Engineers Reduce Radiation to Kill Pathogens on Produce

A team of engineers with the Texas AgriLife Extension Service in College Station has developed a way to cut by as much as half the amount of irradiation needed to kill 99.999 percent of salmonella, E. coli, and other pathogens on fresh produce.

By packing produce in a Mylar (polyester plastic) bag filled with pure oxygen, AgriLife Research food safety engineer Carmen Gomes and her colleagues found they could significantly reduce the amount of radiation needed to kill those pathogens. Gomes says reducing the amount of radiation is not so much a safety measure as it is a way to preserve quality of the produce.

The U.S. Food and Drug Administration approved the use of irradiation at dosages of up to 4,000 gray on leafy greens such as spinach. A gray is a measure of ionizing radiation equal to the absorption of 1 joule of ionizing radiation by 1 kilogram of matter.

Though being exposed to a gray of radiation would be lethal for a human, the radiation leaves no residue on the vegetables, and the vegetables are perfectly safe for human consumption after the process. Also, the energy of the electrons used is too low for the materials to become radioactive.

However, radiation alone at those levels has one problem: it would also cause the vegetables to decay and lose their freshness, which consumers would not like. The researchers knew that electron beams also generate ozone, a corrosive form of oxygen, which is a drawback for many applications.

In this case, however, ozone has an anti-bacterial function, which could help kill any pathogens on the produce. And while ozone is considered hazardous to human health, it naturally and quickly converts back to oxygen.

To test the concept of using oxygen with radiation, the researchers packed the vegetables in Mylar bags filled with pure oxygen, a nitrogen oxygen mix or plain air. Prior to bagging, the team uniformly inoculated both fresh and frozen spinach samples with either salmonella or listeria cultures. They then subjected the sample bags to various levels of radiation.

After irradiation, the team tested the samples for pathogens. Their test showed modified packaging containing either pure oxygen or the nitrogen/oxygen mix increased the sensitivity of salmonella or listeria to radiation without changing the way the radiation affected the vegetables, Gomes said. Thus, with the bacteria more vulnerable to the radiation, they could sharply reduce the radiation and still kill the same quantity of bacteria.

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