2SO4 during 5 days at 40ºC, with continuous stirring at 100 rpm. The suspension was successive washed with distilled water and centrifuged until it was neutral and then it was stored at 4ºC as far as being used. Protein films (used as control) and nanocomposites films were prepared by casting from aqueous dispersions containing API (5% w/v), glycerol (1.25% w/v, as plasticizer) and different amounts of starch nanocrystals (0, 3, 6, 9 and 12% w / w, respect to the protein content). Film forming dispersions were cast in Teflon Petri vessels and dried at 40ºC for 4h in an oven with air flow and circulation. The dry films were conditioned 48 h at 20ºC and 58% relative humidity previous being tested. The resultant films were characterized according to their moisture content, thickness, mechanical properties, water vapor permeability (WVP), water uptake (WU), colour and microstructure. All the studied films, regardless of starch source and nanocrystales content, were homogeneous and showed similar moisture content (@ 20% w/w) and thickness (@ 70 mm) in agreement with film processing conditions. Moreover, the addition of both types of starch crystals did not affect the appearance (determined by color and opacity) of nanocomposite films with respect to the control. On the other side, nanocomposites showed improved mechanical properties. When increasing the nanocrystals content, the tensile strength and Young's modulus dramatically increased although the elongation at break gradually decreased. Films reinforced with 12% of nanocrystals improved the tensile strength by 140 and 210% for waxy maize and wheat starch respectively and Young’s modulus by 450% for both types of crystals, compared with control. Improvements in WVP measurements were also observed for some nanocomposite films. Films containing up to 9 % of waxy maize nanocrystals, or wheat nanostarch contents exceeding 9% had lower WVP than protein films. The addition of nanocrystals also enhanced the water resistance of films. The WU of all nanocomposite films decreased about 20% with respect to control, excepting for the corresponding to the film reinforced with 3% waxy maize starch crystals that showed a reduction of 50%. Micrographs (obtained by SEM) of films reinforced with waxy maize crystals displayed a fractured lamella structure which was more evident when increasing the crystals content up to 12%, and a rough surface for this filler content. In contrast, films reinforced with wheat nanocrystals showed different microstructures depending on the crystals content. The surface of films with 3% of crystals was rougher than control , the ones corresponding to films reinforced with 6 and 9% of crystals presented a fractured lamella structure, and finally the corresponding to film with 12 % of crystals became smoother and similar to the control. Different structures and improvements in mechanical and barrier properties were shown for amaranth protein films reinforced with starch nanoparticles extracted from two different starch sources (waxy maize and wheat).