Heat exposure affects crown structure of plant proteins in dietary NPs

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Adsorption of amphiphilic proteins on the surface of titanium dioxide nanoparticles (TiO2 NPs) cause protein crowns that alter their gastrointestinal fate. However, the factors influencing the formation of the protein crown remain unclear. In an article recently published in the Journal of Agricultural and Food Chemistrythe authors explored the influence of temperature on four plant proteins.

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​​​​​​​Study: Impact of heat treatment on the structure and properties of the plant protein crown formed around TiO2 nanoparticles. Image Credit: Vlad Teodor/Shutterstock.com

Application of TiO2 NP in the food manufacturing process

Inorganic nanomaterials are widely used in materials science, pharmaceutical development, chemical engineering, and cosmetic science. A fraction of NP added to TiO powder2 (E171) is used as a food coloring or lightening agent.

The high refractive index and the dimensions of these NPs cause intense light scattering, thus altering the optical properties of the food material. Since inorganic NPs like TiO2 NPs on food products enter the human body, it is essential to understand the behavior and properties of inorganic NPs.

NPs in contact with food materials such as proteins form a coating around them called the protein crown. The presence of corona proteins can impact the human gastrointestinal tract after ingestion. Previous studies have revealed that few protein crowns alter the physicochemical and biological properties of proteins and reduce their bioactivity and bioavailability.

Food manufacturing processes like thermal operations can alter the structure and functionality of proteins. For example, a high temperature increase breaks the disulfide bond in rice gluten, and heat treatments of albumin alter the functional properties of the protein.

Effect of temperature on proteins

In the present study, the authors analyzed the interaction between TiO2 NPs and four proteins (soy protein isolate, glutenin, zein and gliadin). They first analyzed the effect of temperature on the structural properties of proteins.

Later, quartz crystal microbalance with dissipation (QCM-D) was used to understand changes in protein structure and its binding affinity to TiO2 on heat treatment. After heat-treating the four plant proteins at different temperatures, the authors characterized their structural properties using ultraviolet-visible (UV-Vis) spectroscopy, fluorescence spectroscopy, zeta potential, dynamic light scattering (DLS), and dynamic light scattering (DLS). transmission electron microscopy (TEM).

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UV-Vis absorption spectra revealed peaks at 220 nanometers and around 260-280 nanometers due to the backbone structure of the protein peptide bond and aromatic amino acids.

The authors observed an increase in the height of soy protein and glutenin peaks after they were heat treated at 100 degrees Celsius. However, zein showed an initial upward trend and later a downward trend when heated to a higher temperature. Additionally, the authors observed a slight blue shift for the peak at 260 nanometers indicating a change in polarity in the aromatic groups. Gliadin showed an uncertain trend in absorption spectra. The above results indicate protein aggregation and unfolding during heat treatment.

Aromatic amino acids such as tryptophan, phenylalanine and tyrosine lead to protein fluorescence. During the conformational changes, there was a change in the fluorescence spectra due to an altered molecular environment around the aromatic rings.

Heat treatment on gliadin showed no significant effect on fluorescence spectra, suggesting that the protein is heat resistant. For the zein protein, fluorescence intensity increased with temperature, indicating dissociation and unfolding of the protein.

The change in magnitude of zeta potential values ​​for proteins indicates changes in the number or type of charged groups on the surface of the protein upon heating. The authors hypothesized that the heating could have changed the pH of the solvent, thereby altering the ionization of the surface groups.

DLS studies indicate aggregation of all four proteins upon heat treatment, indicated by increased particle size. The authors observed a shift in the particle size distribution to the left, suggesting protein dissociation due to heating. This information revealed the heat resistance of glutenin protein. In the other three proteins, aggregation and dissociation were observed. DLS results indicate that protein type is critical in determining its fate towards heat treatment.

TEM images for glutenin revealed a smooth sphere which showed reduced size after heating. The large irregular clusters seen before heating in soy protein transformed into smaller irregular fragments upon heat treatment. Before heating, gliadin proteins were small spherical particles, and they turned into large irregular clumps after heat treatment. Similarly, large smooth spheres of zein protein particles observed prior to heating transformed into spherical clusters upon heat treatment.

Conclusion

In conclusion, the authors investigated the impact of temperature and protein type on protein structural properties and their effects on protein corona formation using inorganic NPs.

QCM-D monitoring revealed that temperature affects protein corona formation in total protein adsorption mass and protein fractions in hard and soft layers. Temperature also impacted protein behavior such as aggregation, unfolding and dissociation. This study makes it possible to demonstrate the interaction between the ingredients of the food matrix and inorganic NPs.

Reference

Jiang B, Zhao Q, Shan H, Guo Y, Xu X, Mcclements DJ (2022). Impact of heat treatment on the structure and properties of the plant protein crown formed around TiO2. Nanoparticles https://pubs.acs.org/doi/10.1021/acs.jafc.2c01650

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