Food preservation has been practiced by humans for millennia through fermentation, salting and drying. The industrialisation of food manufacture brought processes like canning and freezing to control microbial safety and enzymatic spoilage of foodstuffs. However, this often comes at the expense of nutritional and sensorial quality attributes and, thus, novel food processing technologies continue to be developed to serve the increasing demand for healthy and ecofriendly food products. In

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... to thermal processing, these new technologies make use of physical stressors other than just heat to kill microorganisms, using high pressure, electric fields, cool plasma or ultraviolet irradiation. The underlying inactivation mechanisms, efficiencies and limitations of these technologies are currently still under investigation and will be highlighted in this paper. High pressure processing High pressure processing (HPP) is a way to modify and preserve food without using heat. HPP normally involves subjecting food to hydrostatic pressures of 300 to 700 MPa for periods of a few minutes. This treatment inactivates vegetative microorganisms and some enzymes at room temperature, whilst valuable low molecular constituents, such as vitamins, colours and flavourings, remain largely unaffected. Therefore, HPP is increasingly used by the food industry to produce safe and fresh-like food with enhanced nutritional and functional properties and extended shelf life. Currently, there are approximately 200 industrial HPP systems installed worldwide, producing more than 300,000 tons of food per annum. In the Australian market, HPP food includes small goods, fruit juices, vegetable purees, and wet salads. The efficacy of HPP is governed by Le Chatelier's principle, which states that reactions or phase transitions associated with a decrease in volume are favoured, whilst those accompanied with a volume increase are inhibited. Low molecular weight molecules in food such as peptides, lipids and saccharides are rarely affected by HPP because of the very low compressibility of covalent bonds at high pressures 1 . On the other hand, macromolecules, such as proteins and starches, can change their native structure during HPP, in a manner analogous to thermal treatments 2 . The viability of vegetative microorganisms is affected by inducing structural changes at the cell membrane or by the inactivation of enzyme systems which are responsible for the control of metabolic actions 3 . At pressures higher than 300 MPa, significant inactivation of vegetative bacteria, yeasts and viruses has been observed at ambient temperature. The rate and magnitude of microbial inactivation is dependent on the applied pressure and temperature as well as environmental factors such as pH, water activity, salts and other antimicrobials. Foodborne pathogens such as enterohemorrhagic Escherichia coli and Listeria monocytogenes, and food spoilage organisms including Lactobacillus spp. (in acidic food), often exhibit high pressure tolerance compared with other bacteria; possibly because of their relatively higher tolerance to other physical and chemical stressors such as heat or acid. Bacteria may also develop increased resistance to pressure due to their prior growth history, e.g. growth of L. monocytogenes at higher temperatures 4 or stationary phase cells being more pressure resistant 5 . High pressure thermal processing Low-acid food (LAF) that is microbiologically safe and stable is not obtainable by HPP at low or ambient temperature. High pressure thermal (HPT) processing can inactivate bacterial spores through high-pressure treatment at 600 MPa with initial temperatures above 608C 6 . Accelerated and homogeneous heating and cooling of food occurs during HPT processing from the increase in temperature Under the Microscope