Toughening Starch-Based Bioplastics with Soy Amyloid Fibrils Produced from Tofu Wastewater

The present study explores the potential of soy amyloid fibrils produced from tofu wastewater as sustainable reinforcements for starch-based edible bioplastics. The soy amyloid fibrils, exhibiting a persistence length of 1.2 μm and β-sheet structures, were incorporated into an aqueous solution containing starch, pectin, and plasticizers (glycerol/sorbitol = 2:1), which was subsequently cast into films. The soy amyloid fibrils could interact with plasticized starch and pectin chains, yielding a difference in glass transition temperature per percentage of plasticizer content of 1.7 °C and forming microvoids, which were found to be beneficial for toughening the films. Under tensile stress, soy amyloid fibril-reinforced starch–pectin films featured a 3-fold enhancement in their ductile toughness relative to the control films, with the elongation at break increasing from 11.7 to 34.5% and the toughness increasing from ≈411 to 1141 MJ/m3, respectively. These films also demonstrated oxygen and water barrier performances comparable to nonbiodegradable films based on polyamide-6. In addition, the blends introduced in this work could be processed both into films by casting and into filaments via extrusion. Importantly, a basic cost analysis of the soy amyloid fibril-reinforced starch–pectin composites revealed that they fall within the same cost range as other bioplastics, providing a versatile solution to expanding bioplastic processing using tofu wastewater as the starting feedstock.