High Pressure Processing FAQ

The High Pressure Processing (HPP) technology is a hydrostatic process that relies on the compression of water to transmit pressure. Therefore, it is recommended that foods have a water activity (a_w) above 0.96 to maximize the lethal effect on microorganisms.

In other words, HPP is more effective in high a_w products because the product will have a higher content of free water available to transmit the pressure, leading to a higher microbial inactivation and extended shelf-life.

Keep in mind that moisture content is different from a_w. A product can have a high moisture content and a low a_w.

HPP has spread to a wide range of foods and beverages: from the most consolidated HPP applications, such as juices & beverages, meat, and avocado products, to the latest trends such as ready-to-eat (RTE) meals, dips of vegetable origin, baby food and pet food.

To enjoy the advantages of HPP, products should have a high water activity content to maximize the lethal effect on microorganisms. In addition, this technology can lead to changes in the texture of some ingredients in the absence of a liquid or dressing surrounding them. Therefore it is not recommended to apply HPP to spices, powders, dry nuts or fruits, cereals, whole fruits, vegetable leaves and leafy salads, bread and pastries and other ingredients that fit in this description.

Foods must be thawed for HPP as the technology does not work for frozen foods. Volume changes due to compression/decompression alter the cellular membrane of microorganisms and the stability of key cellular components like enzymes or proteins. Ice is less compressible than liquid water, which protects microorganisms. It is also possible that microorganisms adapted to cold/frozen environments present a more flexible cellular membrane and generate solutes to stabilize key components in cold environments, which may also confer pressure resistance.

HPP is the most known and used among non-thermal food processing technologies (e.g. pulsed electric fields, ultraviolet radiation, ionizing radiation or membrane filtration) in the USA, Europe, Asia and Oceania. Unlike the other non-thermal technologies, HPP is completely non-thermal and used for liquid and solid foods. Moreover, as a post-packaging intervention, it allows to remove additives and to obtain a clean-label product.

HPP has minimal effects on vitamins, antioxidants, other micronutrients, and flavor and aroma compounds. Furthermore, it helps to better retain these compounds compared to conventional thermal processes. This is because HPP does not break covalent bonds, and only affects weaker non-covalent molecular interactions like hydrogen bonds, van der Waals forces, electrostatic and hydrophobic interactions. These interactions are responsible for the stabilization of the secondary and tertiary structure of proteins, complex carbohydrates, or biological structures such as lipid bilayers of cell membranes.

HPP is an ‘’In-pack’’ process, meaning that the products need to be packaged when processed, usually in their final package. Packaging materials must be flexible (to withstand compression), elastic (to recover their original shape after decompression), and waterproof (as they will be submerged in water). These considerations make plastic polymers the most versatile option.

Hiperbaric has developed the ‘’In-Bulk’’ technology, a breakthrough innovation that allows processing liquids without being packaged, i.e. in bulk before packaging. This opens the range of other packaging solutions that can be used like carton bricks, glass bottles, and metal cans.

HPP is a non-thermal technology as the temperature during the process is below 40 °C/100 °F.

Still, it is true that during this process a slight temperature increase occurs inside the vessel. This phenomenon is called adiabatic heating and is associated with water compression. In the particular case of water, the temperature increases around 2-3 °C for every 100 MPa/1000 bar/15,000 psi of applied pressure. Therefore, taking into account the maximum operating pressure of industrial equipment (600 MPa/6000 bar/87,000 psi), the food/water temperature would only increase 18 °C/64 °F during holding time, which is typically between 2-6 min. Furthermore, the adiabatic heating is completely reversible upon pressure release, and the product returns to the temperature level before the HPP cycle.

HPP technology allows the inactivation of vegetative microorganisms, parasites and viruses by applying 400 MPa (4000 bar/58,000 psi) to 600 MPa (6000 bar/87,000 psi), for a few seconds to around 6 minutes. However, as with thermal pasteurization, some pressure resistant microorganisms remain viable in the product after processing, as well as bacterial spores which are not inactivated by HPP.

Furthermore, HPP does not completely inactivate most enzymes, and for this reason, HPP does not allow the development of stable foods marketed at room temperature. On the other hand, HPP-treated products better retain sensory and nutritional properties in comparison to the conventional thermal process.

In conclusion, HPP technology delivers “fresh-like” products with similar microbiological results as heat pasteurization, however it is not a sterilization technique.

Bacterial spores are highly resistant to high pressure. In fact, at the maximum operating pressure of industrial equipment (600 MPa/6000 bar/87,000 psi), spores are not inactivated. However, considering other hurdles such as a low pH (<4.6) and refrigerated storage (4-6 °C), could keep the product within safe margins by preventing germination and spore growth during food storage.

Combining high pressure with mild or high temperature levels in a process known as pressure assisted thermal sterilization (PATS), or pressure assisted thermal processing (PATP) could be a good strategy to achieve spore inactivation and ensure food safety with a milder impact nutritional and sensory quality. Nonetheless, this technology is not yet at an industrial application level.

HPP inactivates pathogenic and spoilage microorganisms, as well as some tissue enzymes. HPP effects on the enzymes depend on multiple factors, including the type of enzyme, the processing conditions, and the characteristics of the food matrix. Hence, most enzymes will only be partly inactivated and remain active during refrigerated storage.

If desired, some strategies to increase enzymatic inactivation is combining HPP with other hurdles, such as lowering the pH, adding antioxidants, or blanching the raw materials before HPP. However, in some cases, this partial inactivation of the enzymes can be a desirable phenomenon, opening the possibility of developing new products with a different texture.

It is recommended to store HPP products in refrigeration (4-6 °C), since the cold chain will act as a hurdle to slow down undesirable microbial growth enzymatic activity, and chemical reactions during the shelf-life.

This is easily understood, since some microorganisms can be resistant to high pressure and recover during the shelf-life. The same happens with the enzymes that are not completely inactivated: if the cold chain is maintained their residual activity will be lower. Likewise, chemical reactions that are not related to HPP but lead to sensory or nutritional changes, take place at faster rates under temperature abuse (10 °C) or room temperature levels.

Definitely. HPP guarantees food safety and achieves an increased shelf life, while maintaining the optimum attributes of fresh products. In addition, HPP is highly recognized by numerous food safety authorities (FDA, EFSA…). Food safety is achieved by inactivating vegetative pathogens, including bacteria, viruses, molds, yeasts and parasites by applying 400 MPa (4000 bar/58,000 psi) to 600 MPa (6000 bar/87,000 psi), for a few seconds to around 6 minutes.

However, since there are some pressure resistant microorganisms and enzymes, and even at the maximum operating pressure of industrial equipment (600 MPa/6000 bar/87,000 psi), spores are not inactivated, other hurdles should be taken into account. For example, low pH (<4.6), presence of natural antimicrobials and refrigerated storage (4-6 °C), could help to further guarantee food safety together with HPP technology.