Welcome to the first series of Back to school with HPP by Hiperbaric. In these blog series we show all the basics about High Pressure Processing technology, the science, physics and how the microbial cells and spores are affected.

High pressure

High pressure processing (HPP) units uses cold water (4-25 °C; 40-77 °F) to generate high hydrostatic pressure. The etymology of hydrostatic comes from the Greek “hydro” (water) and “static” (stable), indicating that pressure is transmitted equally throughout the HPP unit. Water is the main component of most foods, therefore pressure is also exerted uniformly in the product regardless of its geometry. This feature significantly simplifies operation at an industry level, since the same unit can process basically any type of food product like juices, purees, meats, seafood, dairy, etc.

Hydrostatic pressure, indicating pressure is transmitted equally.

Much has been published about the food safety, health benefits and premium quality of HPP foods in the news and science journals. Technical experts throughout the world have covered the basic principles of HPP, but in this publication Hiperbaric shares its knowledge to a more general audience to generate more consumer awareness.

As the worldwide leader manufacturer of HPP units, Hiperbaric presents a series of basic principles for a general audience, starting with the basics behind microbial inactivation.

The microbial cell

The cell is the basic structure of every living organism, from complex multi-cellular organisms like the human body, to single cell entities like spoilage microorganisms and foodborne pathogens. Inside the cell, organelles are the machinery that regulate the metabolism and other physiological functions that keep microorganisms alive.

The plasma membrane plays a key function in cells by protecting organelles and genetic material from the surroundings. The plasma membrane also regulates the intake of nutrients, excretion of residues and keeps away harmful compounds.

An overview of the microbial cell.

HPP disrupts the plasma membrane of microorganisms, which may be like bursting a balloon. Depending on the applied pressure level, bacterial cells may repair the damage and resume growth. Lethal effects depend on multiple factors, but as a rule of thumb, increasing the pressure level and extending pressure holding time will positively influence microbial inactivation. Some microorganisms may be more resistant than others as well. Yeasts, molds and some foodborne viruses are inactivated between 3,000-4,000 bar (44,000-59,000 psi). Bacteria are more pressure resistant, cellular damage starts around 2,000 bar (29,000 psi) and for most pathogens such as Listeria monocytogenes and Escherichia coli O157:H7 becomes irreversible at 5,000 bar (70,000 psi) or higher pressure.

Bursting balloons: analogy for HPP microbial inactivation. Image source: Flickr

Microbial spores

Despite the widespread application of HPP, this outstanding food processing technology also has some limitations. The most important is that pressure levels currently applied in commercial applications do not inactivate bacterial spores. Spores can be thought as bunkers in which microorganisms enclose to protect against adverse conditions. Metabolism also slows down, allowing spores to survive freezing, chemical compounds, lack of nutrients, and other extreme conditions for years. Once the surroundings of bacterial spores provide ideal conditions, microorganisms will stop hiding inside the spore and resume growth, a biological process known as “spore germination”.

Among spore former bacteria, Clostridium botulinum spores are of concern due to the potential generation of the botulinum toxin. Up to date, the botulinum toxin is the most potent poison known to man-kind. The toxin inhibits the electrical signal, leading to muscle paralysis.

The microorganism grows in oxygen-depleted environments, low-acid conditions (pH > 4.6), room temperature (3-48 °C; 38-118 °F), and in the absence of other microorganisms, scenarios likely found for HPP foods packaged in bottles or vacuum sealed packaging. Unfortunately, HPP does not eliminate bacterial spores and additional control measures.

This is the first series of our “back to school with HPP”, Keep an eye out for our next publications related to High Pressure Processing basics and more from Hiperbaric.