Subjecting live tissue and other cellular structures to isostatic pressure of between 300 to 600 MPa for just a few minutes denaturalises some of the proteins they contain. This process can still not be explained in all its individual details but it seems that longer molecule chains in the proteins are destroyed leading to the modification of their outer coatings and membranes. Watching this process under a microscope reveals that they partly actually “dissolve”. The cell membranes thereby lose some of their original properties, particularly their firmness and stability. This effect which has been known for nearly a hundred years can be put to excellent use in the food processing industry.
One of the possible applications is lengthening the shelf-life of seafood and other foods: the ultra high pressure of about 300 MPa inactivates or kills microorganisms such as bacteria, viruses, funguses and yeasts as well as parasites. This effect can be compared with pasteurisation, only that during High Pressure Processing (HPP) no high temperatures are necessary. Analogous to thermal pasteurisation, HPP is thus also called cold or pressure pasteurisation. The term “pascalisation” is occasionally used, too. This refers to the pascal, the unit for measuring pressure. HPP is an ultra modern, environmentally friendly technology which, compared to conventional pasteurisation, offers some convincing benefits. And it is just as safe because it reliably knocks out dangerous germs (e.g. Listeria, Salmonella, E. coli) which lengthens the shelf-life of the products at least twofold and sometimes even threefold. In contrast to heating, pressure treatment maintains a product’s freshness much better. Sensory properties like flavour and texture, but also colouring, nutritional value and vitamins remain largely intact. High pressure technology enables the production of “clean label” products that do without preservatives and other additives. Furthermore, the technique gets good marks for environmental friendliness, too, because the pressure pumps only need water and electricity, i.e. no chemicals are required.
Pressure treatment possible in airless packs
HPP thus enables the production of products the way today’s consumers want them: fresh and natural, additive-free, germ-free, safe, produced in an environmentally friendly way, with a high convenience grade and a long shelf-life. The food industry’s first attempts to make practical use of HPP technology go back about two decades.
This technology has since asserted itself and is being used increasingly during food production processes: for example, for meat and ready-to-eat products, fruit and vegetables. HPP is a particularly good alternative here because, by avoiding heat, ingredients that are sensitive to higher temperatures such as vitamins and antioxidants remain more intact. The high pressure’s sterilising effect has a preserving effect and so the products do not need to be salted as much which is of benefit with regard to a healthy diet. HPP is also ideally suited to sliced or diced products that are packed in flexible airless bags (e.g. vacuum packs). The pressure takes effect through the elastic cover and kills any germs it contains. The volume of “pressure pasteurised” products is growing continuously: They include high-quality fruit juices and milk products, as well as certain pharmaceutical and cosmetic products. Although the market for HPP products is estimated at only 2 billion US dollars it is growing worldwide at an above-average speed.
High pressure processing offers particular advantages in the seafood sector where in the case of shellfish and crustaceans it offers not only its well-known sterilising effect but also destroys the proteins that are responsible for attaching the meat to the shell. This means that the meat is released completely so that it can be removed fully intact. Mussel and crab meat thus suffers no mechanical damages giving it a more attractive appearance. Yield rises, too, because no meat at all is left in the shell. In the case of scallops and oysters there are not even remains of the muscle that is responsible for closing the shell, and in the case of lobster and spiny lobster the meat can even be removed easily from delicate parts such as the legs or the antennae. The meat from the tail and claws almost falls away from the shell on its own. HPP thus enables nearly 100% yield. This was previously impossible to achieve with raw products, even after meticulous handwork. With the help of HPP it is said that nearly 25% more meat can be gained from lobster claws. Pressure apparently also improves the ability of muscle proteins to bind water which prevents drying of the meat during storage and cooking. And it is of course also of advantage that crab or mussel meat is sterilised during HPP which lengthens shelf-life.
