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Formation mechanisms, handling and digestibility of food protein nanofibrils

https://doi.org/10.1016/j.tifs.2015.05.005Get rights and content

Background

Globular proteins including whey proteins, soy proteins and egg white proteins self-assemble into fibrillar structures with several nanometers thickness and several micrometers length by prolonged heating at very acidic conditions. These in vitro synthesized fibrils resemble amyloids associated with various neurodegenerative diseases and hence have received special attention by bioscientists. The synthesized fibrils are however of widespread potential for food and colloid sciences applications. Characteristics such as aspect ratio, and persistence and contour lengths of the artificially-synthesized fibrils could be elaborated by appropriate selection and employment of parameters of fibrils formation process.

Scope and approach

The fibrillation process of whey proteins, proposed mechanisms and potent inhibitors of the process, as well as, alternative enzyme-based routes of fibrillation are overviewed in the present article. Then post-formation treatment and applications of globular proteins fibrils and their gastric digestibility behavior are briefly referred followed by representing some future trends in this field.

Key findings and conclusions

The building units of protein fibrils are hydrolysis-generated polypeptides rather than parent intact protein monomers. It was hypothesized that proteins fibrillation is an oxidation-triggered process and may be inhibited by antioxidant agents that suppress the generation of reactive oxygen species. Whey protein-originated fibrils may be exploited in formation of heat-resistant protein-stabilized emulsions and nanoemulsions. However, more effort is required to characterize the interfacial behavior of fibrils in comparison with native and heat-denatured whey proteins. Gastric and intestinal digestion fate of protein nanofibrils is also a hot topic for upcoming research studies.

Introduction

Heat denaturation of proteins can form a wide variety of structures such as fibrils, flexible strands, branched structures and random aggregate (Bolder, Hendrickx, Sagis, & van der Linden, 2006) depending on the pH value, salt type and concentration, heating conditions and protein concentration (Nicolai, Britten, & Schmitt, 2011), among which, the greatest impact belongs to heat (Pearce, Mackintosh, & Gerrard, 2007). The pH value of protein solution is also of profound influence on the organizational arrangement of super-structures. The architecture and life span of these structures are determined by balance between attractive hydrophobic and repulsive electrostatic interactions. At low ionic strengths and at pH values far from the iso-electric point (pI), electrostatic repulsive forces overcome the attractive interactions resulting in formation of fibrillar structures with high aspect ratio (length versus diameter); whilst, at pH values close to pI, spherical microgels (with sub-micron diameters) and random aggregates are formed (Fig. 1) (Bolder, 2007, van der Linden, 2006, van der Linden and Venema, 2007). The stimulant-induced self-assembly of globular food proteins to fibrillar structures has received considerable attention owing to the ability of the fabricated structures to display novel profitable characteristics (van der Linden, 2006). In the assembly process, protein monomers are organized into giant supramolecules in which non-covalent interactions such as hydrogen bonds, and electrostatic, hydrophobic and van der Waals interactions hold the building units together (Bolder, 2007). Fibrils formation may involve covalent linkages (disulfide bridges) in addition to non-covalent interactions depending on pH. Heat treatment at pH 3.35 assembled fibrils not only by hydrogen bonds and hydrophobic interactions but also by limited number of disulfide bridges; whilst, by heating at pH 2.0 merely non-covalent hydrophobic interactions contributed in fibrils formation (Mudgal, Daubert, Clare, & Foegeding, 2010).

Section snippets

The essential characteristics of fibrils

Several types of food proteins such as milk proteins, soy proteins and egg white proteins can assemble in vitro at appropriate condition into fibrillar structures with micrometer length and nanometer thickness (Bolder, 2007, van der Linden, 2006). These objects resemble amyloid fibrils that are associated with neurodegenerative diseases like Alzheimer's, Parkinson's, Huntington's and Creutzfeldt–Jakob diseases. Accordingly they are often called “amyloid-like” (Jones & Mezzenga, 2012). The

Process overview

Beta-lactoglobulin is a globular whey protein with molecular mass of 18400 Da and radius of ∼2 nm (Bolder, Vasbinder, Sagis, & van der Linden, 2007). It exists in three genetic variants A, B and C which differ from each other at the subsititutions in their amino acid sequences. The variants A and B contain Gln at residue 59 while variant C contains His. As well, variant A contains respectively Asp and Val at residues 64 and 118 versus Gly and Ala in variants B and C (Dave, Loveday, Anema,

Post-formation processing and application of nanofibrils

Whey protein nanofibrils may undergo various post-formation processes including storage, high pressure treatment, sonication, drying and freezing. In the study of Bolder, Vasbinder, et al. (2007) post-formation behavior of protein fibrils as function of storage pH was investigated. The viscosity of the fibril sample stored overnight at pH 7.0 increased significantly and the sample stored overnight at pH 10.0 gelled. No remarkable change was however observed for the fibril solution stored at pH

Fibrils digestibility

Bateman, Ye, and Singh (2010) were the first investigators who carried out some work on protein fibrils digestibility. Incubation of β-lactoglobulin-originated fibrils by pepsin within a simulated gastric fluid caused complete digestion of the fibrils very quickly within 2 min. In addition to disintegration of the fibrils, peptides (2000–8000 Da) that build up the fibrils were digested to smaller peptides (<2000 Da). The high gastric digestibility of the fibrils-forming peptides was ascribed to

Concluding remarks and future trends

Whey proteins fibrils are artificially-synthesized analogues to neurodegenerative disease-related fibrils. Various attributes including the aspect ratio and length distribution indices impart the functionality of fibrils providing possibility to generate fibrils with characteristics of interest. The irreversibly formed fibrils have been exploited in preparation of cold-set hydrogels at considerably low concentrations. It has been extensively emphasized that the building units of the fibrils are

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