Detection of gamma-irradiated red peppers using a combination of 5,6-dihydrothymidine and thymidine as irradiation indicator

A method for determining the irradiation histories of plant-based food items was developed. The developed method was based on that used for determining the irradiation history of animal-based food items. The liquid chromatography–tandem mass spectrometry (LC-MS/MS) technique was used to analyze the total amount of (5S)- and (5R)-5,6-dihydrothymidine (sum of (5S)- and (5R)-DHdThd; total DHdThd), which are radiolytic products of thymidine (dThd). Red peppers were used as a model for plant-based food items. Powdered or homogenized red peppers were processed using lysis buffer-containing proteinase K, RNase A, and cellulase to obtain crude DNA. The crude DNA was further purified using a solid-phase extraction column. The DNA was enzymatically digested to produce nucleosides. Following this, the concentration of total DHdThd to dThd ratio (total DHdThd/dThd) was analyzed as an irradiation indicator using the LC-MS/MS technique. The total DHdThd/dThd in red peppers was increased in a dose-dependent manner. The presence of trace amounts of total DHdThd/dThd (<1/16th–1/4th of that formed from approximately 1 kGy-irradiated red peppers) was detected in samples produced from non-irradiated dried red peppers. It was assumed that the product was formed during the drying process. The presence of trace amounts of total DHdThd does not hinder the process of determination of irradiation history (in the presence of total DHdThd/dThd). Finally, the total DHdThd/dThd, as an irradiation indicator, can be applied to a wide range of animal- and plant-based food items.