In modern plants, sophisticated electronically-controlled milling equipment rapidly replaces the large stone grinders that have been used for centuries to crush olive drupes. In addition to speeding up the processing, this new technology allows producers to tailor operating times, temperatures, atmospheric composition and other factors to optimize quality.
The process starts when the freshly picked olives are transported to the mill in the shortest amount of time possible to avoid damage or bruising that can degrade flavor. Once the olives are at the mill, they’re deposited into a hopper from which a conveyor system transports them up on to a vibrating grid that sorts the olives. Fruit falls to the lower level while larger sticks and twigs fall off to the side. The olives are then washed and crushed into a paste-like consistency. Rosenthal also adds any specialty flavored oils to the mix at this point (if using). Then the olives are stirred in what is called a malaxer, which progressively joins microscopic oil drops into bigger ones so that they can be separated mechanically from the water and other plant matter. The paste is kept at a relatively warm temperature during this step to help the oil assume its full flavor, but not so hot that it begins to ferment.
The resulting oil is then run through a centrifuge where the difference in densities between the water and the oil forces them apart. The resulting liquid is pure extra virgin olive oil, or olio nuovo. This is what we often call “first press.” Historically, the oil would be heated to separate it more easily, but this degrades the taste and yields less-pure oil, so most places today just spin it.