High-Pressure Processing, also known as High Hydrostatic Pressure (HHP) or Ultra High Pressure (UHP), is a non-thermal technology for food preservation that inactivates foodborne pathogens and spoilage bacteria. Compared to thermal processing, high-pressure processed foods have less negative impact on taste, texture, appearance, and nutritional value. Therefore, HPP is gaining attention as an alternative to thermal sterilization techniques. This article organizes the basics of high-pressure processing from the perspective of microbial sterilization.
Sterilization of food is commonly achieved through thermal processing, with pasteurization being fundamental. However, thermal processing inevitably reduces the flavour and texture of food. High-pressure processing is anticipated as a technology that compensates for these drawbacks.
Overview of the Technology
High-pressure processing is a non-thermal treatment that applies isotropic pressure to food, suitable for both solid and liquid foods. The pressure is applied quickly and uniformly so that all parts of the food are subjected to the same pressure simultaneously.
Typically, this process is conducted at refrigeration or mild processing temperatures (below 45°C) with pressures ranging from 400 to 600 MPa. The common hold times range from 1.5 to 6 minutes.
Overview of High-Pressure Processing Equipment
The high-pressure processing equipment includes methods such as direct pressurization, where a piston is used to decrease the volume of the pressure vessel and increase the pressure, and indirect pressurization, where a high-pressure medium is pumped into the vessel. Equipment used in the food industry generally employs the indirect pressurization method.
In commercial applications within the food industry, pressures up to 600 MPa are used (laboratory devices can achieve up to 1,400 MPa).
The pressure in a high-pressure processing system is transmitted by a medium with low compressibility (most commercial systems use water as the pressure transmission fluid). The circulating water can often be cooled down to 10-15°C. The process consists of three stages and their associated timings:
- Compression stage: Using pumps or intensifiers to feed the liquid medium into the pressure chamber.
- Holding time: Maintaining the target pressure.
- Pressure release stage: Rapid decrease in pressure.
During compression, pressure increases and adiabatic heating raises the temperature of the food, the pressure medium, and the interior of the container. If the temperature is constant, increasing the pressure increases the degree of molecular ordering of a substance. Thus, as long as the pressure increases, the temperature rises (about 2-3 °C for every 100 MPa). In the case of homogeneous foods like juices and beverages, the temperature change is typically about 3°C for every 100 MPa increase from an initial temperature of 25°C. Conversely, during decompression, if heat has dissipated from the food during the hold time, the temperature of the food may be lower than before compression.
Typically, products are packaged before undergoing high-pressure processing. Generally, packaging for high-pressure processed foods is vacuum-sealed to remove air.
Sterilization Mechanism
High-pressure processing causes the disruption of hydrophobic and ionic bonds. Therefore, it induces partial or complete denaturation of the three-dimensional structures of enzymes and proteins in bacterial cells. Importantly, high-pressure processing does not involve the breaking of covalent bonds, such as peptide bonds.
The mechanism of microbial inactivation by high-pressure processing affects many cellular targets including the cell wall, cell membrane, nucleic acids, ribosomes, and various proteins. Thus, it is not a case of bacteria being inactivated by a single specific mechanism. However, protein denaturation is considered to be the primary mechanism by which bacterial cells are inactivated under high pressure.
Additionally, it is believed that microbial inactivation occurs not only due to pressure alone but also through the combined effects of other physical impacts such as cavitation, shear stress, turbulence, collisions, and friction-induced temperature increases.
Factors Influencing the Effectiveness of High-Pressure Processing
Magnitude of Pressure
Most studies on high-pressure processing have demonstrated that the inactivation of foodborne pathogens increases proportionally with both pressure level and exposure time.
For example, Lavinas et al. reported that the D-value (the time required to reduce the microbial population by one logarithmic cycle) of Escherichia coli in apple juice at approximately 106 CFU/mL was 16.4, 11.3, 2.4, and 1.2 minutes at pressures of 250, 300, 350, and 400 MPa, respectively.
Compiled from data in Lavinas et al.（2008）
Processing Time
Moreover, as the duration of high-pressure processing increases, even at the same pressure, the reduction in microbial count also increases. Lavinas et al.（2008）have shown that the logarithmic reduction of E. coli in apple juice processed at 400 MPa for 0.5, 1.0, and 1.5 minutes was 2.19, 2.95, and 4.2, respectively.
Compiled from data in Lavinas et al.（2008）
Temperature and Depressurization Rate During High-Pressure Processing
Various studies have been conducted regarding the temperature and depressurization rate during high-pressure processing, presenting contrasting results. At present, it is difficult to conclusively determine the relationship between temperature or depressurization rate and sterilization efficiency during high-pressure processing. These issues are discussed in detail in the following review:
Factors Affecting Microbial Inactivation during High Pressure Processing in Juices and Beverages: A ReviewJ Food Prot (2020) 83 (9): 1561–1575.
