Cooking oils, even high quality ones, once opened, will begin to go rancid with exposure to air and light over time.

This is especially so with olive oil, which should always be purchased packaged in tinted glass or opaque bottles.

Whenever you open a new bottle of cooking oil, break open several capsules of Vitamin E and squirt it directly into the bottle. Vitamin E acts as an additional antioxidant to quench free radical production as the bottle is open over time.