After olives are picked, any leaves, twigs, and stems are removed, and the olives are washed. Then it's time for pressing. Back in the old days, processors used stone or granite wheels to crush the olives.

Today, stainless steel rollers crush the olives and pits and grind them into paste. The paste then undergoes malaxation, a process in which water is slowly stirred into the paste. Malaxation allows the tiny oil molecules to clump together and concentrate.

The mixture is stirred for 20 to 40 minutes. Longer mixing times increase oil production and give the oil a chance to pick up additional flavours from the olive paste. However, the mixing also exposes the oil to air, producing free radicals that poorly affect its quality.

Modern systems use closed mixing chambers filled with a harmless gas to prevent oxidation. This method increases yield and flavour and preserves quality. The mixture may be heated to about 82 degrees Fahrenheit, which further increases yield but does allow some oxidation. This temperature is low enough to be considered "cold pressed."


The Traditional Method


The olives are put into a shute that allows easy entry into the process either from a trailer, sacks or large square plastic baskets. The first part of the process is to remove all the unwanted twigs leaves and other unwanted solids,  



The olives are then passed onto a crusher that squashes the olives to a pulp. Unlike the grindstones of a mill, which are horizontally mounted, with one turning atop the other, the grindstones of an olive press are vertically mounted and rotate in a tub, crushing the olives against the floor of the tub. It usually consists of three identical, vertical stone wheels each slightly different radius from the centre that revolve around on a flat plane.


The paste goes into a second tub called a gramolatrice, where it is stirred by several rotating paddles. The stirring breaks up the water-oil emulsion derived from the grinding process, and thus forms droplets of oil that can be more easily extracted from the paste during the subsequent pressing. Again, the stirring phase -- simple mechanical action with no heat or addition of water.


When it's done,  the paste is put onto stainless steel round pads called Fiscoli, which are the stacks in the press.

It is very important the olive paste does not oxidise too much, although a little is good as it appears to promote the development of the compounds that give olive oil its distinctive (and captivating) aromas.  The solid material that remains after the extraction of the oil is called pomace, and it contains residual oil. Some manufacturers will use steam, hexane, or other solvents to squeeze more oil out of the pomace. This low-quality oil must be labeled as pomace oil. Olive presses squeeze hard! This isn't wine making, two atmospheres max. Rather, the olive press cranks up to 400 atmospheres (400 k/square cm, close to 900 pounds) and maintains that pressure by continuing to lift the floor of the press as the oil seeps out. It takes about a half hour to press the stack.


Oil may then be refined, bleached, and/or deodorized. Refining reduces acidity and any bitter taste. Bleaching removes chlorophyll and carotenoids (naturally occurring pigments that give plants their colors) and possibly pesticides, resulting in a light-colored oil with fewer nutrients. Deodorizing removes the fragrant aroma of the olive oil.


In the final process, oil is stored in stainless steel containers at about 15 degrees Centigrade to prevent breakdown before it is bottled and shipped.




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Olive Oil - The pressing process


Types  of Olive Oil

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