This fact sheet provides information on how high-pressure processing (HPP) is used to pasteurize food, improve food safety, and enhance food quality.
What Is High-Pressure Processing?
High-pressure processing (HPP) is a food pasteurization method where food is subjected to elevated pressures (up to 87,000 pounds per square inch, or 6,000 atmospheres, or 600 MPa), at ambient or chilled temperatures, to alter the food’s attributes to achieve consumer-desired qualities. HPP also inactivates harmful and spoilage microorganisms, facilitates food safety, and extends food shelf life while retaining food quality and maintaining natural freshness. Since pressure treatment at ambient or chilled temperatures does not inactivate bacterial spores, treated products are typically distributed under a refrigerated environment. The process is also known as high hydrostatic pressure processing (HHP) and ultra-high-pressure processing (UHP).
How Does High-Pressure Processing Work?
High-pressure processing is a batch operation. Typically, the product is packaged in a flexible container (a pouch or plastic bottle), and is then loaded into a sample basket. The sample basket is then loaded inside a high-pressure chamber filled with a pressure-transmitting fluid such as water. The pressure-transmitting fluid in the chamber is pressurized with a pump, and the pressure is transmitted through the package into the food. During pressurization, there is a transient temperature rise in foods (3 degrees Celsius/100 MPa) due to adiabatic heating. The product is pressurized, and held under pressure for a specific time, usually 2–5 minutes, before being depressurized. The product temperature then returns close to its initial value. The processed product is then removed and stored or distributed under refrigerated conditions.
How Does This Technology Benefit Consumers?
Most foods are thermally processed to kill spoilage or pathogenic bacteria, which often diminishes product quality. High-pressure processing (HPP) provides an alternative means of killing bacteria by using pressure as the lethal agent. HPP conducted at refrigerated (or ambient) temperatures prevents thermally induced off-flavors and retains quality attributes, especially in heat-sensitive products. Pressure treatments can eliminate or reduce the need for synthetic additives in product formulation. This helps the food processors to satisfy consumer demand for clean-label products. During HPP, the treated products receive minimal or reduced thermal exposure. This helps retain the natural characteristics of the foods’ appearance, texture, and nutrition.
What Type of Foods Can Be Processed by High Pressure?
Like heat, high-pressure treatment is a versatile pasteurization method for a variety of value-added liquid, semi-solid, and solid foods. Deli meat, guacamole, seafood, ready-to-eat meals, sauces, juices and beverages, jams, salsa, pet foods, baby foods, fruits, and vegetables products are some representative product categories that are pressure treated. Acidic foods (pH < 4.6) are particularly good candidates for HPP technology.
Are Any Products Not Suitable for Pressure Treatment?
Like any other processing method, HPP cannot be universally applied to all types of foods. Foods with entrapped air pockets such as breads, cakes, mousse, strawberries, and marshmallows are not suitable for HPP. These foods might be deformed under pressure due to the compressibility difference between air and food. In addition, foods with low-moisture content such as spices, powders, and dry fruits are also not suitable for HPP because the microbial lethality of pressure diminishes under low-water activity conditions.
What Type of Packaging Material Can Be Used for Pressure Treatment?
During pressure treatment, the food and packaging material may undergo a 15% volume reduction and then return to its original volume upon depressurization. This requires that the packaging be flexible enough to withstand a transient volume reduction while under pressure. HPP products are typically vacuum-packaged using flexible pouches or containers. Polyethylene terephthalate (PET), Polyethylene (PE), Polypropylene (PP), and Ethylene vinyl alcohol copolymer (EVOH) are the commonly used packaging materials. At least one interface of the package need to be flexible enough to transfer pressure. Rigid packaging containers such as glass bottles or tin cans cannot be used for HPP due to the inability to transmit pressure to the food product. The packaging material must also be able to survive intensive pressure treatment and needs to have high-barrier properties towards moisture and oxygen transmission.
What Regulatory Approval Is Required for Commercializing an HPP Product?
