Let's start with the basic question: what is olive oil? Unlike most other common cooking oils, olive oil is really a form of fruit juice squeezed from olives, the fruit of the olive tree. Traditionally, olive oil was extracted by pressing or crushing the olives with heavy stones. Today, the more modern centrifuge is the common industrial method, improving efficiency and quality.
There are two basic types of olive oil: virgin and refined.
Virgin olive oil is that obtained from olives only be mechanical means: traditionally the press or the modern centrifuge. Thus the oil has undergone no chemical alteration, but merely separated from the water, flesh and pit of the olive. Some important virgin oil terms:
Extra-virgin olive oil refers to the highest quality of virgin olive oil. The technical, albeit subjective definition, is oil free from any flavor defects and that has a noticeable fruity flavor. The International Olive Council (IOC) has further defined standards for extra-virgin olive oil, which state that the oil's oleic acidity should be no more than .08 percent. The IOC's lower grades of virgin olive oil are virgin, ordinary virgin and lampante, the last of which is literally "lamp oil" not fit for human consumption. Extra-virgin oil is suitable for a variety of uses, although not high-temperature cooking, since the heat can alter the olive oil's flavor and the oil has a lower smoke point than refined olive oil.
Refined olive oil is olive oil that has gone through a chemical process to remove unwanted flavors and odors. When sold, it is often mixed with some extra-virgin olive oil to add back some flavor, although compared to extra-virgin oil, it has very little olive flavor. It has a high smoke point and lacks flavors that can turn bad from high heat. Thus, this oil is more suited for high-heat cooking, similar to vegetable, canola or peanut oil.
So what do you find in the grocery store? Generally, there's lots of extra-virgin olive oil, with little or no options that claim to be merely "virgin" or "ordinary virgin" (if you see "lampante" in the store, run). Refined olive oils are sold under a variety of names, including light, extra light, pure, classic or just plain "olive oil."
Although many refined olive oils carry the same FDA-approved health claim as extra-virgin olive oils, the refining will likely mean they are lower in antioxidants than extra-virgin olive oil. Antioxidants in extra-virgin olive oil are also destroyed over time and from exposure to light, so it's better to buy oil that's bottled in dark glass. Despite the use of the "light" label, all olive oil has the same fat content.
There is disagreement about whether extra-virgin olive oil should be used for cooking involving heat. Some suggest that refined olive oil should be used instead, since the delicate flavors of extra-virgin olive oil are destroyed and high heat could even make it taste bitter. Others disagree, including myself. I saute with extra-virgin olive oil on medium heat all the time and my food turns out great. If I was doing something with high heat, I would use another oil, however (like vegetable or canola) but for my usual purposes, I see no reason to substitute refined olive oil.
Later in the week, I'll do a taste test of a selection of California and European oils.