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Shock waves tear food bugs apart
13 November 2004
From New Scientist Print Edition.
Andy Coghlan

Shock waves have been used for the first time to destroy a host of common food bacteria. If the technique can be perfected, it could one day be used instead of pasteurisation to sterilise baby foods, dairy products and fruit juices without spoiling their taste.

The process is being developed by Achim Loske and colleagues at the Autonomous University of Mexico's Centre for Applied Physics and Advanced Technology in Querétaro.

Loske subjected vials of bacteria to shock waves in a device called an electrohydraulic generator, which generates shocks with pressures of up to 1000 atmospheres, accompanied by intense flashes of visible and ultraviolet light. This combination, Loske says, killed bacteria in the vial. "A possible advantage of the treatment is that, as far as we know, shock waves don't change the taste of the food," he says.

The pressure waves cause microscopic air bubbles in the liquid surrounding the bacteria to expand momentarily and then violently collapse - a process known as cavitation - generating small regions of intense heat. This, along with the pressure of the shock wave and the intense pulses of visible and ultraviolet light are what kill the bacteria. "It's a combination of compression, cavitation and electromagnetic radiation," says Loske, whose results will be published in a forthcoming edition of the journal Innovative Food Science and Emerging Technologies.

The system needs further development as it doesn't yet kill enough bacteria to be useful. The best results were with Listeria monocytogenes, a food-borne bug which can trigger miscarriages. Least affected were E. coli 0157:H7 bacteria, which have caused fatal food poisoning outbreaks. At best, populations of bacteria shrank a thousandfold following some 350 shock waves given over 15 minutes.

Loske is confident of achieving million-fold reductions, which would be enough to make food safe. "It's a matter of increasing the shock wave energy and dose, and has been achieved recently in our lab with Listeria monocytogenes," he says. His team is also investigating just how the shock waves kill the bacteria. "We still don't know whether this would be cheaper than conventional technologies," he says.

Morse Solomon, a food safety expert at the US Agricultural Research Service lab in Beltsville, Maryland, says that understanding exactly how the bacteria are killed is essential if the technique is ever to be commercialised. Solomon's team once tried tenderising meat using shock waves from a small dynamite explosion (New Scientist, 23 December 2000, p 10). That also killed some bacteria, but not in practical quantities.

From issue 2473 of New Scientist magazine, 13 November 2004, page 26

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