Pressure-Assisted Thermal Sterilisation (PATS)

Pressure-assisted thermal sterilisation (PATS) — also known as pressure-assisted thermal processing (PATP), high-pressure thermal processing (HPTP), or high-pressure thermal sterilisation (HPTS) — is a food preservation technology that combines elevated hydrostatic pressure with moderately elevated temperature to achieve commercial sterility in low-acid foods. Unlike conventional high pressure processing (HPP), which operates at ambient or chilled temperatures and targets only vegetative microorganisms, PATS applies pressure and heat simultaneously, exploiting their synergistic effect to inactivate bacterial endospores — the principal microbiological obstacle to shelf-stable food production without retort sterilisation.

The technology occupies a distinct niche in food preservation: it achieves the microbial safety requirements of commercial sterilisation (including inactivation of Clostridium botulinum spores) at significantly lower peak temperatures than conventional retort processing (121°C or higher), and with greater volumetric uniformity than surface-dependent heat transfer. This combination offers the prospect of shelf-stable products with substantially better retention of flavour, colour, vitamins, and texture than thermally sterilised equivalents.

Nomenclature

The technology is known by several overlapping names in scientific and regulatory literature:

Historical development

Origins in the Netherlands (late 1990s – early 2000s)

The conceptual foundations of PATS emerged in parallel in Europe and the United States during the late 1990s. In the Netherlands, a research consortium at ATO-DLO (Agrotechnological Research Institute, Wageningen) — which later became part of Wageningen University & Research — undertook pioneering work on combining pressure and temperature for sterilisation. Researchers involved in this Dutch development phase included Ludo van Schepdael, Wouter de Heij, and Hans Hoogland, who worked on the fundamental concept of using pressure to destabilise bacterial spores and render them susceptible to thermal inactivation at temperatures well below 121°C. This work took place approximately between 2000 and 2002.

Industrial partners in the Dutch programme included Unilever Research Vlaardingen and Stork Food & Dairy Systems, providing both the motivation of product quality improvement and the engineering expertise for process development. The 2003 publication “High-pressure sterilisation: maximising the benefits of adiabatic heating” (Wageningen University & Research) is a key output from this period, describing the thermodynamic principles that make the technology viable.

US development and military involvement

Simultaneously, interest grew in the United States, driven in part by the United States Army Natick Laboratories (US Army Research, Development, and Engineering Command), which sought high-quality shelf-stable rations for military use. Conventional retort meals — the basis of military field rations — suffered from the quality penalties of prolonged high-temperature sterilisation. PATS offered the possibility of shelf-stable products with significantly better palatability.

US academic research was centred at institutions including Ohio State University, Illinois Institute of Technology (IIT), Washington State University, and the National Center for Food Safety and Technology (NCFST) at IIT. Key researchers in the US programme included V.M. “Bala” Balasubramaniam (Ohio State), who became a leading authority on PATS process engineering, and D.F. Farkas, among others.

FDA regulatory acceptance (2009)

In 2009, the National Center for Food Safety and Technology (NCFST) at Illinois Institute of Technology submitted the first-ever petition to the US FDA for commercial authorisation of PATS for a low-acid food. The petition demonstrated a 6-log₁₀ reduction of heat- and pressure-resistant Clostridium botulinum spores in commercially packaged mashed potato (145 g packs processed at 600 MPa, approximately 120°C). The FDA issued a letter of no objection, representing the first regulatory clearance for a shelf-stable PATS product anywhere in the world. The process was required to comply with 21 CFR Parts 108 and 113 (low-acid canned foods regulations).

Physical mechanism

Adiabatic compression heating

A defining physical characteristic of PATS is adiabatic heating: when a liquid or food product is pressurised in a closed vessel, the mechanical work of compression is converted to heat within the product itself. The temperature rise is approximately:

This effect is exploited in PATS process design: a product pre-heated to approximately 75–90°C and then pressurised to 600–700 MPa will reach 90–120°C near-instantaneously and near-uniformly throughout its entire volume — because pressure, unlike heat, is transmitted isostatically (instantaneously and equally in all directions). This achieves a temperature distribution far more homogeneous than is possible with surface heat transfer in retort processing, where the thermal centre of the package always heats last.

On decompression at the end of the hold time, the product cools equally rapidly, minimising cumulative thermal exposure.

