The Effect Of PH On Proteins

How pH affects the stability of proteins

It is essential to understand the stability of proteins during different processing conditions because this affects the safety and quality of food products. Changes in pH can significantly affect the stability of proteins, especially for high-sugar or low-acid foods. It is because the pH controls how many hydrogen ions are present in the solution, determining how much interaction will occur between different amino acids in the protein. For proteins to form correctly, amino acids must interact in specific ways. Thus, changes in pH can cause these interactions to form incorrectly and break apart, leading to denatured proteins that are no longer functional.

Consequently, an amino acid must have a primary amino group, an acidic carboxyl group, and one hydrogen atom. It suggests that amino acids, depending on their structure, may display both acids and bases (amphoteric). A slight rise in pH causes amino acids to protonate and deprotonate, resulting in structural changes.

Figure 1:effects of PH on amino acids

Casein proteins are inactivated by heat, while those with a pH less than 4.6 are denatured. When exposed to acid, phosphate groups form bonds with calcium ions and casein chains, resulting in milk curdling. When an acid is injected into casein proteins, the inter-ionic connections between the casein phosphoryl groups and calcium ions are dissolved (Li et al., 2020). Additionally, caseins serve as a substrate for the protease rennet, which is added to milk early in creating fresh cheese.

Water-soluble whey proteins are available in various forms and are naturally sulfur-dense. At elevated temperatures, these proteins denature, resulting in milk skin. They remain stable in the whey solution due to their lower acid sensitivity than caseins. Other proteases, such as those produced by yogurt-making bacteria, may eventually be able to target these proteins.

The pH of soy protein ranges from 3.9 to 9, which means that soy proteins can be acidic or alkaline. The stability of amino acids depends on the pH of the solution—most amino acids are stable at a neutral pH (pH 7) but become unstable as the pH changes (Jaramillo et al., 2011). The stability of an amino acid also affects its ability to bond with other amino acids to form longer chains or peptides.

Protein denaturation can be applied to any change in structure resulting from disruption to its native conformation. Heating a protein can cause it to denature, as can chemical treatments such as acid or alkali. Denatured proteins should not be confused with folded proteins in shapes other than their native shape but are still helpful for some functions. Denatured proteins lose part and sometimes cease to be identifiable as a particular protein. Consequently, during the industrial processing of soy products, the heat treatments of the products at alkali levels destroy the amino acids. Thus, at  PH, more than 9.0 arginines are converted to urea, which is toxic to human foods.

An essential component in the solubility of egg white proteins is the PH of the solution. As the PH increases from 4.6 to 9.0, there is a corresponding increase in protein solubility; at a PH below 4.6, the proteins are mainly insoluble (Santos et al., 2014). Consequently, egg white proteins are readily available and contain highly nutritious eggs. The albumins of the egg proteins have an isoelectric point (PH) of 4.6 which governs its solubility at different PH values. This is exploited in industries; the PH is applied to the egg white to alter and control the extraction, for example, lysozyme and avidin.

References

Jaramillo, D. P., Roberts, R. F., & Coupland, J. N. (2011). Effect of pH on the properties of soy protein–pectin complexes. Food Research International, 44(4), 911-916. https://doi.org/10.1016/j.foodres.2011.01.057

Li, X., Cheng, M., Li, J., Zhao, X., Qin, Y., Chen, D., Wang, J., & Wang, C. (2020). Change in goat milk protein's structural and functional properties due to pH and heat. Journal of Dairy Science, 103(2), 1337-1351. https://doi.org/10.3168/jds.2019-16862

Santos, D., Coimbra, J., Teixeira, C., Barreto, S., Silva, M., & Giraldo-Zuniga, A. (2014). Solubility of proteins from quail (Coturnix coturnix japonica) egg white is affected by agitation time, pH, and salt concentration. International Journal of Food Properties, 18(2), 250-258. https://doi.org/10.1080/10942912.2012.654557