The Hazard Analysis Critical Control Points (HACCP) regulations used by the Food and Drug Administration (FDA) recognize HPP as a pasteurization process to satisfy a 5-log reduction of pertinent pathogens in juices. Similarly, HPP is recognized by the FDA as a decontamination process for Vibrio bacteria in raw oysters and seafood. The United States Department of Agriculture Food Safety Inspection Service (USDA FSIS) recognized HPP as a post-lethality treatment for the control of Listeria monocytogenes in ready-to-eat (RTE) meat products. Health Canada, the European Commission, and other regulatory agencies have recognized HPP as a food pasteurization process. Food processors should, however, conduct appropriate validation studies. Since pressure-pasteurization treatment does not inactivate bacterial spores, pressure-pasteurized products should be distributed under refrigerated conditions.
Can HPP Be Used for Commercial Sterilization of Low-Acid Foods?
Bacterial spores can survive in low-acid (pH > 4.6) products. Simultaneously combining pressures (58,000 to 87,000 psi or 400-600 MPa) with temperatures (90–120 degrees Celsius) over a short time (3–15 minutes) inactivates various pathogenic and spoilage spores. This is called the pressure-assisted thermal process (PATP). The FDA’s regulations for low-acid foods packaged in hermetically sealed containers (LACF) were updated to recognize PATP as a viable processing option. The FDA issued letters of no objections for pressure-assisted, thermal-sterilized, potato and seafood products. Currently, there are no known shelf-stable PATP low-acid products that are commercially available.
What HPP Mechanism Inactivates Microorganisms?
High-pressure treatment modifies the cellular morphology of microorganisms and damages cell membranes, ribosomes, and enzymes, including those involved in DNA replication and transcriptions. An insufficient intensity of pressure treatment, however, may only injure a proportion of the bacterial population. The injured bacteria in HPP processed foods may then recover during storage time.
Generally, gram-positive bacteria are more resistant to high pressure than gram-negative bacteria. This resistance may be attributed to their thicker cell walls. High pressure above 60,000 pounds per square inch inactivates most vegetative bacteria, but spores survive pressure treatment at ambient temperatures.
What Is the Shelf Life of an HPP Product?
HPP treatment can extend the shelf life of food products up to 120 days depending upon the choice of process parameters and product formulation. The product’s shelf-life extension is based on the parameters of the process (pressure, temperature, and holding time) and product (acidity, water activity, and composition). Choice of packaging materials must also be considered. In addition, the storage temperature used after HPP is another factor that can most influence the shelf life of the product.
How Are Pressure-Treated Foods Stored and Distributed?
High-pressure pasteurization kills vegetative bacteria. On the other hand, pressure treatment at ambient temperatures does not inactivate bacterial spores. As a result, HPP products that are currently marketed worldwide need to be refrigerated during their distribution. In some cases, this is necessary for safety (to prevent the outgrowth and germination of spores in low-acid foods).
What Is the Effect of HPP on the Quality of Food Products?
HPP does not break the covalent bonds in foods and therefore has a limited effect when compared to thermal processes on low-molecular-weight compounds such as flavor compounds, vitamins, and pigments. As a result, the quality of HPP-treated food is similar to fresh food products, and their quality degradation is influenced more by their storage and distribution after processing. HPP also provides a unique opportunity to create new food textures in protein- or starch-based foods. In some cases, pressure can be used to form protein gels and increase product viscosity without using heat.
Will HPP Damage the Food Product?
During HPP, pressure is uniformly applied around and throughout the food product. In contrast, imagine how a grape placed between fingers can be easily squeezed and broken. This is because the pressure is not applied evenly from all sides simultaneously.
However, if this same grape is squeezed from all sides simultaneously, it will not be crushed. This can be demonstrated by placing a grape inside a container filled with water. By squeezing the container, the water inside as well as the grape is pressurized. Yet the grape is not damaged even with hard squeezing. Similarly, foods processed by high pressure are not damaged.
Is Commercial-Scale Equipment Available?
Yes. Commercial-scale equipment (Figure 1) is available with vessel volumes ranging from 35 to 525 liters. Notable HPP equipment manufacturers are Quintus Technologies (Figure 1a), Hiperbaric (Figure 1b), Avure Technologies (Figure 1c), and ThyssenKrupp.
What Is Bulk High-Pressure Processing?
