Glycoalkaloids and phenolic compounds in gamma irradiated potatoes; a food irradiation study on radiation induced stress in vegetable products

Irradiation is a recent preservation method. With the aid of ionizing radiation microorganisms in food can be killed or specific physiological processes in vegetable products can be influenced.In order to study the effects of metabolic radiation stress on quantitative chemical changes in vegetable products, specific target compounds were investigated in stored irradiated potatoes. These target compounds, i.e. glycoalkaloids and phenolic compounds were choosen with a view to food toxicology and food sensoric quality.Much attention has been spent on quantitative analyses of the pre-selected target compounds in the potato samples. Enzymic changes of the polyphenolic compounds were kept to a minimum by the direct freezing of potato slices in liquid nitrogen, freeze-drying and extraction by boiling in 80% ethanol.A quantitative assay for solanidine potato glycoalkaloids was developed from a pre-existing method with minor changes, by which the rapidity of the assay is improved, without affecting its sensitivity. As a specific application, this method was used for the analysis of solanidine glycoalkaloids in industrial potato protein. Because of objections to the colour assays for glycoalkaloids, concerning their specificity (Fitzpatrick & Osman, 1974; Coxon et al., 1979), these quantitative reactions were investigated.Quantitative analyses of phenolics and coumarins were done, starting from the alcoholic extracts. Qualitative data were obtained by analysis of UV spectra and fluorescence of diluted extracts and TLC chromatography on cellulose plates. For quantitative analyses a method was developed by HPLC chromatography of alcoholic extracts. Evidence with respect to identification of scopolin and scopoletin in the alcoholic extracts was obtained by comparison of extracts with and without enzymic hydrolysis with B-glycosidase. Scopoletin was also directly identified by UV, IR and Mass Spectra.Results of glycoalkaloids, analyzed over several seasons, show no significant changes with regard to irradiation dose or storage time. On the other hand a change in phenolic compounds and coumarins was observed.A 10 to 30 fold accumulation of scopolin was found in irradiated (3 kGy) potatoes of the Eba variety after approx. one month's storage at 10 °C, 90% RH and also a decrease in the chlorogenic acid content. For irradiated (3 kGy) potatoes of the Bintje variety, the increase in fluorescent compounds was smaller. Several unidentified phenolic compounds increase in 3 kGy irradiated Eba potatoes, which were detected by UV absorbance at 310 nm.Chemical analyses of samples of irradiated Eba potatoes indicate a dose threshold for accumulation of fluorescent compounds. Below 0.5 kGy no increase is observed.By using the high fluorescence of the accumulated coumarins it was possible to detect accumulation of fluorescent compounds by simply examining potato halves under long wave UV light. The results agree with the chemical analyses of extracts of irradiated potatoes of the Eba and Bintje varieties. In this way potatoes of several varieties irradiated with a dose of 3 kGy and stored for at least 2 weeks at 10 °C could be examined. The results indicate pronounced differences between varieties. Further, a threshold for the accumulation of fluorescent compounds in irradiated Eba potatoes could be determined. In Eba potatoes irradiated above 0.5 kGy accumulation of fluorescent compounds could be seen. By fluorescence microscopy of sections of tubers it was observed that specific cells accumulate fluorescent coumarins. Examination of the fluorescent cells after plasmolysis, indicates a vacuolar origin of these compounds. Similar results have been reported for fungal infected potatoes (Clarke, 1973; Clarke & Baines, 1976). The increase in scopoletin can be explained by the increase in phenyl ammonia lyase, which has been shown to increase in irradiated citrus fruits (Riov et al., 1972) and irradiated potatoes (Pendharkar & Nair, 1975).In view of conflicting earlier reports of increases in mutagenic compounds in alcoholic extracts of irradited potatoes, glycoalkaloids and phenolic compounds as well as the alcoholic extracts of irradiated and control potatoes were examined using a bacterial mutagenicity test system. No increased mutagenicity of extracts or reference compounds were found. These results aqree with the negative findings of Levinsky & Wilson (1975) for mutagenic evaluation of extracts of irradiated potatoes and mutagenicity studies on irradiated potatoes and chlorogenic acid by Hossain et al. (1976).Radiation-induced increase in coumarins is to be expected primarily in vegetable products having a natural coumarin content. Chemical changes as a result of radiolytic processes in a theoretical foodstuff can be estimated. Diehl & Schertz (1975) calculated 55 mg/kg radiolytic decomposition products for a 5 kGy dose. In comparison, in this study an increase in scopolin content from 2 to 60 mg/kg was found in 3 kGy irradiated Eba potatoes after a month's storage at 10 °C caused by metabolic stress.