Food Processing for Safety: What is High-Pressure Processing (HPP)?
Food processing has been around for centuries to improve food quality and extend its shelf life. However, removing food pathogens and maintaining the structural integrity and nutritional value of food is a thin line to cross. Some food processing methods, especially the ones involving heat, can kill spoilage bacteria, but they can also affect food texture. What's more, they can remove some of the nutrients, vitamins, and fiber content in food products.
Food science is an ever-evolving field, and to solve this problem, scientists continue to develop new methods to ensure food safety without changing the integrity of foods. High-pressure processing (HPP) may have the best of both worlds.
What is High-Pressure Processing?
High-pressure processing (HPP) is a food processing method that inactivates spoilage microorganisms and foodborne pathogens by using cold water and extremely high pressure instead of heat. HPP allows food producers to promote food safety while increasing shelf life and maintaining the original, desirable food qualities.
HPP follows specific steps, similar to the sequence of food irradiation used in the 1950s. Through the cold pasteurization technique used in HPP, food products are:
- Sealed in their final packages
- Introduced into a vessel
- Subjected to a high level of isostatic pressure (300-700 MPa) through a transmitting medium, usually water (Min and Zhang, 2005)
Afterward, the processed foods are unloaded from the pressurizing vessel. This approach inactivates the vegetative flora in the food product, including bacteria, viruses, yeasts, molds, and parasites.
How does HPP work?
Advantages of Using HPP for Food
HPP improves food safety and extends shelf life by inactivating the vegetative flora in food products. High pressure destroys pathogenic microorganisms using different mechanisms—changing the cell morphology, damaging the cell membranes, inducing cell elongation, cessation of their movement, and inducing the collapse of the intercellular gas vacuoles.
All these mechanisms result in the reduction of microbial populations in food products. What's more, as HPP is a non-thermal technique and doesn't use chemical preservatives, it helps maintain food quality while having minimal effect on food chemistry.
Minimally processed foods through non-thermal processes like irradiation, pulsed light technology, pulsed electric field technology, and finally, HPP retains their quality and natural freshness. In the absence of heat in food processing, there's minimal damage to HPP-processed foods like salads and ready-to-eat (RTE) meals (Pandrangi and Balasubramaniam, 2005 ).
Also, compared to other preservation methods, HPP helps retain the functional properties of food. HPP has been used to increase the shelf life of milk. The pressure applied only lasted for a few seconds to minutes. Therefore, it only affected the noncovalent bonds in lipids, proteins, and starch. Thus, it does not affect the covalent bonds of vitamins and minerals, contributing to the maintenance of functionality.
Possible Disadvantages of Using HPP
There are some exceptions to the rule. Not only does it affect the overall cell morphology of food pathogens and spoilage microorganisms, but it can also affect them on a structural level. High pressure can deprotonate charge groups and disrupt the salt bridges and hydrophobic bonds between amino acids. This changes the conformation of proteins and, in effect, denatures them.
While this can also aid in antimicrobial effects of HPP, protein denaturation can also cause undesirable sensory changes in food. These changes can be visually apparent in protein-rich foods like eggs and delicate foods like leafy vegetables and strawberries. But this doesn't stop food scientists from improving the HPP process to accommodate delicate and protein-rich food products.
How is HPP Used in Food & Beverage Manufacturing?
With the honing of HPP food processing, various foods, including those considered delicate, can now be preserved without damaging their desirable characteristics.
HPP in Food Manufacturing
Because HPP uses physical compression with water, products go through an increase in temperature only during the treatment process, leading to uniform pressure application and minimal heat damage. This leads to the retention of the sensory and functional properties of the food (Pandrangi and Balasubramaniam, 2005). For instance, RTE food products and salads now offer greater convenience, longer shelf life, fresh-like attributes, and natural coloring using HPP.
In North America, HPP has also helped address product recalls due to pathogen contamination—particularly Listeria monocytogenes—in food. As such, the US Department of Agriculture (USDA) Food Safety and Inspection Service (FSIS) created an interim rule in 2022 to control the population of L. monocytogenes in RTE meat and poultry products.
This requires the application of high pressure (600 MPa) applied for several minutes at initial temperatures less than 10°C is enough to reduce the presence of L. monocytogenes, thus, extending the shelf-life without affecting the product quality (Patterson and Knoerzer, 2016). HPP also inactivates Salmonella Typhimurium in chicken. In sea bass, it was able to extend shelf-life by an extra week (Ezeh and Niranjan, 2018).
HPP in Beverage and Sauce Production
Aside from meat products, HPP is also used in vegetable and fruit products like juice, salsa, dressing, and guacamole. Commercially pasteurized fruit jams and vegetable sauces are already available in Japan and Italy in 1990 (Salvi et al., 2016).
Moreover, HPP is popular in avocado products as it helps extend shelf life by inactivating spoilage microorganisms and polyphenol oxidase (PPO) enzymes, responsible for the brown color of avocado during oxidation. As such, the products retain their original color, flavor, and nutrients.
Juices and smoothies can also be HPP-pasteurized to retain their fresh-like sensory qualities and extend their shelf life. HPP-pasteurization of beverages is already widely used in the United States, Europe, Australia, and New Zealand. However, the US FDA prohibits labeling HPP-pasteurized juices and smoothies as fresh.
Soups, salad dressings, salads, cheese, baby food, yogurt, dips, and salsa are also processed through HPP-pasteurization. Lastly, the South Wales Food Authority in Australia approved cold pasteurization of raw milk, making cold-pressed milk the latest HPP product in the market as of 2016 (Salvi et al., 2016)
Further Considerations in Applying HPP to Food
High-pressure processing of food products is an effective way to inactivate food pathogens and spoilage microorganisms without changing the sensory and functional properties of food products. Improving this technology is a continuing area of interest as different food products require different pressurization conditions for the technology to work.
While not all products can be processing with this technology, it is a great example of newer technology that can deliver high quality and safe food and beverage products.
Validation of HPP in Food Production and Preservation
When it comes to testing, be sure to have your or your co-manufacturers process parameters validated to ensure its safety and meeting required parameters for control of undesirable microorganisms. This may be achieved through a combination of detailed processing evaluation and risk assessment by a Processing Authority, and shelf-life evaluation of product across many different atmospheric conditions.
If you would like to learn more about evaluation of high-pressure processing systems of products with respect to a specific matrix or process, our experts are here to help.
