By subjecting foods to high pressure (HP), microorganisms and enzymes can be inactivated without the degradation of flavour and nutrients associated with traditional thermal processing. The technology was first commercialised in Japan in the early 1990s for the pasteurisation of acid foods for chilled storage. In spite of massive research efforts, particularly in Europe and the USA, commercial development outside Japan has been slow so far, mainly because of the very high investment and processing costs of HP processing as well as regulatory problems in regions such as Europe. Recent examples of HP processing of fruit products are shown in Table 17.1.
The HP process is non-thermal in principle, even if the pressure increase itself causes a small rise in temperature. HP affects all reactions and structural changes where a change in volume is involved, as in the gelation of proteins or starch. The mechanism behind the killing of microorganisms is a combination of such reactions, the breakdown of non-covalent bonds and the puncturing or per-meabilisation of the cell membrane. Vegetative cells are inactivated at about 300MPa at ambient temperature, while spore inactivation requires much higher pressures (600 MPa or more) in combination with a temperature rise to 60-70°C. Certain enzymes are inactivated at 300 MPa, while others are very difficult to inactivate at all within the pressure range which is available today. Moisture level is extremely important in this context, little effect being noticeable below 40% moisture content.
Table 17.1 Recent examples of HP processing of fruit products (adapted from Cheftel, 1997)
Role of HP
Pokka Corp. (ceased in 1998)
Wakayama Food Ind.
Nisshin fine foods
Ehime Co. Takansi
Pon (test market in 2000)
Orchard House Foods Ltd (UK)
United States Avomex
Fruit-based products (pH < 4.5); jams (apple, kiwi, strawberry); jellies; purees; yoghurts; sauces Grapefruit juice
Mandarin juice (winter season only) (only about 20% of HP juice in final juice mix)
Sugar-impregnated tropical fruits (kept at —18°C without freezing) for sorbet and ice cream Ice-nucleating bacteria (for fruit juice and milk) Japanese mandarin juice Fruit juice Orange juice
Fruit juice (orange, grapefruit, citrus, mixed fruit juice) Squeezed orange juice
Avocado paste (guacamole, chipotle sauce, salsa) and pieces
400 MPa, room temperature
Pasteurisation, improved gelation, faster sugar penetration; limiting residual pectinmethylesterase activity Reduced bitterness
Reduced odour of dimethyl sulphide; reduced thermal degradation of methyl methionine sulphoxide; replace first thermal pasteurisation (after juice extraction) and final pasteurisation before packing: 90°C, 3min Faster sugar penetration and water removal
Inactivation of Xanthomonas, no loss of ice nucleating properties Cold pasteurisation Cold pasteurisation /
Inactivation of microflora (up to 106cfug—1), partial inactivation of pectinmethylesterase Inactivation of microflora and enzyme, keeping natural taste
Microorganism inactivation, polyphenoloxidase inactivation, chilled process
Pulsed or oscillating pressurisation is more effective in spore inactivation than continuous pressure. Rapid decompression increases the impact force on the spore coat much more than the preceding compression and makes possible sterilisation at lower pressures than continuous pressure. At low pressures, 50-300MPa, considerable germination of spores can occur, strongly influenced by temperature and pH, which allows organisms to be killed by moderate pressures. In the case of a large variety of moist products, pressurisation to above 100MPa in less than 30 s, mainly at a temperature around 90°C with a holding time of only a few minutes, resulted in complete inactivation of even thermoresistant spores. The combination of nisin (a bacteriosin), high pressure and lowered temperature may allow for a considerable reduction in processing time and/or pressure in HP treatment. Microbial kill is completed without the frequently encountered survival of some pathogens (Hauben et al., 1997; Garcia-Graells et al., 1998).
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