Ozone technology advance for packaged foods
By Michael Pollitt
GUARDIAN NEWS SERVICE: Packaged foods have had a rough ride but new ozone technology is delivering a healthier approach.
Ozone in a bag. That's the novel method being developed by the food process engineer Dr Kevin Keener, of Purdue University, Indiana, to eliminate harmful bacteria on packaged foods such as spinach, tomatoes, and whole fruit. But rather than use an ozone generator to pump it in, Keener creates the ozone inside the bags only after they are sealed.
The rising popularity of fresh, packaged foods, from mixed-herb salads to fruit has its health dangers. Supermarket suppliers may use water washes enhanced with chlorine or ozone to eliminate possible bacterial contamination before packaging. However, a quick rinse might not be enough to dislodge powerful pathogens such as E coli O157.
Keener's idea of creating ozone inside food packaging came from his work with atmospheric, non-equilibrium plasmas (ANEP). Ionisation generates free electrons that react with gas molecules in the air to become reactive molecules such as ozone, which then attack bacteria. To form ozone, oxygen (O2) splits into single atoms that reform in threes as ozone (O3). But ANEP requires a special treatment chamber for containment.
"I started thinking about what we could do to recreate ANEP without the costly chamber process," says Keener. "My question became: 'Can we place the generation method outside the container and, if so, what would be our limitations?'"
He turned to the Purdue University research engineer Paul Klockow for help and, using off-the-shelf components costing less than $1,000, built a demonstration device. The prototype generates a high voltage potential (12kV) between electrodes consisting of coils of wire wrapped around dielectric (insulator) plates.
The electrode plates are placed above and beneath a sealed food bag and, once the device is turned on, generate a room-temperature plasma field inside, partly ionising the air and producing ozone. While the outside of the package may increase slightly in temperature, the contents are unaffected by this.
Bacteria are normally killed by a combination of concentration and time so a quick ozonated rinse before bagging won't necessarily do the job. "In a standard food manufacturing process using ozone, wash water or ozonated rinses only contact fruit or vegetables for seconds to minutes. Thus limiting the total concentration-time effect," says Keener.
Those limits are now overcome as the ozone reverts to oxygen over many hours. "In our system, because we are doing it in-package, we can deliver a specific ozone concentration which natural decay processes will convert back to normal after a known time," says Keener.
The longer the gaseous ozone remains present, the more bacteria - such as E coli and salmonella - will be killed. To test their system, Keener and Klockow placed a deadly strain of E coli O157 on spinach leaves. Food safety experts talk of achieving a minimum "5-log" standard, which means reducing bacterial numbers by 100,000-fold - killing an impressive 99.999% of them.
Ozone-in-a-bag technology achieves that and more, and the only drawbacks, so far, are spinach leaf discolouration. "The power required for this process is less than 50W to treat a gallon-size container. It is very low power, indicating a very efficient generation process," adds Keener.
By the time the treated food reached the supermarket it would be free of harmful bacteria and the ozone in the sealed package would also be gone. Keener's current research is looking at how to limit any adverse quality effects, such as reduced shelf life, by adjusting parameters such as gas composition and relative humidity.
"We are still in the evaluation phase for a lot of food applications," says Keener, who has received enquiries from equipment manufacturers and international companies. "We have done treatment with tomatoes and saw little to no changes in the quality." However, when starting with oxygen rather than ordinary air, green spinach was bleached white - finding the right parameters for foods is important.
Gaseous ozone would also, Keener suggests, have an advantage over rinses in penetrating "nooks and crannies" in vegetables such as cauliflower or broccoli. "Can we generate [ozone] inside or have infiltration of the ozone molecules into those places to kill residual bacteria?"
Another step to overcome will be the move from the laboratory to the food packing line. "Our process can generate significant concentrations of ozone within seconds (>100 ppm) and should be adaptable to existing conveyor speeds. It only requires a specified voltage and plate configuration designed to a specific product-package combination," says Keener.
Ian Connerton, professor of food safety at the University of Nottingham, says there are difficulties in dislodging certain bacteria from fresh food such as salads. "Most of the treatment is washing in potable [drinking] water," says Connerton. "The problem is that bacteria can get inside the salad leaf or stuck on the salad leaf so hard you cannot remove them."
Dr Gilbert Shama at Loughborough University is a chemical engineer with research interests in microbial decontamination. He maintains plasmas in air create short-lived charged oxygen and nitrogen species. "Some of these may combine to produce ozone but it would be difficult to attribute any microbial inactivation observed to one particular compound when so many lethal chemical species are produced," says Shama. "The principle, though, is a good one." However, he believes vitamin content, appearance, and taste of treated food would have to remain unaltered: "Consumers will not buy a tomato that looks as if it has been bleached."
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