HPP enables the development of high-convenience products
So HPP products have a natural, perfect appearance, as well as a longer shelf-life and this gives them premium character. Apart from that, this technology enables the development of new kinds of products with a high level of convenience. One such is “easy open” oysters which are produced by putting an elastic band around the oyster prior to high pressure processing to keep the shells together. After HPP, the meat is no longer attached to the shell but it remains “ready to eat” in its natural “packaging”. To enjoy it, the consumer only has to remove the elastic band, after which the upper shell can then be lifted without the use of a knife, and the meat slides out. This is a very consumer-friendly product because the easy opening mechanism prevents the risk of accidents that can occur when newcomers to oyster opening try their hand with a knife. Yield is high and after pressure pasteurisation the oysters keep for up to two weeks.
The Canadian company Gourmet Chef Packers has developed a whole range of convenience products based on lobster meat with the intention of shifting lobster out of its “noble” niche. The high pressure technique enables the removal and use of meat from complicated body parts such as the legs and this is noticeable in the price asked for such products. The spectrum of these products includes seasoned lobster meat and lobster skewers as well as lobster carpaccio (pure lobster meat in the shape of a sausage that can be sliced), and formed sticks of pure lobster meat. This product is ideal for the gastronomy sector in that it is relatively low-priced and very versatile in its use.
High pressure processing is an amazingly uncomplicated process. At the heart of the technology are special high pressure machines that are in the meantime available from nearly all manufacturers as either vertical or horizontal constructions. With these machines extremely high pressures of between 300 and 600 MPa can be reached, as are necessary to achieve the described effects. In comparison: 600 MPa correspond to a sea depth of 60 km, more than five times the pressure at the deepest point in the ocean, the Mariana Trench which is about 11 km deep. The products are put into the pressure chamber and, after this has been filled with water and tightly sealed, are subjected to pressure for a certain time. Two to three minutes are usually sufficient. The water in the chamber ensures even distribution of the pressure which can then take effect on the product from all sides. This also explains why the products are not squashed despite the immense pressure and can at the end of the treatment be taken out of the chamber without any deformation.
High costs currently impede wide usage
In spite of all the merits of High Pressure Processing there are several drawbacks that should not be overlooked. The relatively low product throughput is certainly one of the reasons why this technology is not suited to broad use within the food industry. Manufacturers like the Spanish NC Hyperbaric or the US-American Avure offer high pressure systems with capacities ranging from just a few litres (mainly for use in the research and development sectors) to several hundred litres, but even the biggest pressure chambers are by no means sufficient to cover the requirements of large processing companies. Apart from that, pressure treatment is only possible in batches which is not particularly good for continuous mass production. At the moment the technology is mainly used by small and medium-sized companies that produce high-value products in not very large numbers.
Furthermore, scientific tests point to the fact that some foods are more strongly modified by HPP than was presumed so far. As stated above, high pressure is known to influence proteins and the functionality of cell membranes which also play important roles in cell metabolism. Experiments with fish that is rich in fat have revealed that the activity of lipases, enzymes which break down fat, is not slowed down by high pressure but, on the contrary, is accelerated. In the first three to six days after treatment, fat oxidation rises rapidly. This can be seen, for example, in a reduced Omega 3 fatty acid content. This would, of course, not be very desirable because the health value of fish and seafood is a key sales argument. Further, the researchers criticised that some fillets looked “cooked” in places, despite the fact that the temperature used in the experiments did not rise above 35°C. They admit, however, that so far little is known about the processes that take place during pressure treatment and that, by changing the basic conditions, it was very likely that the results could be optimised.
The main obstacle to the use of HPP, however, is the high costs of this technology. The investment costs alone for a small plant with a throughput of 150 kg per hour are about half a million dollars. The price of a plant with a throughput of 1,500 kg per hour already exceeds the one million dollar mark. Added to this are the running costs for processing which, depending on product throughput, are said to be between 4 and 12 cents per kg (the bigger and the more efficient the system, the lower the cost). So although high pressure processing has a lot of advantages, in the face of such high costs a lot of small potential users will probably decide against it. One solution to this dilemma might be to set up specialised companies that offer high pressure processing as a service. The pay-per-use principle would make the technology more attractive for a lot of customers, particularly since HPP products are very suitable for exporting to demanding markets with ambitious requirements... and HPP technology is recognised by the FDA, the USDA and other international food safety authorities.