# Photo-Stimulated Luminescence Approach for Effective Identification of Irradiated Fruit
## 1 Introduction  
Treatment with ionizing radiation is widely used against microbiological contamination, to preserve the hygienic quality of food during storage and transport and ensure shelf life [1, 2, 3, 4, 5, 6, 7, 8]. In the European Union, food irradiation is regulated by the Directives No. 1999/2/EC e No. 1999/3/EC [9, 10], which state that products treated with ionizing radiation must be labeled as irradiated and that label compliance must be verified, every year, by each Member State, through checks on food at the market stage. Ten analytical methods based on physical, chemical or biological techniques have been standardized by the European Committee for Standardization (CEN), each one being applicable to certain food products. Five of these methods have been validated for fruit in international trials: CEN EN 1788, based on thermally stimulated luminescence or Thermodynamicinescence (TL); CEN EN 1787 and CEN EN 13708, based on Electron Spin Resonance (ESR); CEN EN 1784, based on Gas Chromatography-Mass Spectrometry (GC-MS); CEN EN 13784, based on DNA Comet Assay [11, 12, 13, 14, 15]. Apart from CEN EN 1788, the other analytical methods are applicable only to a few types of fruit and do not always give reliable responses. In particular, the TL technique is applicable to all fruits because it detects the irradiation in theminerals (silicates) of the dust, which is present everywhere. In fact, as shown in Figure 1, the minerals can keep track of irradiation for a long time due to the trapping of the electrons released by ionizing radiation in the crystalline defects (impurities, atom vacancies, etc.). The original arrangement of the charges can be restored by heating the minerals, which gives the electrons the energy necessary to leave the traps coming back to the original sites. When specific sites (the so-called luminescent sites) are involved, these processes are accompanied by light emissions (thermally stimulated luminescence) which allows us to detect irradiation. The TL method is extremely reliable but time-consuming, since measuring the intensity of luminescence emitted by the heated silicates up to 400 \({}^{\circ}\)C and over requires the separation of the minerals from the organic matrix that would produce spurious TL signals at such high temperatures. In addition, to check the quantity of silicates collected from the sample, the CEN EN 1788 Standard procedure [11] requires a second calibration measurement on the same aliquot after laboratory irradiation (1 kGy), which prolongs the duration of the analysis. Finally, such analytical procedure requires expensive equipment, including an irradiation facility and skilled personnel. Consequently, the method has mostly been used for confirmation of the positive or inconsistent results obtained by other methods. Another standardized protocol, CEN EN 13751 [16], also based on the measurement of the luminescence emitted by the mineral contaminants, could be applied, in principle, to all fruits growing in open air. This method, which detects the luminescence stimulated by infrared radiation (photo-stimulated luminescence, PSL) has the advantage of facilitating the analysis on the whole sample directly, including the organic matrix, thus avoiding the long mineral extraction pre-phase and making the analysis procedure very fast (only 60 s for the measurement) and suitable for sample screening. However, this advantage may also represent a limitation in the case of \"clean\" matrices, where the quantity of minerals in the single test aliquot can be very low and insufficient for the analysis. This makes the reliability of the PSL method crucially linked to the mineral contamination that varies among the products, depending on their exposure to dust throughout the entire production chain. As an example, the PSL signals recorded with different irradiated herbs and spices can vary even by a factor of 10\({}^{6}\)[17; 18; 19].