New food safety treatment

Open up a punnet of strawberries and more often than not you'll find a fuzzy berry or two in the mix. A blast of chlorine dioxide gas, however, promises to not only keep those berries fuzz-free, but also to kill off harmful bacteria living on their surface more efficiently than methods currently used by the food industry, say Purdue University researchers.

"Strawberries are tricky," said Rich Linton, professor of food science and one of the leaders of the current study on decontaminating strawberries. "They're notoriously difficult to clean, and their surface composition actually encourages bugs to grow."

Those bugs can include potentially lethal bacteria, such as E.coli, as well as viruses including hepatitis A, which caused an outbreak linked to frozen strawberries in 1996.

"The issue with strawberries is that they're easily contaminated," Linton said. "They're grown in close association with soil, where they may pick up pathogens such as E.coli from manure-based fertilisers, and they're hand picked, providing another avenue for contamination."

Linton and his colleagues at Purdue's Center for Food Safety Engineering, who already have demonstrated the efficacy of using chlorine dioxide gas to kill pathogens on the surface of apples and green peppers, have shown the treatment also removes significantly higher levels of pathogens than the current industry-standard chlorinated water rinse.

Linton's study, published in the Journal of Food Protection, compares two different chlorine dioxide treatments, called 'batch processing' and 'continuous processing'. Both treatments provide greater than a 5-log, or 99.999%, reduction in the numbers of E.coli and Listeria monocytogenes on strawberry surfaces.

Not only does Linton's treatment significantly reduce the number of potentially harmful pathogens growing on strawberries, it also extends their shelf life without sacrificing quality attributes such as colour and taste.

The two methods Linton used differ in the way the berries are exposed to the chlorine dioxide. In a batch system, the strawberries are placed in a sealed container and a set amount of chlorine dioxide gas is applied once and then allowed to remain in the chamber for a period of time. Continuous treatment involves constant delivery of gas into the chamber over time.

Batch treatment required higher concentrations of chlorine dioxide treatment for longer amounts of time than continuous treatment, but both methods achieved more than a 5-log reduction in pathogens, Linton said.

He found that either 30 minutes of batch treatment, or 10 minutes of continuous treatment, produced effective levels of decontamination.