A bulletin for the Australian Food Industry November 1999
Contents: Genetically modified foods | International Food Safety Conference | Foodborne outbreaks in Australia | Salmonella in unpasteurised orange juice - US | Preliminary treatment of fruit for fresh juice | High pressure processing of foods | Listeriosis from fruit salad | Cold Chain Guidelines
High pressure processing of foods
In the last few years there has been growing research and commercial interest in non-thermal techniques to pasteurize foods. High pressure and pulsed electric fields are two such technologies.
The driving force to find new processing techniques is the potential market for convenient foods that more closely resemble fresh foods than processed foods but which still have an extended storage life. This represents a considerable challenge for food microbiologists and technologists. Most of the steps we are seeking to remove, e.g. a thermal process, chemical preservatives, act as safety or stability factors. Refrigeration is still acceptable but cannot be relied upon as the single hurdle for microbial growth.
High pressure alone or in combination with other technologies offers potential for producing foods with the characteristics outlined above. It has already been commercialized to a limited extent, especially in Japan, with the main drawback being the high capital cost of equipment. This is likely to change in the future with new developments in pump and vessel technology. The primary focus for exploitation to date has been with acid products such as jams and fruit juices and refrigerated foods, notably guacamole and some fish products.
High pressure technology utilizes pressures in the range 3400 to 6800 atmospheres. Normally, water is the transmitting medium and the pressure is applied uniformly and instantaneously. Since the pressure is isostatic (uniform throughout the food), the food is preserved evenly throughout. No permanent deformation of a packaged food occurs because water, the main component of most foods, is relatively incompressible.
The lethality of high pressure on microorganisms is primarily attributed to the disruption of the cell membrane and the denaturation of proteins including some enzymes. Microorganisms vary in their sensitivity to high pressure, with Gram negative bacteria being most sensitive and bacterial spores being the most resistant.
The high resistance of spores has to date limited the application of the technology to acid foods or foods which will be refrigerated to control outgrowth.
- some food enzymes are very resistant to pressure. These include polyphenoloxidase and pectin esterase, both important in food spoilage;
- as a result, most foods processed by high pressure require low temperature storage to retain their sensory qualities;
- residual oxygen in the food as well as enzyme activity can lead to degradation;
- many products may require aseptic packaging.
The potential for improved sensory quality outweighs these disadvantages at least some of which can be overcome by combination processes.
Food Science Australia plans to commence a research program in high pressure processing and would welcome input from companies interested in the process. The project will be led by Dr Martin Cole. Dr Cole has recently joined Food Science Australia after extensive international experience including tenures with Unilever Research in the UK, and The Netherlands and Nabisco in the United States.
For further details contact:
Food Science Australia, Sydney
Telephone 02 9490 8333
Food Science Australia
PO Box 52, North Ryde 1670. Tel +61 2 9490 8397 Fax +61 2 9490 8499
Email enquiries@csiro.au