Recently, equipment providers introduced a bulk HPP equipment option to process a large volume of beverages. The liquid food is packaged into a large, pre-sanitized, large-volume polymer bag (up to 500 liters) that occupies about 90% of the pressure vessel. After loading the raw product into the pouch, the contents of the vessel undergo processing similar to batch, high-pressure processing. The pressure vessel is equipped with sterilizable valves for filling the bag with raw product and then discharging the product into an aseptic tank after processing. At the end of processing, the pressure is released, and the processed liquid is discharged through the discharge valve. The liquid is then collected in an aseptic storage tank and transferred to a filling line.
Is HPP Equipment Safe to Operate?
High-pressure equipment design is a mature technology originated in the chemical processing industry. Most high-pressure vessels are manufactured under boiler and pressure vessel code guidelines established by the American Society of Mechanical Engineers (ASME). Processors should also ensure that the vessels are manufactured, installed, tested, and operated according to relevant state regulations. With a little training, food plant personnel can learn to safely operate the equipment.
How Economical Is HPP Processing?
Depending on the volume of the vessel and extent of automation, costs range from $500,000–$4,000,000 USD. Originally introduced with a vertical orientation, most of the commercial-scale, high-pressure equipment is now horizontally oriented. This helps to accommodate higher volumes as well as providing a clear separation between raw and processed product zones. A recent study estimated the cost for HPP pasteurization to be 10.7cents/L for a processing capacity of 3,000 L/h (792.5 gal/h). The cost for comparable thermal treatment is 1.5 cents/L. Factory production rates above 40 million pounds per year are now in operation. As demand for HPP equipment grows, the capital cost and operating cost will continue to decrease. Consumers also will benefit from the increased shelf-life, quality, and availability of value-added products and new types of foods that are impossible to make using thermal processing methods.
What Is Ultrashear Technology Processing?
Ultrashear technology (USTTM) is a next generation, continuous high-pressure processing method for treating liquid foods. The liquid food is pressurized and transferred to a shear valve where it passes through a tiny gap between the valve seat and the ball or needle valve. Thus, like HPP, UST-treated food can be pasteurized or commercially sterilized. In addition, UST treated liquids are reported to have desirable quality changes such as particle size reduction, and changes in rheological, emulsification, and homogenization characteristics. While UST is not commercially practiced today, the technology has the potential to create various clean-label liquid foods, emulsions, and sauces.
What Are Other Applications of High Pressure in Food Processing?
Like heat, pressure treatment has a variety of applications beyond food preservation, such as pasteurization and commercial sterilization. High pressure also assists in performing functions such as meat tenderization, protein denaturation, nutrient infusion, freezing and thawing, extraction, and lipid crystallization which provide great benefits for various food processing operations.
Are Facilities Available for Product Development Before Venturing into HPP Processing?
There are several research facilities throughout the United States and Europe where food processors can evaluate HPP technology. The Ohio State University Center for Clean Food Process Technology Development conducts laboratory and pilot-scale research using batch HPP (Figure 2a) and ultrashear technology equipment (Figure 2b). The university can assist food processors in conducting confidential product evaluations for a nominal fee.
Balasubramaniam, V. M., Martinez-Monteagudo, S. I., & Gupta, R. (2015). Principles and application of high pressure-based technologies in the food industry. Annual review of food science and technology, 6(1), 435–462.
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Janahar, J. J., Balasubramaniam, V. M., Jimenez-Flores, R., Campanella, O. H., García-Cano, I., & Chen, D. (2022). Pressure, shear, thermal, and interaction effects on quality attributes of pea–dairy protein colloidal dispersions. Food Hydrocolloids, 107811.
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The authors acknowledge the contribution of The Ohio State University Food Safety Engineering Laboratory, Center for Clean Food Process Technology Development (u.osu.edu/foodsafetyeng/). The authors also thank Dr. John Fulton, technical editor Tim Vargo, and project coordinator Annie Steel of Ohio State University Extension for their constructive review comments. This work was supported by the USDA National Institute of Food and Agriculture (NIFA) grant 2018-67017-27914. References to commercial products or trade names are made with the understanding that no endorsement or discrimination by The Ohio State University is implied.
Jerish Joyner Janahar, Graduate Research Associate and Doctoral Student, Department of Food Science and Technology, The Ohio State University
V. M. Balasubramaniam, Professor of Food Engineering, Department of Food, Agricultural and Biological Engineering and Department of Food Science and Technology, The Ohio State University.