For oils that you may use infrequently or over a long period of time, such as sesame, walnut, chili, peanut and the like, or big jugs of anything, you can prevent them from becoming rancid after you open them by adding the contents of a capsule of vitamin E, an antioxident, to the bottle and mixing. I though this was New Age hooey when I first read it but it really works. My Mom, not a gifted cook, always cooked in Wesson oil, and for years I avoided it like the plague because of hugley unpleasant childhood memories. It wasn't until years later that I realized that it wasn't that Wesson oil sucked, but the nasty taste and smell were because the oil had turned rancid. Try it!
I've used a can of CO2 wine saver that you discharge in the oil bottle before capping. This displaces the oxygen. Then I keep it in the fridge. I have a two year old can of walnut oil that still tastes fresh.
If you feel the oil has already "gone south" - Is there anything that can be done to rectify it? Must you add the vitamin E when the oil is first opened? Please advise. Thanks