The production of olive oil sounds deceptively simple: crush the olives and extract the oil. In reality it's far from that. The yield and flavor of olive oil depend on the variety and location of trees and on when and how the olive is picked. As olives mature, they progress from green, through a mixed color called veraison, to black. Green olives produce a lower yield of oil that has the most grassy and bitter flavor, and that has the highest antioxidant concentration and shelf life. Veraison olives give perhaps the optimum processing (high yield, high antioxidant concentration and shelf life, and a mixture of fruity and bitter flavors). Black olives give a high yield of “sweet” oil that is lower in antioxidants and shelf life but that is not very bitter.
Less than 24 hours after the harvest, olives must be brought in the mill. The first step in the oil extraction process is cleaning the olives with water and removing the stems, leaves, twigs, and other debris left with the olives. Then the olives are crushed into a paste. The purpose of crushing is to tear the flesh cells to facilitate the release of the oil from the vacuoles. This step can be done with stone mills, metal tooth grinders, or various kinds of hammermills. This paste undergoes a unique process called malaxation. The paste is thoroughly mixed at a controlled temperature under 27°C for approximately 1 h. According to IOOC regulations this is still considered "cold pressed". Malaxing (mixing) the paste allows small oil droplets to combine into bigger ones. It is an indispensible step.
The next step consists in separating the oil from the rest of the olive components. This is done by centrifugation, except in old facilities. Some centrifuges are called three-phase because they separate the oil, the water, and the solids separately. The two-phase centrifuges separate the oil from a wet paste. In most cases, the oil coming out of the first centrifuge is further processed to eliminate any remaining water and solids (pomace) by a second centrifuge that rotates faster. The oil is then left in tanks or barrels where a final separation, if needed, happens through gravity. This is called racking the oil. Finally the oil can be filtered, if desired.
There's just nothing better from fresh extra virgin olive oil!