Water Activity of the Target Food
Water activity (aw) is considered a crucial factor in the inactivation of bacteria and yeast during pressurization.
Generally, as water activity decreases, microbial growth is inhibited while at the same time, it may protect microbes from other environmental stresses such as heat. It is known that a decrease in water activity during high-pressure processing also reduces the sterilizing effect. The protective mechanism is not clear, but it is speculated that proteins stabilize under low water activity conditions.
The diagram below shows the relationship between high-pressure processing of E. coli at 25°C and water activity. For example, at 450 MPa, a reduction of about 3.5 log is observed when the water activity is 0.850, while a reduction of over 5 logs occurs when the water activity is 0.992.
The figure above was drawn from data on the relationship between water activity and high-pressure treatment from the following paper .
Synergistic and Antagonistic Effects of Combined Subzero Temperature and High Pressure on Inactivation of Escherichia coli
Appl.Enbiron.Microbiol.72.150-156(2006)
Why does low water activity weaken the effect of high-pressure processing?
This is not yet clearly elucidated. However, a hypothesis introduced here suggests the following: Water molecules binding around proteins maintain their flexibility. High-pressure processing exposes hydrophobic sites and alters active sites in enzymes.
On the other hand, under low water activity, these hydration waters are reduced, and the structural flexibility of proteins is lost. Thus, the hydrophobic sites of enzymes remain structurally rigid and buried, making them less likely to be exposed by high-pressure processing, thereby making enzyme inactivation less likely.
In any case, the application of high-pressure processing to dry or low water activity foods may not be practical, as microbes and enzymes in these foods are less susceptible to pressure.
The pH of the Target Food
The pH significantly impacts the inactivation of microorganisms during high-pressure processing. It has been widely reported that the lower the pH, the more susceptible bacterial vegetative cells, spores, yeast, and molds are to high-pressure processing.
For instance, the following study example investigates the effect of high pressure on the survival of pressure-resistant Escherichia coli O157:H7 in orange juice within a pH range of 3.4 to 5.0.
Commercially sterile orange juice was adjusted to pH levels of 3.4, 3.6, 3.9, 4.5, or 5.0, inoculated with E. coli O157:H7 at 108 cfu/ml, and then subjected to 400 MPa pressure treatment at 20°C. As indicated in the diagram below, the lower the pH, the greater the sterilizing effect of the high-pressure processing.
The reason why a lower pH enhances the sterilizing effect of high-pressure processing is believed to be due to the inability of the surviving cells to immediately repair damage under low pH conditions.
The diagram above was created using data extracted from the following study:
Inactivation of Escherichia coli O157:H7 in orange juice using a combination of high pressure and mild heat. J Food Prot, 62(3):277-9(1999)
Application Examples of Microbial Sterilization
High-pressure processing technology has been primarily applied to pre-packaged juices, sauces, dips, stir-fries, meat products, and ready-to-eat (RTE) foods. In the EU, there is also growing interest in using high-pressure processing as an alternative to pasteurization for milk.
Over the past 30 years, numerous studies have applied high-pressure processing to food. Here, we will focus on three examples.
Application Example 1: Sterilization of Orange Juice
Juices are susceptible to microbial spoilage and enzymatic activity, limiting their shelf life. High hydrostatic pressure (HHP) can be used to inactivate microbes and enzymes.
In the study presented below, the effect of mild heating combined with high hydrostatic pressure was investigated on Staphylococcus aureus, Escherichia coli O157, and Salmonella in apple, orange, apricot, and sour cherry juices.
In the pressure tests, vials containing fresh or sterilized juice samples were exposed to pressures of 250 to 450 MPa at temperatures ranging from 25 to 50°C for durations of 0 to 60 minutes, depending on the level of enzyme or microbial inactivation required. The results are summarized as follows:
- At 250 MPa and 30°C with a processing time of 20 minutes, it was possible to achieve a 5-log reduction of Staphylococcus aureus, E. coli O157, and Salmonella.
- A 5-minute treatment at 350 MPa and 40°C resulted in complete inactivation of the inoculated microbes.
The above two diagrams were created using data from the following study.
Efficiency of high pressure treatment on inactivation of pathogenic microorganisms and enzymes in apple, orange, apricot and sour cherry juices
Food Control 17(1):52-58（2006）
Application Example 2: Sterilization of Chicken Meat
High hydrostatic pressures of 300, 450, and 600 MPa were used to investigate the effects on microbial populations, meat quality, and sensory properties of chicken breast meat. The summarized results are as follows:
- Pressurization at 450 MPa and 600 MPa nearly completely eliminated harmful bacteria such as Salmonella, E. coli O157, and Listeria, thus improving the safety of chicken breast fillets.
