Whey protein aggregates affect cast film appearance and mechanical properties – Part I: Network formation of straight fibrils and non-amyloid aggregates

The film forming properties of pre-treated whey protein isolate (WPI) were investigated. In part I of this study, WPI was structured into amyloid aggregates via acidic pH-dependent heat treatment. Isolated amyloid fractions (straight fibrils) and non-amyloid fractions containing small aggregates and peptides, or mixtures of both fractions were used for film formation. The films' appearance and mechanical properties were influenced by cohesive and adhesive interactions of mesoscopic materials, affecting delamination, stress control, and peelability. Analysis of surface structure and crack patterns using top-view photographs, scanning electron microscopy, and atomic force microscopy images indicated the interplay between adhesion and cohesion forces, which was supported by measurements of hardness, and elasticity through nanoindentation. Films consisting of straight, long fibrils and non-amyloid aggregates provided the highest mechanical strength and cohesive properties. This was due to the scaffold-forming fibrillar network, in which adhesive non-amyloid aggregates were embedded. Fibrillization led to a more uniform film profile at lower protein concentrations by decreasing vertical and horizontal convection flows. The alignment of aggregates during the drying process demonstrated to play a regulatory role in stress release. As a result, aggregate morphology has been observed to directly affect the nature of the resultant cracks, with straight fibrils resulting in straight and linear cracks. Insights obtained from studying pure amyloid/non-amyloid film networks can optimize WPI film formulations for improved edible film applications. This includes reducing crack formation, enhancing visual appearance, and improving peelability.