High pressure processing

HPP stands for high pressure processing. ‘High’ is extremely high and pressure levels of up to 600 MPa (6000 bar) are achieved. To give you an idea of scale, the Mariana Trench is the deepest part of the ocean reaching depths of 11,000 m. The pressure at this lowest point is around 1000 bar. So the industrial HPP equipment can create pressures 6 times higher.

Depending on the intensity, HPP can be used for a variety of applications from preserving food by inactivating microorganisms to protein denaturation and cell disintegration. Pioneering work on the application of high pressure to microorganisms was carried out by Bert Hite in 1899 in the USA. In his work, he reported that milk was ‘kept sweet for longer’ if a pressure of 650 MPa was applied for 10 minutes at room temperature. Some years later, in 1914, Percy William Bridgeman reported the coagulation of protein albumin in eggs at high pressure. However, the first HPP treated products were introduced into the market in Japan as recently as the 1990s, including jams and sauces. These were followed, in the later 1990s, by HPP treated meat products in Spain.

Take a look at this video to see what an industrial HPP machine looks like.

©Hiperbaric.

Small industrial scale HPP machine for treating food. ©DIL

During HPP treatment, the product (already sealed in its final packaging) is submerged in a liquid – usually water – which acts as a pressure transmitting medium. Based on Pascal’s principle, pressure applied to the vessel is transmitted uniformly and immediately to the product. By subjecting the food to high hydrostatic pressures of between 400-600 MPa for a short holding time (usually 1 to 5 minutes), microorganisms are inactivated, while the effects on low-molecular-weight compounds (eg sugar, vitamins, pigments, flavour compounds) are minimal. Applying pressure to a system shifts the equilibrium towards the state that occupies a smaller volume, meaning that high pressure favours any phenomenon that is accompanied by a decrease in volume (phase transition, conformation of molecules, chemical reactions, etc). This can be exploited to produce a range of different outcomes. Changes to molecular structures such as proteins and hydrocolloids can be achieved by applying lower pressures for shorter times.

As the process leads to a very limited increase in temperature (3-4 °C per 100 MPa), HPP is considered a non-thermal preservation technique. And unlike thermal treatments, the effects of increased pressure are instantaneous and independent of product size and geometry.

There are some practical requirements for both product and packaging though, for high pressure techniques to be successful. The product must have a high enough moisture content for pressure to be transmitted equally throughout. And the product should not have a porous structure or contain air pockets, as air and water have different compressibility. Bread, for example, would not be suitable for HPP treatment as it would completely loose its structure. On the other hand, liquids such as juices are highly suitable, and also meat products, ready-to eat-meals, etc. On the packaging side, flexible airtight packaging should be used such as PET-bottles or vacuum-packages. Glassware and metal cans are obviously not suitable.

A close-up of the container holding PET-bottles of carrot juice, about to enter an HPP machine. ©DIL

While many other preservation technologies are carried out in a continuous process, HPP is performed in a batch mode, which means that at the end of each HPP cycle a finite amount of product is produced. This makes it relatively costly and it’s best suited to high value products. Industrially HPP is used to preserve fruit and vegetable products (juices, smoothies, guacamole), meat products, sea food and ready-to-eat meals.

Research has shown the technique has the potential to influence the structure and sensory characteristics of some products. An example is the high-protein-snack, salami. Normally, this product contains a considerable amount of fat visible as white spots. However, if raw meat is treated with gradually increasing pressure, the colour of the meat changes from red to off-white, just as it does in cooking due to the protein denaturing. Similarly, if you cook or pressure-treat a raw egg, the colour of the albumen changes, in both cases due to the denaturation of the proteins. So HPP-treated meat (which is pale in colour) can be used as an optical fat replacer in high protein low fat snack salamis. Another application currently being investigated is the possibility of reducing salt levels in meat products by applying high pressure. Salt has conventionally been used in meat processing to maintain structure and sensory characteristics, but HPP processing may allow these to be maintained using less salt.

HPP is a non-thermal process with huge potential, especially for the premium market as it preserves food whilst maintaining desirable health and sensory characteristics.