Growth Inhibition of Grain Spoilage Fungi by Selected Herbs and Spices Essential Oils

Abstract

Natural plant extracts are promising alternatives for chemical food additives and synthetic pesticides. In this study, essential oils of selected herbs and spices were tested for their antimicrobial activities against Aspergillus flavus and Aspergillus niger, two of the most common food spoilage  microorganisms. Agar disk diffusion assay was used for screening of the most effective essential oils, agar dilution assay was used to determine Minimum Inhibitory Concentration (MIC) of the essential oils and broth dilution assay was employed to the spore germination inhibition assay. Tests were also conducted to examine the effects of the essential oils for sorghum kernel protection against the tested fungi, and the optimal  protective dosages on the sorghum grains were also determined. From the preliminary tests, essential oils of Cinnamomum zeylanicum (Cinnamon) and Thymus schimperi (thymus) were found to be the most effective. However piper nigrum (black pepper) had no effect on the test organisms. In MIC, spore germination inhibition and grain protection assay, cinnamon essential oil was found to be superior where its MIC on the isolates was found to be 0.0156% and its optimum protective dosage on the sorghum grain was 5%. It inhibited spore germination at a concentration of 3ìL/mL. The effect of thymus oil was also very much comparable to these results (no significant difference at P>0.05). Finally, it was concluded that essential oil extracts of cinnamon and thymus can be a useful source of antifungal agents for protection of grain spoilage by fungi.