Irradiation enhances fruit and vegetable safety.

Nonthermal processing techniques, such as irradiation processing, have the potential to improve product safety without reducing the fresh-like qualities of minimally processed vegetables. Combining treatments can also improve a product's shelf life.

Researchers at Chapman University have applied irradiation to various fresh-cut products to improve their sensory and quality attributes. Using a protocol designed to recover injured bacterial cells, the investigators evaluated the survival of E. coli O157:H7 in shredded iceberg lettuce and cilantro that had been inoculated with a six-strain cocktail of E. coli O157:H7. This mix was chlorinated at 200 mg/mL, and then it was irradiated at 0.5 kGy using a cobalt 60 source of irradiation.

The combination of chlorination and irradiation led to more than a 4.5 log reduction in E. coli O157:H7 levels for both the cilantro and lettuce. Using irradiation at this level did not cause any softening or wilting of the product. The sensory attributes of either product were not adversely affected.

On another front, diced Roma tomatoes and diced bell peppers were irradiated at various low levels and evaluated for changes in microbial, physical, chemical and sensory properties. The total plate counts and the amount of yeast and molds were significantly reduced in the irradiated samples. At dose levels greater than 1 kGy, the loss of firmness was a limiting factor in the acceptability of the product. However, at low-dose levels, the change was not detectable by a nine-member trained sensory panel.

The scientists also observed a six-log reduction in L. monocytogenes inoculated into refrigerated avocado pulp after it was irradiated at 3 kGy. No significant reduction was observed at 1 kGy. Adjusting the pH to as low as 4.5 did not inhibit Listeria survival in avocado pulp and did not enhance irradiation sensitivity. Although the activity of polyphenoloxidase was slightly enhanced, there was no perceptible change in the rate of browning. Other sensory attributes, such as flavor and aroma, were not affected by irradiation.

Generally, these researchers optimize irradiation techniques for various products, including fruits and vegetables. Some products, such as papayas from Hawaii and imported fruits and vegetables, can be irradiated to ensure that no pests are brought into the country from overseas. In the United States, minimally processed products would be covered under a petition that is not yet approved, but which may be approved soon. The investigators work mostly on minimally processed products.

Further information. Denise Foley, Chapman University, Department of Biological Sciences, One University Dr., Orange, CA 92866; phone: [phone]; fax: [phone]; email: [e-mail].