Research – The effectiveness and safety of high pressure food treatment

High pressure treatment (HPP), also known as “high pressure hydrostatic treatment (HHP)” or “ultra pressure treatment (UHP)”, is a non-thermal (<45 °C) food preservation technology that inactivates forms vegetative pathogens and spoilage microorganisms – does not inactivate bacterial spores – using high pressures with minimal effects on taste, texture, appearance or nutritional value.

High pressure treatment is not specifically regulated within the EU, however, according to Regulation (EC) 852/2004 regarding the hygiene of food products, HPP is considered a processing. Any relevant food safety legislation is applicable to HPP ─hygiene requirements, microbiological criteria, food contact materials, traceability and labeling requirements. The guidance document on the application of certain provisions of Regulation (EC) 852/2004 introduces, in section 9.6, clarifications on the implementation of the HPP.

The European Food Safety Authority (EFSA) was asked to issue a scientific opinion on the efficacy (reduction of foodborne pathogen levels) and food safety of HPP. Specifically, the terms of reference of the mandate required: to provide an overview of the foods to which the HPP is or could be applied, together with the processing conditions; list the intrinsic and extrinsic factors that may influence the effectiveness of HPP; and assess the potential chemical and microbiological risks to food safety of HPP-treated foods compared to untreated foods or foods that are routinely applied in order to increase their microbiological food safety.An evaluation of the use of HPP was also requested for two specific purposes: as an alternative to the pasteurization and ultra high temperature (UHT) treatment of raw milk and raw colostrum; and for the control of Listeria monocytogenes in ready-to-eat foods. Quality aspects and organoleptic properties were beyond the scope of this mandate.

Virtually all types of food can be treated with HPP. However, foods that contain trapped air (eg, bread, cakes, whole and freshly cut fruits and vegetables) are not suitable for HPP because their porous structure will be adversely affected. Low-moisture foods, such as powdered products and nuts, are not usually treated with this technology due to low microbial inactivation when the water content is less than 40%.

According to the data collected through a questionnaire in food operators, the relative importance between the types of food that are currently treated with HPP is as follows:

• High – Cooked meat products (including sliced, hot dog , etc.) ready-to-eat, raw-cured meat products (fermented or dried); acidic fruit and vegetable juices, guacamole and ready-to-eat pre-cooked meals.
• Medium – Fruit purees, moist salads (pH <5), and other spreads (eg, hummus, pesto); crustaceans, shellfish, molluscs and derived products; baby food.
• Low : fish and fishery products; milk, raw milk and pasteurized cheese, processed cheese in sauce or spread and dairy products (except cheese).

In the industrial context, to inactivate microorganisms, pressures of between 400 and 600 MPa are applied, for 1.5 to 6 minutes. The water used as a pressure transmission fluid for HPP is often pre-cooled to 4-8 °C.

Typically, products (liquid, semi-solid and solid foods) are packaged in flexible plastic materials prior to HPP to prevent recontamination of the product after HPP. Equipment for processing liquids in bulk before packaging is also available, but is currently rarely used.

According to EFSA’s scientific opinion, the main intrinsic factors in food that influence the effectiveness of HPP in terms of reducing vegetative microorganisms are water activity (a _w ) and pH. Microbial inactivation increases with high values ​​of a _w and low pH values. Carbohydrates, proteins and lipids have a protective effect on microorganisms, which reduces microbial reduction. The main extrinsic factors are pressure and target pressure retention time. The type of microorganism, the taxonomic unit and the strain and the physiological state of the microorganisms to be inactivated also affect the effectiveness of the HPP.

The efficacy of HPP in different food matrices is variable due to the interactions between specific intrinsic factors, which makes it difficult to predict the efficacy of HPP in a complex food matrix.

EFSA recommends that the interactions of the different components be considered in the planning of the assessment of the impact of intrinsic factors on the effectiveness of PPPs and that validation studies be performed on actual food matrices.

Food HPP poses no additional microbiological risk to food safety (eg, spore activation, induction of sublethal cell damage, conversion of normal form of prions to amyloid forms, and induction of virulence, gene expression for toxins and cross-resistance to other stresses) compared to other treatments commonly applied to these foods (eg, thermal pasteurization).

