Hydrothermal Treatments Cause Wheat Gluten-Derived Peptides to Form Amyloid-like Fibrils

Formation of amyloid fibrils (i.e., protein structures containing a compact core of ordered β-sheet structures) from food proteins can improve their techno-functional properties. Wheat gluten is the most consumed cereal protein by humans and extensively present in food and feed systems. Hydrolysis of wheat gluten increases the solubility of its proteins and brings new opportunities for value creation. In this study, the formation of amyloid-like fibrils (ALFs) from wheat gluten peptides (WGPs) under food relevant processing conditions was investigated. Different hydrothermal treatments were tested to maximize the formation of straight ALFs from WGPs. Thioflavin T (ThT) fluorescence measurements and transmission electron microscopy (TEM) were performed to study the extent of fibrillation and the morphology of the fibrils, respectively. First, the formation of fibrils by heating solutions of tryptic WGPs [degrees of hydrolysis 2.0% (DH 2) or 6.0% (DH 6)] was optimized using a response surface design. WGP solutions were incubated at different pH values, times, and temperatures. DH 6 WGPs had a higher propensity for fibrillation than did DH 2 WGPs. Heating DH 6 WGPs at 2.0% (w/v) for 38 h at 85 °C and pH 7.0 resulted in optimal fibrillation. Second, trypsin, chymotrypsin, thermolysin, papain, and proteinase K were used to produce different DH 6 WGPs. After enzyme inactivation and subsequent heating at optimal fibrillation conditions, chymotrypsin and proteinase K DH 6 WGPs produced small worm-like fibrils, whereas fibrils prepared from trypsin DH 6 WGPs were long and straight. The surface hydrophobicity of the peptides was key for fibrillation. Third, peptides from the wheat gluten components gliadin and glutenin fractions formed smaller and worm-like fibrils than did WGPs. Thus, the peptides of both gluten protein fractions jointly contribute to gluten fibrillation.