Olive oil is formed of triglycerides.  Tryglycerides are triesters of
glycerol (an alcohol) and fatty acids (oleic in olive oil). While values of
oils are returned in the form of percentage of fatty acids they contain
almost none in the free
form.   To be consumable the free acid content must be
low.
A sample of olive oil is tested with a standard dose of KOH to dertermine
it's acid index.  This index is used to determine the percentage of free
acid in the oil.

"Extra Virgin olive oil
Virgin olive oil which has a free acidity, expressed as oleic acid, of not
more than 0.8 grams per 100 grams (0.8%), and the other characteristics of
which correspond to those fixed for this category. Extra Virgin olive oil
accounts for less than 10% of oil in many producing countries. Used on
salads, added at the table to soups and stews and for dipping.

Virgin olive oil
Virgin olive oil which has a free acidity, expressed as oleic acid, of not
more than 2 grams per 100 grams (2.0%) and the other characteristics of
which correspond to those fixed for this category in this standard.

Ordinary Virgin Olive Oil
Virgin olive oil which has a free acidity, expressed as oleic acid, of not
more than 3.3 grams per 100 grams and the other characteristics of which
correspond to those fixed for this category in this standard. Ordinary oil
may still be fine for frying or where flavor is not wanted or needed."

"Storage: Olive Oil can be stored in containers as mundane as plastic or as
indestructible as stainless steel. Researchers have shown that oil stored in
polyethylene bottles exposed to light can develop unacceptable limits of
peroxide in as little as 20 days while if stored in dark it can last 120-190
days
Oil deteriorates through the action of lipase and other enzymes in the oil
and the action of oxygen. Oxidation or rancidity speeds up with light and
heat exposure. Keeping your oil next to the stove 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.
"Auto-oxidation" occurs in the absence of air and is prevented by natural
anti-oxidants in the oil. Oil from green olives have higher levels of
anti-oxidants 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 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.
www.oliveoilsource.com
"