How does food irradiation work? Is it safe?

Sam Beattie, a food science professor at Iowa State University, zaps a reply (as told to Jordan Lite):

Irradiation treatments expose food to a dose of ionizing radiation, which disrupts the DNA or proteins of bacteria that make people ill.

Anytime you break bonds in chemicals you are bound to introduce changes, so it is critical that those alterations do not impart any toxicological effects to the food. Irradiation appears to be safe in that sense. The process does produce some unique by-products, but no evidence exists that these cause human illness at the levels found in irradiated food. There was some thought that 2-alkylcyclobutanone, a by-product derived from a fatty acid, could cause cell mutations that might lead to cancer, but the most recent findings suggest otherwise.

Two primary sources provide irradiation: radioactive elements—such as cobalt 60—and the electron beam, or e-beam. Cobalt 60 is an isotope, a traceable radioactive version of the element, that emits gamma rays, whereas the e-beam is an electron-based source. Cobalt 60 has a lower dose rate, so it takes longer—a span of minutes. The e-beam is more intense, with a higher dose rate, so it works in a matter of seconds. We are also experimenting with x-rays as a potential new approach.

In the U.S., irradiation is not as common as it is in some other countries, where preventing spoilage is an important contributor to food security. Among fresh produce, the Food and Drug Administration has approved irradiation to reduce food-borne illness only in leaf spinach and iceberg lettuce. For other food management purposes, however, it is approved for a variety of foods. On imported produce, for instance, irradiation can be employed to knock out pests or to control sprouting and ripening. With meat, it is an approved pasteurization process for killing organisms such as Escherichia coli or Salmonella.

Irradiation is not a blanket treatment. Practitioners tune the dosage and duration for the pathogen that is riskiest and most likely to be found in a given food. In pasteurizing meat, for instance, the radiation might be targeted to kill E. coli rather than Clostridium botulinum spores, which produce the toxin that causes botulism, because E. coli is more likely to be present and to cause problems. Irradiation does not work as well against viruses, although viruses typically appear in food service settings, where personal hygiene comes into play, rather than in processed foods.

Although irradiation is effective in many circumstances, it would not necessarily have prevented the recent Salmonella outbreak in peanut butter. Products high in fat are not as amenable to the treatment, because fats produce unwanted flavors when they break down. After Salmonella outbreaks in 2001 and 2004, the U.S. Department of Agriculture now requires California almonds to be heat-pasteurized or chemically treated. Peanuts are likely to see similar regulation mandating a thermal process—roasting in dry heat or immersion in hot oil.

Indoor plants tend to grow toward the light, so why do trees outdoors grow straight instead of leaning toward the equator? —W. Anderson, Sacramento, Calif.

Edgar Spalding, a botany professor at the University of Wisconsin–Madison, sprouts off:

A plant on a windowsill experiences a stronger light gradient than does a tree outdoors, where gravitational cues can overpower more subtle light-direction cues. Indoor plants get a lot more light on one side than on the other, which activates photoreceptor molecules to a much greater extent on the lit side. This difference is biochemically translated as a growth response, known as phototropism, which makes the plant bend toward the light.

Trees growing at a latitude of, say, 60 degrees are also asymmetrically illuminated because of the slant of the noon sun—approximately 55 degrees at the beginning of the summer growth season—but the difference in light intensity there is smaller and more variable. The modest light gradient experienced by the tree is counteracted by a continuous gravitational influence, known as gravitropism, which guides plant growth upward. The strength of gravitropism trumps phototropism in the tree scenario but not on the windowsill.

The edge of forest gaps provides a good place to observe light-guided tree growth at any latitude. There the effect of a strong light gradient can be seen in the reaching of trees into the gap.

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