Effect of varying relative humidity on the rancidity of cashew (Anacardium occidentale L.) kernel oil by lipolytic organisms
Post harvest deterioration by microbes due to improper storage condition is considered to be the major cause of spoilage and rancidity of most oil-bearing seeds like cashew nuts through lipolytic action of lipase enzyme. Roasted cashew nuts were subjected to four different storage conditions with different relative humidity of 30, 70, 80 and 90%, respectively for a period of twelve days. Oil extracted from cashew kernel at each storage condition was examined for rancidity. Eight fungal species and three bacteria were obtained and identified as Aspergillus niger, Rhizopus sp., Penicillium sp., Trichoderma sp., Botryodiplodia sp., Fusarium sp., Aspergillus flavus and Aspergillus ochraceous while the bacteria isolates were identified as Bacillus subtilis, Bacillus licheniformis and Staphylococcus sp., respectively. Rancidity of extracted cashew oil was observed with cashew kernel stored at 70, 80 and 90% of relative humidity with increase in time while no level of rancidity was detected at 30% of relative humidity judging by the peroxide value, free fatty acid value, specific gravity and acid value of the oil. Isolates with the highest frequency of occurrence were found to be A. niger and Rhizopus sp. for fungal and B. subtilis and B. licheniformis for the bacterial isolates. These four isolates were screened for conditions for optimal growth and lipolytic activities in a chemically defined medium. Result of the assay shows that lipase enzyme produced by A. niger had the highest lipolytic activity at 37°C.
Key words: Cashew kernel, storage conditions, rancidity, lipolytic.