I think you need to further research on the purity of the oil , may be by using more optimized chromatographic separation technique (GC-MS for example). From there, you may know which compound is responsible for the off odor. Alternatively you need to know the fatty acid profile of the oil and determine their structure , just to make sure that the flavour is not due to an intramolecular functional group of the fatty acids constituting the oil.
May be you think in this line if you were not. Hope it will help.