When storing olive oil keep in mind that Heat,Air,Light and time are the biggest factors speeding oxidation, allowing olive oil to change flavor or become rancid.Beware of flavored oil,sometimes flavored olive oil is created to hide the rancid taste. Be careful if you try to store the olive oil in the refrigerator,storing oil in the icebox will not harm the oil but, may allow condensation inside the bottle,effecting flavor. Also when water is introduced to oil either through condensation or flavoring as is the case with the "flavored olive oil" bacteria may start growing.
The best way to guarantee fresh oil is to buy olive oil in smaller quantities and more often, this would ensure that the antioxidants retain there full strength.Remember do not use anything plastic to store your oil.Dark glass and stainless steel are the best containers.
Buttery tasting olive oil is rancid oil.Rancid olive oil is bad for you,it oxidizes in your system and creates free radicals,you want to kill free radicals with antioxidants.Oxidization is a fairly slow process and the following multiple factors will effect the speed of the process.And will determine how fast an olive oil will oxidize.
1. Heat...The ideal temperature for storing olive oil is 57°F or 14 degrees C, although a normal room temperature of 70ºF works very well if the olive oil is stored in a dark area where the temperature remains fairly constant.
2. Exposure to light...A kitchen cabinet located away from the stove and away from direct sunlight will work quite well. If you have a wine cellar, store your olive oils there and keep a small amount in your kitchen. Do not put olive oil in a container without a tight cap.
3. Exposure to air, buy a small can of inert gas at the wine store and replace the air in the bottle.
4. Time...Olive oil really does not like to sit around, the longer it sits the more it oxidizes.
5. Time of year picked - Olives picked earlier in the year may have more polyphenols and a longer shelf life(stored longer). Low polyphenol olive oils are often blended with high polyphenol olive oils for longer life.
6. Picking method - I hand pick on my Farm but, in some parts of the world, nets are placed under the trees and the olives are allowed to drop for weeks as they ripen. Remaining olives are beaten from the tree. Some olives may have been off the tree for weeks or months, spoiling in the interim. Olives which are badly bruised during picking will become more rancid.
7. Time to Milling - Live olives start to die when picked. The longer it takes to get to the mill, the more oxidized the oil. Most producers who are trying to produce a premium extra virgin oil will get the olives to the mill within 9 hours of picking.
8. Milling method can be a negative when storing olive oil- During the milling process the olive paste may be exposed to air. The old fashioned stone wheel and hydraulic press with jute mats may be very picturesque but exposes the paste to way too much air.