Like all oils, olive oil is a mixture of monounsaturated, polyunsaturated, and saturated fatty acids. What makes olive oil so healthful is its high percentage of oleic acid, a monounsaturated fat that helps protect the heart and in some research seems to lower cancer risk. The amount of oleic acid varies from 55 percent to 85 percent of the total fat content, depending on the type of olives and the site and soil in which they were grown. But oleic acid isn’t the only reason to like olive oil: It also contains heart-healthy flavonoid antioxidants and vitamin E. Extra-virgin olive oils—they’re made from the first pressing of the choicest olives—contain the highest levels of these beneficial substances.
As far as cooking is concerned, when any oil reaches its smoke point (about 410 degrees for olive oil), it will degrade, oxidize (a process that damages the fat molecules), and partially hydrogenate, creating harmful trans fats. But you don’t need to get oil that hot to sauté vegetables (300 degrees) or even to fry breaded items (340 degrees). Although those lower temperatures may damage some of the flavonoids, the loss will be trivial. And by starting with a healthy oil, your dish will still be more nutritious than if you cooked with, say, butter.
As for storage, olive oil keeps well—and maintains its healthful properties along with its great taste—for up to two years if unopened. Once opened, it can last a year if kept well sealed in a cool, dry, dark cabinet (even longer if refrigerated or frozen). If the oil smells like old French fries or has a buttery taste, it has probably gone rancid and should be tossed.
Many consumers purchase olive oil without really knowing what they’re buying, when in fact there exist many different qualities of olive oil. The highest quality of olive oil is extra virgin olive oil.
By definition, extra virgin olive oil must meet very restrictive requirements, which includes being extracted at a lower
temperature of 27°C (81°F) and within 24 hours from picking. The oil must be obtained by mechanical cold pressing — without the addition of solvents — during the first pressing of the olives and bottled immediately.
In Tuscany, a further guarantee of high quality is the D.O.P. appellation — a European appellation — whose regulation is even stricter, for example, acidity cannot exceed 0.5%. For D.O.P. Chianti Classico Extra Virgin Olive Oil, the law states: “the extraction of the oil “Chianti Classico” must be effected after washing olives with room temperature water, by mechanical continuous process; extraction plant temperature must be controlled and must not exceed the 27°C (80.6°F).
Going off of the previous statement, when choosing an olive oil you want to make sure it has a low acidity. A low acidity is associated with a higher quality, which leads to a more pleasant taste. Moreover, a low acidity typically means the producer pressed the olives shortly after picking, which is crucial for high quality olive oil.