When olive oil is too old and has oxidized, it is usually rancid. Rancidity is most commonly detected by taste but a chemical test can also check for rancidity. The "rancimat" chemical method is mostly used for large industrial frying operations. Oil doesn't suddenly go rancid, it slowly becomes more oxidized and as it does, the flavor suffers.
Different oils age at different rates. Some olive varieties make oil with more natural antioxidants which resist ageing. These oils may be good for up to 3-4 years if properly stored in unopened containers. Other oils, particularly unfiltered oils, may be unpalatable in a year even if stored well.
A two year old olive oil may taste rancid to some while others don't mind it. Most people would be put off by the taste of any vegetable oil more than 4-5 years old. Rancid oil has fewer antioxidants but is not poisonous. A good percentage of the world's population routinely eat rancid oil because of lack of proper storage conditions and some actually prefer the taste. In historical times olives which had dropped to the ground or which may have spoiled were made into olive oil which was stored in open-mouthed earthenware vats. Practices like these encouraged rancidity. People have come to expect non-rancid oil in the past 50 years because of chemical refining and better production and storage methods.
Fatty acids are oxidized by one of the following mechanisms:
1. "Auto-oxidation" occurs in the absence of air by reactive oxygen species or "free radicals". It is temporarily prevented by natural anti-oxidants in the oil which absorb these free radicals. When the antioxidants are used up, the oil ages quickly
2. Photo-oxidation occurs when a double bond interacts with singlet oxygen produced from O2 by light. This can be 30,000 times faster than auto-oxidation (Frenkel EN et al. Lipids 1979, 14, 961).
3. Enzymatic peroxidation. lipoxygenase and cyclooxygenase enzymes are naturally found in plants and catalyze reactions between oxygen and polyunsaturated acids (http://www.cyberlipid.org/perox/oxid0006.htm#3)
Olive oil will oxidize with exposure to air and dry mixes have very high
surface areas for exposure. Using air-tight or inert gas packaging will help
prevent oxidation. Anti-oxidant food additives will slow oxidation.
Unfortunately auto-oxidation happens in the absence of oxygen and may be
accelerated by exposure to other ingredients.
Many people ask about the proper storage temperature for olive oil. Heat speeds up all of the above reactions. Keeping your oil next to the stove in a clear bottle will quickly age it. Better to keep a large container in a dark, cool cupboard and pour a small amount into a dispenser for everyday use. Olive oil can be put into the refrigerator or freezer without harm, which will greatly extend its shelf life. Waxes in the oil may crystallize out into needles or a slurry when the oil is chilled. Warming the oil back to room temperature will re-liquefy it.
Oil from green olives have higher levels of anti-oxidants such as carotenoids and some varieties naturally have higher levels than others. Blending an oil high in antioxidants with a more bland oil can greatly extend its shelf life. Auto-oxidation proceeds slowly until all anti-oxidants are used up at which time the free radicals attack the fatty acids and the oil quickly becomes rancid. This can happen in 1 to 3 years depending on oil storage conditions and variety. Sometimes an old oil will taste fine when first exposed to the air but a few weeks later can taste old and oxidized whereas a new oil will last for months after opening because it's natural antioxidants have not been used up. Look for olive oil brands which date their oil. Note that for oil made in the northern hemisphere and sold in the year 2005 will often have been picked in the fall and winter of 2004. It is the freshest oil you can buy even though it may be dated the year before.
Olive Oil can be stored in containers as mundane as plastic or as indestructible as stainless steel.
Metallic drums lined with epoxy resins resist light and impart little flavor however resin coatings can peel after several years, exposing metal surfaces which impart off odors and flavors. Most large producers feel that the newer plastic drums are excellent for long term storage and are inexpensive but unattractive.
Glass is an excellent storage container if it is tinted to exclude light but is not practical for bulk storage.
Stainless steel is much better and is considered one of the best storage methods, but has been considered expensive. Custom stainless steel tanks fabricated domestically can cost tens of thousands of dollars. Imported standard sized containers are now fairly reasonable. The Olive Oil Source is currently importing stainless steel tanks from Italy for those who like the look, durability and keeping characteristics. The tanks are specifically made for olive oil in the Imperia region. 500 liter tanks with conical bottoms, welded steel legs, 2 stainless ball valves, a site glass to determine oil level and a floating air-tight lid (see containers). 1000 liter tanks end up being 67 inches tall. 2000 liter tanks are also available. The floating lid has an inner-tube type gasket which can be inflated to exclude air. As the oil level drops, the top drops too, keeping oxygen out. Tanks without floating lids can be filled with nitrogen or inert gas to exclude oxygen. Tanks can be ordered with extra access hatches for easier cleaning. Flat bottom tanks are considerably cheaper than the conicals. A Florentine swirl finish adds 6% to the price.
Stainless Fustis are also available in a variety of sizes from 5 to 100 liters with airtight lids and stainless spigots. These look something like milk cans with handles on the top and are highly polished. Some producers are offering their oil in the smaller fustis or are allowing retail locations to refill the customer's empty bottles out of the larger ones. This encourages brand loyalty and makes buying oil an event.
Bottling equipment is now available which will put a charge of inert gas into the airspace above the oil in the bottle which delays oxidization and rancidity. See bottling equipment and look for sparge options.
Information on this page updated: 10/15/2005