large millstones at a corporate oil mill. The olive paste generally stays under the stones for 30–40 minutes. This has three objectives: Ensure that olives are well ground Allow enough time for the olive drops to join to form the largest droplets of oil Allow the fruit enzymes to produce some of the oil aromas and taste Olive oil mills very rarely use a modern crushing method with a traditional press. In modern-day mills, after grinding, the olive paste is spread on fibre discs, which are stacked on top of each other, then placed into the mechanical press. In modern times, these discs are made of synthetic fibres which are easier to clean and maintain. These discs are then put on a hydraulic piston, forming a pile. Pressure is applied on the disks, thus compacting the solid phase of the olive paste and percolating the liquid phases (oil and vegetation water). The applied hydraulic pressure can go to 400 atm. To facilitate separation of the liquid phases, water is run down the sides of the discs to increase the speed of percolation. The liquids are then separated either by a standard process of decantation or by means of a faster vertical centrifuge. The traditional method is a valid form of producing high-quality olive oil, if after each extraction the discs are properly cleaned from the remains of paste; if not the leftover paste will begin to ferment, thereby producing inconsistencies of flavors (called defects) that will contaminate the subsequently produced olive oil. A similar problem can affect the grindstones that, in order to assure perfect quality, also require cleaning after each usage. Grades of olive oil In ancient Palestine and the Levant, three methods were used to produce different grades of olive oil. The finest oil was produced from fully developed and ripe olives harvested solely from the apex of the tree, and lightly pressed, "for what flows from light pressure is very sweet and very thin." The remaining olives are pressed with a heavier weight, and vary in ripeness. Inferior oil is produced from unripe olives that are stored for extended periods of time until they grow soft or begin to shrivel to become more fit for grinding. Others are left for extended periods in pits in the ground to induce sweating and decay before they are ground. According to the Geoponica, salt and a little nitre are added