DEFINITION OF HIGH-PRESSURE PROCESSING (HPP):
High-Pressure Processing or (HPP) is the cold treatment technique in which very high pressures (100MPa to 1000MPa) are applied to food material to induce various changes in it by disrupting the non-covalent interactions.
MECHANISM AND PRINCIPLE
According to the isostatic principle, the pressure is applied uniformly in all directions and due to this, high pressure does not damage the food at macroscopic levels. The changes brought about by HPP are a result of volume change, which is governed by le Chatelier’s principal. According to this principle, high pressure will favour any change in the food matrix which is accompanied by volume change. The reaction rate constant of a protein solution under high pressure is governed by the state transition theory, which works by lowering down the free energy of the reaction.
When high pressure is applied to the protein matrix, it will bring some changes in the matrix. This is due to the volume of protein solution that consists of the volume of its atoms and the volume of the cavities, which makes the whole structure. Hence, any suppression in volume causes compression in the cavities. High pressure also leads to rupturing and formation of non-covalent interactions (intermolecular hydrophobic and electrostatic interactions). High pressure affects hydrogen bonds only at the higher pressure values (above 1000MPa), which is not usually operated during HPP treatments. It does not affect covalent bonding, as a result of which primary structure of protein usually remains unaffected by high-pressure treatment, although, it would affect secondary, tertiary and quaternary structures significantly. The secondary structure is usually affected at pressure values above 400MPa, wherein, non-reversible denaturation takes place. Within the secondary structure of proteins, beta-sheet regions are less sensitive to pressure, as they are less sensitive to deformation compared to the alpha helix. With the resulting conformational changes and exposure of hydrophobic groups to the surface, due to the unfolding of protein structure, surface hydrophobicity usually increases. Sulphydryl groups and disulphide bonds, which are responsible for the weak secondary covalent bonds and which help maintain the protein’s tertiary structure are reported to increase up to a certain pressure (300MPa). After a certain pressure – SH groups exposed at the surface starts decreasing lightly, due to further aggregation of proteins after denaturation/unfolding.
The above factors bring out certain changes in the Protein Functionality and they are as follows:
Solubility
Solubility is the ability to dissolve in a solvent (usually water). Generally, High-Pressure Treatment lowers the solubility, but in rare cases, depending on the type of protein, it can also increase solubility at lower pressures, which is most likely due to enhanced interactions between protein and water due to low pressures. But at higher pressure values (above 400 MPa), there is a formation of insoluble macro aggregates, due to further increase in intermolecular interaction of hydrophobic residues that are exposed on the surface. The formation of these aggregates further decreases the solubility of the proteins.
Water Holding Capacity
It is the amount of water that a protein can hold after dissolving and centrifugation at some constant values. High pressure increases the water holding capacity exposure of polar amino acids induced by unfolding and thus creating a more polar environment. But after certain pressure (above 600 MPa), there is a decline in the water holding capacity. This likely takes place, due to more extensive denaturation and high surface hydrophobicity.
Emulsifying Activity
Emulsifying activity is the extent to which proteins are involved in emulsion formation. It can be measured in terms of emulsifying activity and emulsion stability. Emulsion stability is the measure of “how long the emulsion is sustained after standing for some time.” High-Pressure Treatment improves the emulsifying activity of the proteins but not the emulsion stability. It is because of an increase in hydrophobic interactions and molecular flexibility.
Foaming Activity
Foaming activity is the ability to retain the air cells in the protein matrix. The adsorption of proteins at gas-liquid interfaces improves, due to a decrease in interfacial tension. This may be due to the increase in surface hydrophobicity and partial unfolding of proteins as a result of high pressure.
Gelation
High Pressure enables aggregation and formation of three-dimensional protein network, due to unfolding of the proteins and dissociation of polymeric structures and partial denaturation. This may be due to hydrophobic interactions, hydrogen bonds and electrostatic effects that are altered by high-pressure treatments.
Conclusion
As per the recent scenario of increasing awareness about health and food among the people, the demand for healthy and high-quality food is increasing. At the same time, animal proteins are shown to be associated with different diseases and hence the demand for plant proteins is continuously increasing. But the major challenge in the application of plant proteins is their lower degree of functionality, which limits their application in various food products. Further, the functionality of proteins majorly depends on the source of plant proteins and the subsequent extraction processes. Plant proteins can be modified either by chemical or physical methods. The disadvantage of chemical methods is that they may leave the chemical residues in the final product which is not desirable. Similarly, the heat treatment may also degrade the structure of proteins which is again not desirable, Hence, the trend is going towards using some novel or green technologies which are appropriate to provide high-quality food. Nowadays, consumers are even willing to pay a higher price for high-quality food products. In this respect, HPP treatment may play an important role in providing high-quality food products by least affecting or even modifying the properties of food items alike. The conventional treatment by which heat is generally applied to food may degrade its quality. In conclusion, HPP not only improves quality, but also improves the shelf life and safety of food products. Hence, it is a technique that might have a greater scope in the food industry, while at the same time being environment-friendly.
References:
• Queirós, R. P., Saraiva, J. A., & da Silva, J. A. L. (2018). Tailoring structure and technological properties of plant proteins using high hydrostatic pressure. Critical reviews in food science and nutrition, 58(9), 1538-1556.
• Chao, D., Jung, S., & Aluko, R. E. (2018). Physicochemical and functional properties of high pressure-treated isolated pea protein. Innovative Food Science & Emerging Technologies, 45, 179-185.
• Sim, S. Y., Hua, X. Y., & Henry, C. J. (2020). A Novel Approach to Structure Plant-Based Yogurts Using High Pressure Processing. Foods, 9(8), 1126.
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