EFSA has also assessed the risk associated with mycotoxins and chemical processing contaminants by establishing that PPH-treated foods do not present a higher risk compared to conventionally treated foods.

HPP does not generate additional chemical food safety hazards from food contact materials compared to foods treated under similar temperature and weather conditions without HPP.

When raw milk, colostrum, dairy products or colostrum products are subjected to a heat treatment, such as pasteurisation or ultra-high temperature (UHT) treatment, the treatment must comply with the conditions laid down in the Regulation (CE) 853/2004. According to this Regulation, if pasteurization is used for these products, food operators must ensure that the following specifications are met: a high temperature for a short period of time (at least 72 ° C for 15 seconds), a low temperature for a long period of time (at least 63 °C for 30 minutes), or any other combination of temperature and time conditions to obtain an equivalent effect.

There is a growing interest in the use of HPP as an alternative treatment to pasteurization and UHT because it is expected to maintain properties closer to those of raw milk and colostrum.

According to the data collected and evaluated by EFSA, it was determined that HPP could not achieve logarithmic reductions (log ₁₀ ) equivalent to those achieved by thermal pasteurization of milk (more than 10 log ₁₀ ) or by UHT (log 10). more than 12 log ₁₀ ). However, HPP conditions could be identified to achieve reductions equivalent to those recommended by international agencies as benchmarks of performance criteria for pasteurization (eg, reductions of 5, 6, 7, and 8 log _{10 ).} ).From the mathematical models obtained, several examples are provided of the minimum requirements (combination of pressure and time) of the HPP that, with a high certainty, would allow to reach the different criteria of operation.

Under the most stringent industrially used HPP conditions (600 MPa for 6 minutes), reductions of 5 log ₁₀ for Mycobacterium bovis , 8 log ₁₀ for Shiga toxin-producing Escherichia coli (ECTS or STEC),Listeria monocytogenes , Salmonella spp . and Campylobacter spp. , and 6 log ₁₀ for Staphyloccoccus aureus .

According to EFSA, no data were found on the impact of HPP on the reduction of Brucella melitensis and tick-borne encephalitis virus (TBEV) and therefore no conclusions could be drawn for these. dangers.

EFSA evaluated several milk and colostrum compounds to determine their suitability as indicators of HPP efficacy, including the endogenous alkaline milk phosphatase enzyme (ALP) – widely used to verify the proper thermal pasteurization of milk. γ-glutamyltransferase (GGT), xanthine oxidase (XoX), β-lactoglobulin (β-Lg) or lactoferrin (LF).

In view of the available evidence, EFSA concludes that none of the evaluated indicators can currently be proposed as an appropriate indicator for use under the commercially viable technological conditions of HPP applied to industry (400 and 600 MPa for 1.5-6 minutes) and recommends further in-depth studies to determine the suitability of such compounds as indicators of HPP efficacy.

The most relevant foods associated with human listeriosis in the EU that are also relevant to be treated with HPP to increase microbiological food safety include ready-to-eat cooked meat products, soft and semi-soft cheeses, fresh cheeses and smoked or marinated fish. .

For ready-to-eat cooked meat products, the minimum requirements (combinations of pressure and retention time) were derived, which would achieve reductions of 1 to 5 log ₁₀ for L. monocytogenes . For the other types of ready-to-eat foods relevant to listeriosis, the high uncertainty of the data did not allow the establishment of generic minimum HPP conditions, so specific validation studies are required for each specific product.

Salmonella spp. and E. coli were identified as the most relevant additional hazards, apart from L. monocytogenes, in ready-to-eat foods associated with human listeriosis. In the foods mentioned, these pathogens ( Salmonella and E. coli ) are generally more sensitive to HPP than L. monocytogenes and are thought to be inactivated to a similar or greater extent.

According to the EFSA report, further studies on the inactivation by HPP of L. monocytogenes and other pathogenic bacteria relevant to ready-to-eat foods, such as smoked fish, marinated fish, soft and semi-soft cheese, would be needed to establish the generic minimum requirements for HPP to ensure the safety of these foods.