Date of Award:

1969

Document Type:

Dissertation

Degree Name:

Doctor of Philosophy (PhD)

Department:

Nutrition, Dietetics, and Food Sciences

Department name when degree awarded

Food Science and Technology

Advisor/Chair:

D. K. Salunkhe

Abstract

In vitro investigations were conducted to study the lethal effect of gamma radiation on the fruit spoilage fungus Penicillium expansum under certain environmental conditions, and to study some physio-chemical changes in the fungus induced by the radiation which may be related to the death of the fungus or its ability to invade fruits.

The radiation sensitivity of P. expansum conidia was influenced by such factors as nutritional condition of the post-irradiation media, age of the spores, delayed plating, heat treatment, and chemical sensitizers. Higher survival was obtained when irradiated conidia were plated on Czapek solution agar (sugar-salts medium) than on potato dextrose agar; yeast extract added in Czapek's medium hastened colony growth but reduced survival. The 6-month-old conidia were more radiation sensitive than the 1-week-old conidia. Recovery from the radiation injuries was observed when irradiated spores were held in the suspension for several days at 23o C. No recovery was found at 1o C. Heat treatment (58o C; 4 minutes) before or after irradiation increased the radiation effect. Difolatan (10 ppm), iodacetic acid (50 ppm), and secondary butylamine (500 ppm) were effective radiation sensitizers to P. expansum. The colony growths were inhibited for various periods following irradiation of the mycelia. The duration of inhibition was influenced by the dose applied and the nutrient of the medium tested. Irradiation of the Czapek solution agar slowed the initial growth of unirradiated fungus. No such effects were found with the other media tested. From these results, it can be concluded that under proper conditions, the fungus can be effectively inactivated by low doses of gamma radiation and thus retain the quality of fresh fruit with a minimum irradiation cost.

Abnormal colonies and mutants were induced by the radiation. Some mutants were no more radiation resistant than the normal strain. The culture grown from irradiated conidia produced more of pectolytic enzyme than did the control after 3 days of post-irradiation incubation. The enzyme production was stimulated by the irradiation.

Nucleic acid content of the mycelia grown from irradiated spores was related to the growth stage of the fungus and not directly influenced by the irradiation. The slowed growth rate of the irradiated fungus indirectly influenced the nucleic acid content. When isotope labeled mycelia were irradiated, no radiation induced degradation of nucleic acid was detected in the fungus during 8 hours of post-irradiation incubation, either by chemical or isotopic analysis.

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