November 22, 2004

Fuzz-free strawberries forecast with new food safety treatment

WEST LAFAYETTE, Ind. - Open up a pint of strawberries from the grocery store, 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.

Yingchang Han
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"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. coil from manure-based fertilizers, 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 current issue of 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 percent, reduction in the numbers of E. coli and Listeria monocytogenes on strawberry surfaces.

Food safety experts assess decontamination efficiency with a measurement called "log reduction," which indicates how much contamination can be reduced after a decontamination treatment. A log, or logarithm, is a power of ten; thus a 1-log reduction is a 90 percent reduction; a 2-log reduction is a 99 percent reduction, and a 5-log reduction is a 99.999 percent reduction.

While current methods for removing pathogens on strawberries yield about a 2.5 log reduction in bacteria levels, the Food and Drug Administration has stated produce treatments should achieve a 5-log reduction in pathogens.

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 color and taste.

"The berries last a lot longer after this treatment-in fact, we've had strawberries in the refrigerator for more than six weeks with no mold growth," Linton said.

"If this process can give consumers even one or two more days before the strawberries they buy get fuzzy, that's huge. Think about it - how many strawberries do you have to throw away in a pint? If we could reduce that number, it would be a great advantage for consumers and the industry."

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.

Linton's team currently has funding through the United States Department of Agriculture to scale up this technology and further develop it for use by the food industry.

"We see this technology as a potential intervention for security applied to our food system," Linton said. "It may be possible to develop this technology so that we can begin decontaminating produce while it's in transit.

"Much of our produce comes from other countries where we may have less control over sanitary practices in the field. If we could use technology like this to seal up produce and treat it as it travels from point A to point B, it's a great application for protection of our nation's food supply."

Also participating in this research were Yingchan Han, post-doctoral research associate; Travis Selby and Krista Schultze, graduate students in the Department of Food Science; and Phil Nelson, professor of food science. The U.S. Department of Agriculture Cooperative State esearch and Extension Service and the Food and Drug Administration provided funding for this work.

Writer: Jennifer Cutraro, (765) 496-2050, jcutraro@purdue.edu

Source: Rich Linton, (765) 494-6481, linton@purdue.edu

Ag Communications: (765) 494-2722; Beth Forbes, forbes@purdue.edu
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PHOTO CAPTION:
Yingchang Han, post-doctoral research associate in the Center for Food Safety Engineering at Purdue University, prepares strawberries for a decontamination treatment with chlorine dioxide gas. A new study shows this treatment to be significantly more effective than the current industry-standard chlorinated water rinse. (Purdue Agricultural Communications Photo/Mike Kerper)

A publication-quality photograph is available at https://ftp.purdue.edu/pub/uns/+2004/linton-strawberries.jpg.

Efficacy of chlorine dioxide (ClO2) gas in reducing Escherichia coli O157:H7 and Listeria monocytogenes on strawberries was determined using batch and continuous flow ClO2 gas treatment systems. Effects of continuous ClO2 gas treatment on total aerobic plate count, color, and residual ClO2 and chlorite on strawberries were also evaluated. Strawberries were spot inoculated with 7 to 8 log CFU per strawberry of each pathogen (E. coli O157:H7 and L. monocytogenes), stored for 1 day at 48C, and treated at 228C and 90 to 95% relative humidity with 0.2 to 4.0 mg/liter ClO2 gas for 15 or 30 min using a batch treatment system or with 0.6, 1.8, and 3.0 mg/liter for 10 min using a continuous treatment system. Surviving microbial populations were determined using a membrane-transfer plating recovery method. Increased ClO2 gas concentrations resulted in increased log reductions of each pathogen for both the batch and continuous systems. A batch treatment of strawberries with 4 mg/liter ClO2 for 30 min and continuous treatment with 3 mg/liter ClO2 for 10 min achieved greater than a 5-log reduction for both E. coli O157:H7 and L. monocytogenes. After continuous exposure to 3.0 mg/liter ClO2 gas for 10 min followed by 1 week of storage at 48C, no aerobic microorganisms were detected and the color of the strawberry surface did not change significantly (P . 0.05). Residues of ClO2 and chlorite on strawberries after the treatment were 0.19 6 0.33 mg ClO2 per kg and 1.17 6 2.02 mg Cl2 per kg, respectively, whereas after 1 week of storage no ClO2 residues were detected and residual chlorite levels were down to 0.07 6 0.12 mg Cl2 per kg. These results suggest that ClO2 gas treatment is an effective decontamination technique for improving the safety of strawberries while extending shelf life.

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