Efficacy of cocoa (Theobroma Cacao l.) Leaf extract in the control of storage spoilage microorganisms associated with export grade white yam (Dioscorea rotundata poir.)

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August, 2015
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Ghana is the largest exporter of yam in West Africa. There are constraints associated with yam storage as a result of yam rot during and after the pre-export handling period. The predominant method of control is the use of synthetic pesticides, which could pose health hazard to consumers. Therefore, this study aimed to test the efficacy of cocoa leaves (CL) extracts as an alternative to synthetic pesticides. A preliminary survey was conducted on pre-export handling operations. Fresh cocoa leaves were collected, steam-blanched, dried, powdered and qualitatively analyzed. Polyphenolic compounds were extracted using standard methods. Crude Aqueous CL Extract was also prepared by cold maceration for 7 days. Antioxidant assays conducted on the respective extracts such as total phenol content and total antioxidant capacity. Fungi were isolated and identified from yam collected from marketplaces and exporters’ warehouses. Agar Well Diffusion and Broth Micro Dilution Assays were used to determine the antimicrobial activities and Minimum Inhibitory Concentrations (MICs) in vitro. In vivo, separately treated white yams with extracts were inoculated with test fungi. The experimental design was factorially arranged in CRD. Ethylacetate Purified CL Extract recorded the highest in both total phenol content and total antioxidant capacity whereas the Crude Aqueous CL Extract was the least active. The occurrences of Apergillius flavus, Aspergillus niger, Rhizopus stolinifa and Penicillium spp. isolated and identified from the samples collected, recorded 15.15%, 18.18%, 27.27% and 39.39% respectively. Among the extracts, Ethylacetate Purified CL Extract (5.4mm) was significantly (p< 0.5) effective followed by Acetone Purified CL Extract (3.13mm), while Crude Aqueous CL Extract (0.0mm) showed no inhibition against the four test fungi in the Agar Well Diffusion Assay. Regarding the MICs, the most effective extract was obtained from Mancozeb (2mg/ml) followed Ethylacetate Purified CL Extract (2.5mg/ml) and the least was Crude Aqueous CL Extract (15mg/ml) in vitro. Crud Aqueous CL Extract and Distilled Water treated tubers significantly showed the highest and lowest weight losses respectively. Percentage incidence of rot was significantly higher on tubers treated with Distilled Water (6.94%), followed by Crude Aqueous CL Extract (4.1%), Ethylacetate Purified CL Extract (2.78%) and Mancozeb (2.78%). Hence cocoa leaf extracts especially Ethylacetate Purified CL Extract, which showed similar potential to that of Mancozeb, could be an ideal alternative to synthetic pesticide use in controlling rot-causing pathogens on export grade white yam.
A thesis submitted to the School of Research and Graduate Studies, Kwame Nkrumah University of Science and Technology in partial fulfillment of the requirements for the degree of Master of Philosophy (Postharvest Technology)