Research Matters - Sterilising lettuce by radiation

Recent outbreaks of food poisoning in Europe have focused attention again on whether the systems for sterilising fresh vegetables are adequate. Ionising radiation effectively eliminates human pathogens from fresh produce and it can be used for this purpose in the USA.

In the experiments described here, iceberg lettuce grown in California had their outer leaves removed, were cored and then cut into one-inch-square pieces. Samples (150g) were placed in film bags that were either perforated or not.

The unperforated bags were evacuated and then flushed with nitrogen before both types of bag received 0, 0.5, or 1.0-kilogray (kGy) doses of gamma radiation. The doses were given either within three hours of packaging or the next day. All bags were stored at 4 degsC.

After seven days, the overall quality of the lettuce that received 1.0kGy in sealed bags was adequate, with very little cut-edge browning. This was still the case after 14 days, whereas the quality in most other treatments was poor. Radiation alone was unsatisfactory at preserving fresh-cut iceberg lettuce but it was effective when combined with modified-atmosphere packaging.

Furthermore, there was little difference between lettuce irradiated within three or 24 hours of packaging, which suggests that the irradiation facility could be sited within a 24-hour drive of the packing facility.

Dr Ken Cockshull, Associate Fellow, Warwick Crop Centre, University of Warwick

Effects of Gamma Irradiation, Modified Atmosphere Packaging, and Delay of Irradiation on Quality of Fresh-Cut Iceberg lettuce by Fan and Sokorai (2011). HortScience 46 (2): 273-277. ISHS members can view HortScience from the website

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