Olive oil is the flavorful, monounsaturated oil extracted from ripe olives. Olive oils range in color and flavor depending on the grade, the growing region, the quality of the fruit, and the process by which they are extracted.
All olive oils are graded in accordance with the degree of acidity they contain, but the means and conditions of extraction also contribute to the oil's grade.
According to the International Olive Oil Council (IOOC), virgin olive oils are obtained solely by mechanical means under conditions that do not lead to alterations in the oil. This means that virgin oils are extracted through pressing the olives without the use of heat or chemicals that can compromise the product. There are two main grades of virgin oil: virgin olive oil and extra virgin olive oil.
The IOOC dictates that virgin olive oil can contain no more than 2% acidity. Extra virgin oils, which are finer and more expensive, may have no more than 0.8% acidity, and no defects in flavor or aroma. The United States is not a member of the IOOC, but oils from California are regulated by the California Olive Oil Council, whose standards are even stricter: It allows no more than 0.5% acidity.
Oils labeled simply "olive oil" (or pure olive oil ) are a combination of refined olive oil and virgin or extra virgin oils. This grade of olive oil is mostly recommended for frying, since the nuances of flavor and aroma of extra virgin and virgin oils are lost when heated to high temperatures.
For more information on olive oil grades and uses, check out http://www.internationaloliveoil.org.
Since heat destroys the subtle nuances of flavor and aroma, choose inexpensive olive oils of lower grades for high heat cooking.
For dressings and drizzling on finished foods, use the more expensive, flavorful extra virgin oils.
Olive oil can be stored in a cool, dark place for up to 6 months. It can also be refrigerated, where it will last up to a year. When olive oil is chilled, it becomes cloudy and thick, but it will return to a clear, liquid state when brought to room temperature.