Generally speaking, olive oils fall into one of two broad categories: unrefined (virgin and extra virgin), and refined (pure and light).
Olives used to make the two virgin, unrefined oils are cold pressed and not treated with heat or chemicals. The olives are simply pressed and squeezed to get the oil out. Olives that produce the highest quality oil in terms of rich taste and acidity make extra virgin olive oil. Slightly riper olives, that are also just pressed, produce virgin olive oil.
To be graded extra virgin, the olive oil must have an oleic acid content of less than 0.8%, while virgin olive oil can have as much as 2.0%, or 1.5% under International Olive Council standards.
Extra virgin olive oil is generally preferred for things like dressing and dips, where a flavorful oil is preferable and the oil is not going to be subjected to high heats that would rapidly degrade it.
The refined oils are generally made from oils that would be relatively unpalatable if bottled without further processing. They are treated with solvents and high heat to remove undesirable odors and flavors. The process leaves a relatively neutral-tasting, light color olive oil. Sometimes, in order to make light oils taste a bit like the more expensive grades, a small percentage of virgin olive oil is blended into the refined oil. Some labeled light olive oils are blended with other oils such as canola.
While the refined olive oils are less nutritious, they are also less affected by high temperatures when compared with the virgin oils. For comparison, the smoke point of extra virgin olive oil is approximately 320°F (160°C), virgin at approximately 420°F (215°C), and light at approximately 465°F (240°C). As such, it is typical to use light olive oil for baking, grilling, frying, and sautéing, essentially where high temperatures are required.
During 2015, Italian authorities discovered that 9 out of every 20 bottles of olive oil sold by its top exporters were tainted with other types of oil. Among the companies allegedly selling lesser-quality oils as “extra virgin” were Bertolli, Carapelli and Primadonna.
To get around the problem of mislabeling, in the US the California Olive Oil Council provides olive oil grade certification, with its seal appearing on certified bottles. Beyond looking for these certifications, it is also important to avoid olive oils that lack a harvest date on the label, as extra virgin olive oil’s shelf life is generally only in the range of 18-24 months and a lack of such data could possibly indicate older oil being sold. Choosing oils that include their harvest date on the label ensures fresher oil.
It is best to store olive oil away from light and heat, as well as to limit exposure to air as these factors will rapidly degrade the quality of the oil.