All olive oils are graded in accordance with the degree of acidity they contain.
The best are cold-pressed, a chemical-free process that involves only pressure, which produces a natural level of low acidity.
Extra virgin olive oil, the cold-pressed result of the first pressing of the olives, is only 1 percent acid. It's considered the finest and fruitiest of the olive oils and is therefore also the most expensive.
After extra virgin, olive oils are classified in order of ascending acidity. Virgin olive oil is also a first-press oil, with a slightly higher level of acidity of between 1 and 3 percent.
Products labeled simply olive oil (once called pure olive oil) contain a combination of refined olive oil and virgin or extra virgin oil.
The International Olive Oil Institute recommends using pure olive oil for frying, since the flavor of extra virgin olive oil tends to break down at frying temperatures, making the added expense a waste.
http://web.foodnetwork.com/food/web/encyclopedia/termdetail/0,7770,4171,00.html"Extra virgin olive oil is one of the few oils that can be eaten without chemical processing. (Nearly every other vegetable oil has not been detoxified and refined with steam and solvents). Fresh pressed olive oil can be eaten immediately and retains the natural flavors, vitamins, minerals, antioxidants, and other healthy products of the ripe olive fruit."
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