How High Pressure Processing Inactivates Microorganisms and Enzymes in Food Our expertise

• Alters cell membrane structure. The high pressure disrupts the lipid bilayer and membrane proteins, causing a leakage of intercellular contents. This permeability damage is a major mechanism of inactivation (Rastogi et al., 2007)
• Inhibits enzyme activity. Pressure above 100 MPa can denature proteins like enzymes through breaking hydrophobic bonds and hydrogen bonds (Patterson, 2005). The critical microbial enzymes for metabolism become inactivated
• Damages ribosomes. Ribosomes, the site of cellular protein synthesis, are also disrupted by HPP (Smelt, 1998)
• Induces DNA damage. DNA strand breaks can be caused by HPP through the depletion of adenosine triphosphate (ATP)’s need for repair and the generation of reactive oxygen species (Manas & Pagan, 2005).
• Impacts cell walls. In gram-positive bacteria, peptidoglycan layers in cell walls are weakened by HPP, causing cell lysis. Spores are inactivated by the effects on gram cell wall peptidoglycan (Black et al., 2007)
• Denatures proteins. Beyond enzymes, HPP denatures a wide range of proteins including cytokines, membrane transports, and those involved in transcription and translation (Rastogi et al., 2007)

Pectin Methylesterase (PME) in citrus fruits causes cloudiness in orange juice. To keep the juice clear, this enzyme has to be deactivated or slowed down right after extraction. If not, it alters pectin, a key component for juice cloudiness. This can lead to juice separation or, in concentrated forms, a gel that can't turn back into juice.

Citrus fruits have different types of PME. For example, oranges have a high molecular weight PME that is active at low pH and low temperatures, and it can withstand high heat. However, using heat to deactivate PME can affect the juice's taste.

An alternative method called Manothermosonication (MTS) uses heat, ultrasound, and high pressure to preserve food. This method works well for orange juice because the usual microbes and heat-resistant PME in the juice are both sensitive to ultrasound under pressure at relatively low temperatures. By using MTS at 35°C, 200 kPa pressure, and about 110 μm amplitude, PME can be deactivated as effectively as heating to 80°C.

HPP utilizes elevated pressure of 100–800 MPa to inactivate microorganisms and enzymes through alterations in cell structures and enzymatic systems, providing an alternative to thermal processing. HPP has been effectively applied to preserve quality and safety in products like fruit juice, meats, seafood, and dairy products. Although equipment costs are high, HPP offers major advantages for retaining nutrition and freshness in food.