Radiation induced chemical changes in food

Treatment of food with ionizing radiation is becoming of increasing importance as a method to improve food hygiene, spoilage reduction and extension of shelf-life. In order to check compliance with existing regulations, several methods have to be available for the identification of irradiated foods. For some fruits and vegetables physical methods based on thermoluminescence and electron spin resonance measurements are existing. There is, however, some lack of knowledge on radiation induced chemical changes in this kind of foodstuff. The main goals of this research project are investigations into the radiation induced chemical changes of the phenolic food constituents of fruits and vegetables and into the detection of specific radiolytic products which may be used as marker compounds for radiation processed food. Food under investigation: strawberries, mushrooms, tomatoes and, depending o requirement, other berries and mangoes. Analytical detection methods: HPLC with diode array- and amperometric detection as well as with a coulometric array detector (16 coulometric working electrodes). In the first part, after elaboration of an optimal HPLC method for the detection of phenolic components in unirradiated fruits and vegetables, various food samples are irradiated and the concentration change of the phenolic components in dependence of dose is determined. These components which show significant changes will be investigated individually (model reaction systems) to elucidate the reaction mechanisms and to detect specific radiolytic products. In the second part irradiated food samples are analysed in respect to the detection of specific radiolytically formed products. These marker compounds will be enriched and isolated by preparative HPLC, identified by LC/MS or MS/MS and the dose/concentration relations are evaluated. In the last part analytical methods for identification of radiation processed fruits and vegetables will be developed.

Irradiation treatment of strawberries and tomatoes do not lead to such a lost of their bioactive components that a reduction of quality can be deduced. The radiation induced chemical changes are in the same order of magnitude as their natural variations in concentration. For strawberries a substance could be detected which may be used as marker for an irradiation treatment. As preservation method for food, irradiation is applied in many countries. The main motives for irradiating fruits and vegetables are shelf life extension by reduction of microorganisms causing food spoilage and sprout inhibition. Ionizing radiation, however, gives rise to chemical changes which could affect food quality. The objective of this project was the detection of chemical changes in fruits and vegetables. For food consisting mainly of water only few data are available. Priority was given to the investigation of polyphenols (phenolic acids and flavonoids). These compounds are present only in minor amounts [low ppm-range (mg/kg)], but they are part of the bioactive substances and have anticancerogenic, antimicrobial and antioxidative properties. The most intensive investigations were done with strawberries and tomatoes. Irradiation had been carried out in various steps up to 6 kGy, in practical application the maximum dose is 4 kGy. In strawberries 15 phenolic compounds could be detected. Five showed a decrease and one an increase as a function of dose, nine of them remained unaffected. In tomatoes five phenolic components could be identified, three of them decreased markedly with irradiation. In unirradiated samples the content of phenolic derivatives varied significantly, depending on variety, origin and degree of ripeness. Since the radiation-induced chemical changes are in the same range as the variations of the phenolic substances between the varieties, it can be conculded that irradiation does not reduce the food quality, based on these compounds. In strawberries one compound could be found which may be usable as marker to prove an irradiation treatment. It exhibits a characteristic linear increase with dose, whereas the zero dose level is insignificant. The chemical structure of this compound is not yet known. Although enrichment steps had been carried out its concentration is <1ppm, hence only the molecular weight could be determined.