- At 600 MPa, microbial reduction of 6 to 8 log (CFU/g) was observed over 7 to 14 days, and at 450 MPa, a reduction of 4 to 8 log (CFU/g) was seen over 3 to 14 days.
- A pressure of 300 MPa decreased the flavor intensity, aroma, and juiciness, with 450 MPa producing the weakest aroma. Increasing pressure led to higher L, a, b* values, increased cooking loss, and more intense color changes.
- Increased pressure enhanced firmness, cohesiveness, chewiness, and bite, while reducing volatile basic nitrogen (VBN), thereby improving meat freshness.
- Pressures of 450 MPa and above induced lipid oxidation.
These results indicate that high-pressure processing is an effective technology for reducing bacterial spoilage and extending the shelf life of chicken breast fillets, although it may have negative impacts on certain quality and sensory properties. Therefore, detailed studies on the conditions for microbial sterilization without compromising these qualities are currently being concluded.
The effect of high pressure on microbial population, meat quality and sensory characteristics of chicken breast fillet
Food Control, 22, 6-12(2011)
Application Example 3: Sterilization of Liquid Eggs with a High-Pressure Homogenizer
Recently, due to their convenience and ease of use, the demand for liquid eggs has rapidly increased in both commercial and domestic settings. To obtain safe liquid egg products free from Salmonella, low-temperature pasteurization is necessary. However, heat treatment can compromise several functional properties of egg whites, such as coagulation capacity, foaming during whipping, emulsion formation, increased bonding adhesion, and reduced digestibility.
In the study shown below, the bactericidal effect of passing liquid eggs through a high-pressure homogenizer at 150 MPa multiple times was investigated.
Note: High-pressure homogenizers (HPH) are a promising technology especially suited for the continuous production of fluid foods. Fluids are pushed through a narrow gap in the homogenizer valve, where they are subjected to rapid acceleration, causing simultaneous phenomena such as cavitation, shear, and turbulence.
Summary of results:
- Egg whites inoculated with approximately 6 log CFU mL-1 of Salmonella Enterica 9898 DSMZ were treated, and the microbial inactivation effect of high-pressure homogenization was evaluated.
- A 3.5 log-cycle inactivation was achieved with four passes at 150 MPa.
- Increasing the number of passes to eight resulted in a reduction of approximately 4.7 to 5 log cycles.
Effect of high pressure homogenisation on microbial inactivation, protein structure and functionality of egg white.
Food Research International, 62, 718-725(2014)
Foods Unsuitable for High-Pressure Processing
High-pressure processing can, in principle, be applied to almost all foods. However, the following types of foods are not well-suited for this process:
- Aerated Foods: Foods containing air bubbles, such as bread, cakes, and whole or fresh-cut fruits and vegetables, may experience negative effects due to their porous structures being compromised or disrupted.
Microorganisms Unsuitable for High-Pressure Processing
High-pressure processing cannot inactivate spores (although inactivation may be possible when combined with heat treatment).
Applications of High-Pressure Processing Beyond Microbial Sterilization
Besides sterilizing foodborne microbes, high-pressure processing is also applied in numerous other food technology processes and for the development of new products. These applications include freezing, enzyme control, low-temperature gelatinization of starch, unfolding of proteins, shell peeling of shellfish, and enhancement of mass transfer phenomena. As this blog focuses on food microbiology, these applications will not be discussed further here.
The History of High-Pressure Processing Research in Global Food Studies
As introduced in this article, over the past 30 years, substantial knowledge has been accumulated globally in both the research and industrial application of high-pressure processing of foods. Pioneering research on high-pressure processing of foods can be traced back to Dr. Bridgman (winner of the Nobel Prize in Physics in 1946), who discovered that the application of 500–600 MPa of hydrostatic pressure caused coagulation of both egg yolk and egg white in shell-encased eggs.
However, the practical application of food high-pressure processing was ignited by researchers in Japan. The research in Japan on high-pressure food processing became active after 1987 when Dr. Rikimaru Hayashi, then an assistant professor at Kyoto University's Institute of Food Science, proposed the application of high-pressure processing technology to food processing. Since this proposal, research on high-pressure food processing in Japan commenced. The first commercialized high-pressure processed foods in Japan appeared in 1990, including juices, jellies, and jams made from fruits. Subsequently, the application of high-pressure processing expanded to various foods such as meats, fish, rice pudding, beef ham, and sake. Beyond Japan, this innovative processing technology began to be adopted internationally.
Furthermore, on March 8, 2022, the European Food Safety Authority (EFSA) issued an assessment report on the safety and efficacy of high-pressure processed foods. With the issuance of this report, the practical adoption of high-pressure food processing is expected to accelerate worldwide.
For an overview of the European Food Safety Authority's (EFSA) assessment on the safety of high-pressure processed foods, please refer to the following article: "The efficacy and safety of high-pressure processing of food"
