When olives are six to eight months old, they begin to turn from green to purple. This period is best suited to extract olive oil. The olives are now cleaned, coarsely crushed, with its pit and sometimes with a few leaves from the tree. It is then finely ground to a paste, to break open the cells and free their oils. The paste is then mixed for 30-40 minutes, this separates the oil droplets from the watery olive pulp and the oil droplets coalesce with each other. The paste is then squeezed, to extract oil and water. The oil is then separated by centrifuge (sometimes by other means) and filtered. The oil that is derived by the first cold pressing is called “extra virgin” oil. The oil is required to have less than 0.8% free fatty acids (more free fatty acids means oil is damaged and unstable). More oil is then extracted by heating the paste and pressing it repeatedly, but this oil is of lesser quality. “Virgin” oil has less than 2% free fatty acids. If the oil has higher, fatty acids, then the oil is refined, this removes all impurities, including flavours. Once refined, producers add a bit of “virgin” oil to give it flavour. The greenish golden hue, of virgin olive oils are due to the presence of chlorophyll and carotenoid pigments. The flavours are due to the many phenolic compounds and hexanols. The aroma is due to the many volatile molecules like; flowery terpenes, fruity esters, green smelling fatty acids found in leaves etc. The green chlorophyll that colours the oil, also can damage the oil in the presence of light. Hence these oils are kept in dark opaque cans. Unfortunately, many myths persists around olive oils. Many people, consume olive oil for the anti-oxidants it has, oblivious to the fact that higher quantities of anti-oxidants can be got from fruits and vegetables. I was once shocked to hear, some one tell me that olive oil does not have any calories! Like any other oil, it has 9 Kcal for every gram (1 tsp will have about 45Kcal). So other than flavouring your food, do not expect olive oil to work any miracles upon your health, especially when used for frying. When used for frying, most non-refined, non-hydrogenated oils are as good as olive oil.