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Texas Researchers Devise New Irradiation Method

Food-safety engineers from Texas AgriLife Research at Texas A&M University report they’ve devised a method to reduce the amount of irradiation needed to kill pathogens like Salmonella or E. coli O157:H7 that can contaminate fresh produce.


That’s significant not for safety reasons–irradiation does not cause foods to become radioactive and irradiated food is not harmful–but because cutting the amount of irradiation can keep fresh produce crisp and green while still eliminating illness-causing bacteria.

Two excellent articles on the breakthrough research being done at Texas A&M’s Department of Biological and Agriculture Engineering are Laura Kern’s “Advances in Food Safety: Irradiating Confidence,” and another by Robert Burns.

They describe how the Texas engineers are experimenting not only with lower-dose irradiation, but with microencapsulation, incorporating natural enzymes and extracts such as cinnamon, garlic and cloves that have powerful antimicrobial properties into packaging to help control bacteria and other microbes.  And how the scientists are using CT (computed tomography) scans to help calculate dosimetry so that the smallest, most precise amounts of ionizing radiation can be used to reach every part of a particular fruit or vegetable.

In his article published in AgriLife News, Burns explains that at irradiation dosages of above 1,000 Gray (a Gray is a measure of the ionizing radiation dose), delicate, leafy vegetables begin to discolor, soften and lose some flavor and nutritional value – about what happens when greens are cooked.  Federal regulations permit the use of irradiation at dosages of up to 4,000 Gray on leafy greens such as spinach, but the question has been how to reduce that amount to keep vegetables from turning mushy without reducing irradiation’s effectiveness. 

Texas A&M food safety engineers Rosana Moreira, Elena Castell-Perez and Carmen Gomes wondered if an answer could be found in the packaging process, Burns writes.

The team contaminated fresh and frozen spinach samples with either Salmonella or Listeria cultures, then put the spinach in Mylar bags filled with pure oxygen, a nitrogen oxygen mix and plain air.  They bombarded the bags with an electron-beam gun, a method typically used for irradiation that Burns reports generates ozone.

Ozone is somewhat of a bactericide, so the researchers theorized that if they irradiated a bag of fresh spinach with enough oxygen inside so that it would become ozone, they could use it to work synergistically with irradiation and thus lower the necessary pathogen-killing dose.

The sample bags were subjected to levels of radiation ranging from 0.2 to 1.25 kilo Gray and the researchers discovered they were right – the spinach inside the modified packaging containing either pure oxygen or the nitrogen/oxygen mix was decontaminated but not degraded.

The scientists found they could significantly reduce the amount of radiation and still achieve a 100,000 fold reduction of pathogens–a 99.9999 percent kill rate.

The ozone, the only byproduct of the process that might be considered hazardous to human health, converted back to oxygen within an hour, according to Gomes.

Gomes, in the AgriLife article, notes fresh produce is usually either rinsed with water or water and chlorine to rid it of possible contaminants.  But neither method can completely remove pathogens, which can be lodged in furrows on the outside or even be inside fresh vegetables.

Ionizing irradiation, unlike chlorine washes, can penetrate the produce to zap bacteria, and it  doesn’t leave chemical residues.

Outside of regulatory approaches to ensuring food safety, irradiation is one the best ways to keep contaminated food out of the market, however, it continues to be one of the most misunderstood and controversial issues in food science.  Gomes offers a clear explanation of the process, saying it is analogous to heat treatments that expose milk, juice and cans of vegetables to very high temperatures to kill pathogens.

But unlike ionizing radiation, thermal processes can reduce nutrients such as chlorophyll and carotenoids as well as antioxidants, according to the AgriLife article. And the researchers say they’ve found that  ionizing radiation can actually enhance some nutrients, such as carotene,  in addition to keeping the food fresher longer.

Although irradiated food is increasingly common in the marketplace, consumers still have not fully accepted the process.  And some consumers and food safety advocates reject irradiation as a “techno fix,” arguing that contamination needs to be addressed at the beginning of the food-production chain, not the end.  Officials with the Centers for Disease Control and Prevention have said irradiation offers “great potential” for preventing many of the foodborne illnesses caused by contaminated meat and poultry products, fresh produce and other foods.

© Food Safety News