The aggregation of proteins is of fundamental relevance in a number of daily phenomena, as important and diverse as blood coagulation, medical diseases, or cooking an egg in the kitchen. Colloidal food systems, in particular, are examples that have great significance for protein aggregation, not only for their importance and implications, which touches on everyday life, but also because they allow the limits of the colloidal science analogy to be tested in a much broader window of conditions, such as pH, ionic strength, concentration and temperature. Thus, studying the aggregation and self-assembly of proteins in foods challenges our understanding of these complex systems from both the molecular and statistical physics perspectives. Last but not least, food offers a unique playground to study the aggregation of proteins in three, two and one dimensions, that is to say, in the bulk, at air/water and oil/water interfaces and in protein fibrillation phenomena. In this review we will tackle this very ambitious task in order to discuss the current understanding of protein aggregation in the framework of foods, which is possibly one of the broadest contexts, yet is of tremendous daily relevance.

http://iopscience.iop.org/article/10.1088/0034-4885/76/4/046601/pdf

The self-assembly, aggregation and phase transitions of food protein systems in one, two and three dimensions

Intramolecular folding events, triggered by the presence of salt, induce the self-assembly of β-hairpin peptides into hydrogel networks at physiological conditions. At pH 7.4 and low ionic strength solution conditions, dilute, homogeneous solutions of peptide (≤2 wt %) exhibit the viscosity of pure water. Circular dichroism spectroscopy shows that, at pH 7.4 in the absence of salt, peptides are unfolded. By raising the ionic strength of the solution, electrostatic interactions between charged amino acids within the peptide are screened, and a β-hairpin conformation is adopted. Folded β-hairpin molecules supramolecularly assemble via hydrophobic collapse and hydrogen bonding into a three-dimensional hydrogel network. FTIR and X-ray scattering data demonstrate that these hydrogels are rich in β-sheet. Dynamic oscillatory rheological measurements demonstrate that the resultant supramolecular structure forms an elastic material whose structure, and thus modulus, can be tuned by salt concentration and temperat...

/pdf/salt-triggered-peptide-folding-and-consequent-self-assembly-3878ecnohe.pdf

Salt-Triggered Peptide Folding and Consequent Self-Assembly into Hydrogels with Tunable Modulus

Whey protein ingredients are used for a variety of functional applications in the food industry. Each application requires one or several functional properties such as gelation, thermal stability, foam formation or emulsification. Whey protein ingredients can be designed for enhanced functional properties by altering the protein and non-protein composition, and/or modifying the proteins. Modifications of whey proteins based on enzymatic hydrolysis or heat-induced polymerization have a broad potential for designing functionality for specific applications. The effects of these modifications are demonstrated by discussing how they alter gelation and interfacial properties.

Advances in modifying and understanding whey protein functionality

On the basis of suggestive X-ray data, 14 aroyl l-cystine derivatives were designed, synthesized, and examined for their ability to gelate water. Several members of this amino acid family are remarkably effective aqueous gelators (the best being one that can rigidify aqueous solutions at 0.25 mM, ca. 0.01%, in less than 30 s!). A few of the analogues separate from water as crystals, indicating a close relationship between gelation and crystallization. All effective gelators self-assemble into fibrous structures that entrain the solvent in the capillary spaces among them. Hydrogen-bonding sites on the compounds that might stabilize the fibers were identified from specific substitutions that replace a hydrogen donor with a methyl group, enhance the hydrogen-accepting ability of a carbonyl oxygen, or promote the hydrogen-donating ability of an amide proton. The structural variations were characterized via minimal gelation concentrations and times, X-ray crystallography, light and electron microscopy, rheolog...

Anatomy of a Gel. Amino Acid Derivatives That Rigidify Water at Submillimolar Concentrations

Globular protein gelation - theory and experiment

The effects of ionic strength on fractal structures in heat-induced gels prepared from globular proteins were investigated in the framework of a fractal aggregation of colloidal particles. All gels formed at 90 °C exhibited power law relationships between the storage shear modulus (G‘) and protein concentration. At 25 mM NaCl, the fractal dimension, d (∼2.2), calculated based on the value of the power law exponent agreed with those for reaction-limited cluster−cluster aggregation. Further addition of NaCl (50, 80, 500, 1000 mM) decreased the values of d (∼1.8), which agreed with d for diffusion-limited cluster−cluster aggregation. These results suggest that the predominant effect of an increase in ionic strength on globular protein gelation is ascribed to shielding charges on the surface of the proteins, thereby increasing the reaction probability of protein aggregation. The effective structure-determining rheological properties of heat-induced protein gels are characterized by fractal dimensions deduced ...

Shinya Ikeda

Papers

Rheological Study on the Fractal Nature of the Protein Gel Structure