Synergistic pressure-temperature inactivation of spores

Conventional HPP at ambient temperature is ineffective against bacterial endospores, which survive pressures exceeding 1,000 MPa without germination at low temperatures. PATS exploits a two-stage synergistic mechanism:

Stage 1 — Pressure-induced spore destabilisation: At pressures above approximately 500 MPa combined with temperatures above 70°C, spores undergo rapid release of dipicolinic acid (DPA) and associated divalent cations (Ca²⁺) through pressure-activated channels. DPA is a unique spore component that contributes critically to their thermal resistance. Its release triggers a cascade equivalent to germination, causing the spore to shed its protective cortex and coat structures.

Stage 2 — Thermal inactivation of destabilised spores: Once DPA has been released and the cortex disrupted, the formerly resistant spore structure is far more susceptible to heat. At temperatures of 90–120°C that would be insufficient to inactivate intact spores at equivalent hold times under atmospheric pressure, the destabilised spores are rapidly inactivated.

The combined effect is synergistic: neither the pressure alone (at ambient temperature) nor the heat alone (at 90–120°C) would achieve commercial sterility in low-acid foods, but the combination does. This synergy is the fundamental scientific basis of PATS.

Process parameters

Typical PATS processes operate within the following ranges:

Research systems, including installations at Wageningen University & Research and equipment developed by Resato High Pressure Technology (Assen, Netherlands), extend to 10,000 bar (1,000 MPa), enabling investigation of PATS at pressures substantially above commercial HPP systems. At these elevated pressures, adiabatic heating is proportionally greater, potentially allowing sterilisation at even lower pre-heat temperatures.

Target organisms

Clostridium botulinum — the critical organism

The principal target of PATS is Clostridium botulinum, whose spores are the most pressure- and heat-resistant hazard in low-acid food sterilisation. C. botulinum produces botulinum toxin — one of the most potent biological toxins known — during anaerobic vegetative growth. In sealed, anaerobic packaging, any surviving spores that germinate and grow post-processing represent a food safety risk.

C. botulinum occurs in four toxin-producing types relevant to food safety, which differ substantially in their resistance to pressure-temperature treatments:

For PATS sterilisation validation, proteolytic Type A and B are used as the critical challenge organisms, as they exhibit the highest resistance. Achieving a 6-log₁₀ (6D) reduction of these strains in the cold spot of the most challenging product-package combination is the standard validation requirement.

At 105°C and 700 MPa, the D-value (time for one log reduction) of C. botulinum Type A spores ranges from 0.57 to 2.28 minutes depending on strain — substantially shorter than D-values at equivalent temperature under atmospheric pressure (D₁₂₁°C ≈ 0.1–0.2 min conventionally, but D₉₅°C can exceed 20 minutes), demonstrating the pressure contribution.

Bacillus species

Bacillus cereus and other Bacillus spore-formers are relevant in some food matrices. They generally show somewhat lower resistance to combined pressure-temperature treatment than proteolytic C. botulinum, though validation against the most resistant organism (typically C. botulinum Type A) provides coverage for lesser pathogens.

Comparison with conventional retort sterilisation

The key quality advantage of PATS over conventional retort is the lower cumulative thermal load — the integrated time-temperature exposure experienced by the product. Because PATS achieves sterilisation at lower peak temperatures for shorter durations, heat-sensitive quality attributes (vitamin C and B-group vitamins, heat-labile flavour compounds, pigments, and protein structure) are better preserved.

Quality retention

Published studies on PATS-treated products demonstrate:

• Vitamin C (ascorbic acid): Substantially higher retention than thermally sterilised equivalents, comparable in some studies to refrigerated HPP-treated products
• Colour: Chlorophyll degradation, carotenoid loss, and Maillard browning are reduced
• Texture and firmness: Better structural preservation in vegetable and fruit products compared to retort equivalents
• Flavour: Reduced development of cooked, sulphurous, or oxidised off-notes
• Proteins and enzymes: Reduced denaturation at lower temperatures, though pressure itself can modify protein structure

Packaging requirements

PATS imposes specific requirements on food packaging:

