Extending the shelf-life of foods without the use of artificial chemicals, radiation or genetic engineering is a big challenge for process engineers. Manufacturers worldwide have been researching processes which will allow them to produce long-life foods which can be branded `preservative-free’.
Traditionally, heat treatment has been the most common technique, as with the pasteurisation of milk, but this can damage delicate flavours and textures. Now, the use of pressure is emerging as an alternative and is proving to have widespread applications.
The effect of pressure on food was studied as long ago as the 19th century, but serious research was not undertaken until the 1980. Ten years later Japan’s Meida-ya Food became the world’s first producer of high-pressure treated food, and the technique has since spread to Europe and the US.
High-pressure food processing works because the strong chemical bonds within food components such as carbohydrates, fats and vitamins, are resistant to the effects of high pressure, while the weaker bonds within the cells of food-rotting bacteria are not. A pressure between 3,000-8,000 bar will also kill disease-causing bacteria, including E.Coli and Listeria, by damaging their cell membranes.
Alstom supplies high-pressure equipment for food processing and has adapted technology developed for other applications, says Bruno Pelletier of the Alstom Fluides et Mecanique’s food processing equipment division, based in Nantes. `Alstom has been manufacturing high-pressure equipment for 30 years,’ he says. `It was originally used in powder metallurgy and in the ceramic industry.’
The company realised the potential of cross-fertilising its high-pressure technology and food businesses in 1990. In 1993, it built its first pilot plant, a semi-continuous system for liquid foods. By 1996, its research started to pay off, with the first high-pressure installation being commissioned for freshly-squeezed orange juice supplier Pampryl.
In most systems, the product to be processed is placed in a pressure vessel and submitted for 1-30 minutes to a pressure of several thousand bar. Water is used as the pressure-transmitting medium.
For packed products, either solid or liquid, the pressure treatment can be done right at the end of the process, when the product is already packed. The packaging must be tight and flexible to transmit the pressure: suitable types include plastic bottles, pouches or trays with flexible lids, and vacuum packs. Liquids such as fruit juices can be injected directly into the pressure vessel. A cylinder then compresses the liquid.
Carl Schaschke, a chemical engineering lecturer and researcher at the University of Strathclyde in Glasgow, has been studying high-pressure processing. His work involves studying the effects of pressure on the molecular structure of the chemicals in food.
He claims that high-pressure processing is applicable to all foods with vegetative micro-organisms, and that it can be used to improve the functional or physical qualities of fish, meats, dairy products and fresh juices, purees, sauces, and desserts. This includes tenderisation of meats or changing the viscosity of desserts.
He adds that pressure treatment is an environmentally friendlier alternative to heat treatment as it is much more energy efficient.
Alstom launched its Hyperbar range of cold pasteurisers in 1996. It is based on a horizontal design which allows automatic loading and unloading. In 1998 it delivered the world’s first industrial high-pressure installation for processing meat products. This unit can treat 600kg/hour of vacuum-packed ham at a pressure of 4,000 bar.
The company’s laboratories in Nantes are equipped with Hyperbar pilot plants. It has also developed a mobile Hyperbar plant, with a volume of 50 litres, which is capable of operating at up to 5,000 bar. It can be rented and shipped to the customers’ plant where it is used to perform large scale operations such as market testing or to check the process consistency on large production batches.
Pelletier believes that customer demand will be the main driving force behind the take-up of the technology by the food industry. `As consumer demand for fresher products with fewer preservatives increases, we expect the market for the technique to grow,’ he says.
Despite Pelletier’s obvious enthusiasm, Schaschke says high-pressure processes cost more than heat treatment. Slow production speeds and the cost of equipment capable of withstanding pressures of up to 5,000 bar make the process more expensive. `It’s not easy convincing commercial manufacturers to invest in this type of processing. I’ve had a number of meetings with several companies but still no investment,’ he admits.
In the longer term, however, Schaschke believes that high-pressure food treatment technology will become cheaper, and the process will be adopted more widely. `There is a lot of experimental work on high-pressure processed foods being done in the UK. Commercially, industry just has to pick up the ball and run with it like the Japanese did a decade ago.’