Pro Tip: Electrostatic Interactions Help Understand Protein Functionality | 2022-02-02

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Pro Tip: By understanding the electrostatic interactions in plant proteins, product developers can ensure emulsion stability and gel formation.

Electrostatic interactions in proteins are responsible for emulsion stability and protein-starch interactions, and they influence the formation of gels, but what drives these interactions? Proteins are made up of 20 unique amino acids that combine in virtually unlimited ways to form the amino acid sequence, also known as the primary protein structure. Primary structure informs the physical chemistry of a protein, including hydrophobic interactions, disulfide bonds, hydrogen bonds, and electrostatic interactions.

Electrostatic interactions are dictated by the ratio of positively charged amino acids (lysine and arginine) to negatively charged amino acids (glutamic acid and aspartic acid). The positions of these positive and negative amino acids in protein sequences establish an electrostatic field, and the nature of this field is partially responsible for protein functionality such as solubility.

Like magnets, positive regions repel each other and are attracted to negative regions, and the reverse is also true. Electrostatic forces on a protein are most often measured using a machine to obtain the zeta potential, usually measured by dispersing the protein in water. These machines apply a positive or negative charge to one end of a plastic tube and measure the speed at which the particles are moving towards that side. This quantifies the strength of the load on the protein.

When the pH of the solution is changed, the zeta potential becomes more or less positively charged above and below the isoelectric point of the protein, or where the zeta potential is 0. This is an important point for product developers. For example, vegetable proteins such as lentil and pea are used as emulsifiers because the proteins attach to the surface of oil droplets. For the emulsion to remain stable and prevent the oil droplets from coming together, the zeta potential must be highly negative or positive, which can be accomplished by changing the pH of the emulsion. When the charges on the surface of a protein are the same, the proteins attached to the oil droplets will tend to repel each other, adding to the stability of the emulsion.

In baking, these repulsive forces are part of the changes observed during proofing. As the pH decreases during fermentation, the disulfide-linked glutenin proteins become more negative and repel each other, slightly weakening the gluten matrix. This is part of the reason why dough becomes more prone to crumbling during fermentation and requires emulsifiers to help stabilize rising dough!

Harrison Helmick is a PhD candidate at Purdue University. Login to LinkedIn and check out her other baking tips at BakeSci.com.

His research is carried out with the support of Jozef Kokini, Andrea Liceagaand Arun Bhunia.

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