• High barrier flexible pouches: The dominant format; must withstand pressurisation to 600–700 MPa without rupture, seal failure, or delamination
• Low oxygen and water vapour transmission rates: To maintain shelf stability in the absence of refrigeration
• Volumetric flexibility: Flexible packaging must accommodate approximately 10–15% volumetric compression under pressure and return to original dimensions on decompression without structural damage
• Sterility of packaging: Packaging materials must be pre-sterilised; validated methods include gamma irradiation (4.7 Mrad, achieving a 12D reduction of C. botulinum spores on packaging surfaces) and hydrogen peroxide treatment (≤35% concentration)
• Rigid containers: Not suitable for PATS; glass and metal cannot accommodate the volumetric compression cycle

Validated and studied food products

Equipment and industrial infrastructure

Commercial PATS requires pressure vessels capable of reaching 500–700 MPa with integrated pre-heating and temperature monitoring systems. The equipment base for PATS overlaps substantially with HPP but requires higher thermal engineering capability for the pre-heating and insulation systems.

Key equipment suppliers active in PATS-relevant pressure ranges include:

• Avure Technologies (JBT Corporation, US) — major commercial HPP and PATS research equipment supplier
• Hiperbaric S.A. (Spain) — largest global HPP equipment manufacturer; active in high-pressure thermal processing research
• Stansted Fluid Power (UK) — research and laboratory-scale high-pressure systems
• Resato High Pressure Technology (Assen, Netherlands) — designs systems up to 10,000 bar; specifically positioned for PATS research and development applications at pressures beyond standard commercial HPP

The high capital cost of PATS systems — substantially greater than conventional retort lines of equivalent throughput — has been a persistent barrier to wider commercial adoption.

Limitations

• Capital cost: Pressure vessels and associated systems for PATS are substantially more expensive than equivalent retort capacity
• Batch processing: Like HPP, PATS is a batch process; throughput is limited by vessel volume and cycle time
• Regulatory pathway: FDA acceptance covers specific validated product-process-package combinations; each new application requires individual validation against the critical organism in the target product matrix
• European regulatory status: PATS for shelf-stable low-acid products has not achieved the same degree of regulatory clarity in the EU as in the US; guidelines are evolving
• Process complexity: Precise control of pre-heat temperature, pressurisation rate, hold time, and decompression rate is critical; deviation from validated parameters can compromise both safety and quality
• No direct F₀ equivalency: The synergistic pressure-temperature mechanism means that conventional F₀ (thermal lethality) calculations cannot be directly applied to PATS processes; validation requires direct microbiological challenge studies

Regulatory status

United States: The FDA accepted PATS for the commercial sterilisation of mashed potato in 2009, following a petition by the National Center for Food Safety and Technology (NCFST) at Illinois Institute of Technology. This was the first regulatory acceptance worldwide of a PATS process for a shelf-stable low-acid food. Subsequent applications must individually comply with 21 CFR Parts 108 and 113 and provide process authority documentation including critical factors and inoculated pack studies.

European Union: PATS processes for low-acid shelf-stable products are subject to existing EU frameworks for thermal processing and novel food processes. No equivalent to the US NCFST petition process exists; regulatory acceptance is managed on a product-by-product basis with national competent authorities and the European Food Safety Authority (EFSA).

Key publications

• De Heij, W.B.C. et al. (2003). “High pressure sterilisation: maximising the benefits of adiabatic heating.” Food Technology, 57(3), 37–41. Wageningen University & Research.
• Ting, E. et al. (2002). “Pressure-assisted thermal sterilization validation.” In: High Pressure Processing of Food. Springer.
• Balasubramaniam, V.M. et al. (2009). “Pressure-assisted thermal processing.” Food Science and Technology International, review article.
• Sevenich, R. et al. (2021). “Assessing the pressure’s direct contribution to the efficacy of pressure-assisted thermal sterilization.” Food Engineering Reviews, 13, 726–745.
• Koutchma, T. et al. (2009). “Reaction kinetics analysis of chemical changes in pressure-assisted thermal processing.” Food Engineering Reviews, 1(1), 56–68.
• Albahr, S. et al. (2025). “Storage stability of selected vegetable purees in high barrier pouches processed with pressure-assisted thermal sterilization.” Journal of Food Process Engineering, 48(3).
• High Pressure Research (2017). “Processing of baby food using pressure-assisted thermal sterilization (PATS) and comparison with thermal treatment.” High Pressure Research, 37(4).
• NCFST/IIT press release (2009). “NCFST receives regulatory acceptance for novel food sterilization process.” Illinois Institute of Technology.
• IFT (2003). “High-pressure sterilization: maximizing the benefits of adiabatic heating.” Food Technology, 57(3).