Observed distribution of radiocaesium contamination in shiitake lots and variability of test results

Shiitake mushrooms (Lentinula edodes) cultivated on bed-log are known to accumulate radiocaesium. Since the Fukushima-Diichi nuclear power plant accident (2011), the violation rate has been higher for log-cultivated shiitake than that for agricultural products or other foodstuffs. When testing shiitake mushrooms for radionuclide contamination, the validation of the sampling plan can be severely compromised by the heterogeneous contamination within shiitake lots. Currently, few data are available on the statistical properties of the radiocaesium contamination of log-cultivated shiitake. In this paper, shiitake lots contaminated by radiocaesium were identified and the distribution of the radiocaesium concentration within the lots investigated. The risk of misclassifying shiitake lots was predicted from the operating characteristic curve generated from Monte Carlo simulations and the performance of various sampling plans was evaluated. This study provides useful information for deciding on an acceptable level of misclassification risk.