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Microbiological possibilities to demonstrate that strawberries have been irradiated

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Samples of strawberries were divided into two portions, they were wrapped and one portion was irradiated at 200 krad. A total of 30 samples of Dutch strawberries (10 greenhouse-and 20 outdoor-grown) were treated. The composition of the microflora before and after irradiation was investigated.

It was evident that the flora of the non-irradiated samples was composed mainly of gram-negative rod-shaped bacteria belonging, with only a few exceptions, either to the Enterobacteriaceae-family or to the genusPseudomonas. Also, moulds and yeasts were frequently encountered in varying quantities. Other groups were only incidentally demonstrable.

Moulds and yeasts did not appear to make good indicator organisms. Though their total numbers after irradiation almost always appeared to have undergone a certain reduction, the number seldom became too low to count. Special groups or species of molds and yeasts, showing such sensitivity that they were always absent after irradiation also were not demonstrable.

Gram-negative rod-shaped bacteria on the other hand were always found to be absent after irradiation when solid media were used, even when starting from samples with numbers in the order of magnitude 105–106/gm.

When outdoor-grown strawberries were used this always led to a clear difference from the corresponding untreated samples. Greenhouse strawberries, however, sometimes contain, even if they have not been treated, only a very small number of microorganisms; the possibility should therefore be taken into account that none of the media show any growth without irradiation having taken place.

Consequently, if gram-negative rod-shaped bacteria are present on strawberries (when using solid media) they most probably have not been irradiated. Of course, this does not apply if post-contamination has been possible.

If there is no growth of the above mentioned bacteria on any of the media used, considerable possibility exists that irradiation has taken place.

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Tamminga, S.K., Beumer, R.R., van Kooij, J.G. et al. Microbiological possibilities to demonstrate that strawberries have been irradiated. European J. Appl Microbiol. 1, 79–93 (1975). https://doi.org/10.1007/BF01880622

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  • Mold
  • Special Group
  • Solid Medium
  • Untreated Sample
  • Indicator Organism