The craft of turning olives into oil has been practised in the Mediterranean region over thousands of years, and techniques have been passed down from generation to generation. The key goal when producing quality olive oils is to ensure the olives are picked, cleaned and pressed in the shortest possible time. Below is a guide to how this is done today.
The time at which olives are harvested plays a major role in flavour and chemical structure of the oil. The peak time for harvesting is just as the olives begin to ripen but this period lasts for a matter of weeks before the quality of the olives starts to diminish. The first olives collected in the season, the “Primera Cosecha” is highly valued by experts and are the most expensive.
Traditionally, trees were shaken or beaten with sticks to make the olives drop to the ground. However, this does not make good quality oil as the olives are fragile and once they bruise, the beneficial oils within start to degrade. Today, many large-scale growers use a tree-shaking device and set up nets beneath the trees that catch the olives before they hit the ground.
Some premium producers go a step further and only use hand-picked olives. This typically denotes a better-quality oil as they have been individually picked from the tree and they have not been damaged by falling on the ground.
Growers must be careful when transporting olives from the trees to the processing plant. Olives are best carried in shallow containers so they don't pile up too deeply and crush one another. Any damage to the olives can trigger oxidation and fermentation, which create a rancid taste. Olives should be processed soon after harvest because storing them also diminishes their quality.
Before pressing, any foreign particles (leaves, twigs, and stems etc) are removed, and the olives are washed in water. Then it's time for pressing.
Traditionally, a stone or granite wheel was used to crush the olives and extract the oil.
Today, stainless steel rollers crush the olives and grind them into a paste. The paste then undergoes a process in which water is slowly added and by stirring the mixture, this allows the tiny oil molecules to concentrate into clumps.
The mixture is stirred for 20 to 40 minutes. Modern systems use closed mixing chambers filled with a harmless gas to prevent oxidation. The mixture may be heated to about 27°C (max) which is low enough to be considered "cold-pressed."
Next, the paste is sent through a centrifuge - a compartment that is rotated on a central axis at extreme speed to separate the oil from the solid part of the paste.
The solid material that remains after the extraction of the oil is called pomace, and it contains residual oil. This low-quality oil must be heavily refined using heat or chemicals and when sold, must be labelled as pomace oil.
DECANTATION AND STORAGE
Finally, large decantation tanks are used to separate the oil from any water leftover from the process.
The oil is stored at about 18°C in stainless steel vacuum tanks which remove any air to prevent any further oxidisation before it is bottled and shipped.