The Use of Ionising Radiation from 60co Gamma Source in Controlling Mouldiness in Dried Cocoa Beans

Abstract

Mouldiness in stored cocoa beans in Ghana and the production of aflatoxin have been studied. Based on actual weight of discarded beans, mouldy beans have been estimated to constitute 0.13 % and 0.00002 % of marketable beans at the farmers’ level and the buying agents’ depots respectively in the Tafo District. This is contrasted with an estimated value o f 0.16 % obtained in a questionnaire type study involving farmers. Estimated mouldy beans at the Tema port was 0.69 % per year (based on the cut test) representing a financial loss of $1,688,637.19 per year at S989/T should the mouldy beans be discarded. Fifty-eight (58) internally- and externally- borne fungal species were isolated from dried cocoa beans. Of these, forty-eight (48) were internally- borne and ten (10) were superficial. Twenty-nine (29) o f the internally occurring fungi have been recorded for the first time on cocoa beans in Ghana. Twenty-six (26) o f the fungi isolated belong to Aspergillus grou/?. They included A. parasiticus and A. fla vu s, which can produce aflatoxins. Five (5) belong to Penicillium, eight (8) to Fusarium and nineteen (19) to other species. Ionising radiation effectively controlled fungi associated with mouldiness in cocoa beans in a dose - dependent manner. A radiation dose o f 6 kGy completely inactivated the moulds. A. flavus and A. tamarii were the most radiation resistant moulds encountered. The moisture content o f the beans before, during and after irradiation influenced the effect of radiation. The relative humidity during storage and the type o f packaging also influenced the radiation effect. Conidia of-A fla vu s subjected to moist heat at temperatures 20 °C to 60 °C for 2.5, 5 and 10 min respectively were not significantly affected by heating up to 50 °C. Pleating an aqueous conidial suspension at 60 °C for at least 2.5 min reduced the number of fungal colonies by at least 5 log cycles

when the suspension was assayed on agar plate media. Heating at 59 °C for 10 min completely inactivated the wet conidia of A. flams. When a combination o f moist heat and radiation was applied to A. flavus, in different media, the inactivation dose was different in each medium. In vitro studies on A. flavus in suspension showed that the fungus could be inactivated by applying a combination o f moist heat at 50°C for 10 min followed by 1.0 kGy radiation. A combination of moist heat at 80 °C and radiation at 3.5 kGy produced the same effect with cocoa beans. Storage o f beans for 28 days at 75% RH suppressed fungal growth while storage at 90% RH resulted in mouldiness o f beans previously inoculated with A. fla m s conidia and treated with heat and radiation. No aflatoxin was detected in cocoa beans after 4 weeks following inoculation with conidia of a toxigenic A. f la w s strain and irradiation at 10 kGy. Without irradiation, inoculated beans became mouldy and produced aflatoxin Bi. Irradiation at 10 kGy could not destroy the already formed aflatoxin. Radiation up to 6 kGy gave cocoa butter with free fatty acid, melting point, saponification value, and unsaponifiable matter levels comparable with factory standards. Exposure of beans to radiation did not have any effect on the rancidity o f the extracted cocoa butter. Exposure of beans to radiation only or to radiation after heat treatment also did not affect the rate at which cocoa butter solidified. No significant differences in taste, colour and flavour were detected in cocoa drink prepared from cocoa powder produced from irradiated beans.