Behavior of Radioactive Cesium and Strontium-90 during Processing of Soybean into Traditional Japanese Products

Following the Fukushima Daiichi Nuclear Power Plant accident in 2011, concerns over radionuclide contamination in food, particularly radioactive cesium (r-Cs: ¹³⁴Cs and ¹³⁷Cs) and strontium-90 (⁹⁰Sr), have persisted. While extensive studies have examined the behavior of r-Cs during food processing, limited data exist regarding ⁹⁰Sr, especially in traditional Japanese foods. This study investigated the changes in both the concentration and total amount of r-Cs and ⁹⁰Sr during the processing of soybeans into soy milk, tofu, and yuba (tofu skin). Soybeans cultivated in Fukushima Prefecture were processed using standard domestic methods, and radioactivity was measured using high-purity germanium γ-spectrometry for r-Cs and a low-background gas-flow counter for ⁹⁰Sr. Results showed that approximately 65% and 30% of r-Cs in raw soybeans were distributed to soy milk and okara (soy pulp), respectively. Tofu and yuba, prepared from soy milk, retained 21% and 27% of the r-Cs present in the soy milk, respectively. Notably, the r-Cs concentration in yuba was about 1.7 times higher than in the soy milk due to water evaporation during heating. In contrast, ⁹⁰Sr was only quantifiable in okara, with approximately 64% of the ⁹⁰Sr in raw soybeans retained, suggesting distinct behavior compared to r-Cs. The distribution differences may be attributed to the differing tissue localization of calcium and potassium analogues in soybeans. These findings indicate the need to consider radionuclide behavior during food processing, especially for traditional soy products, to ensure accurate internal dose assessments and raw material management of processed foods.