After the olive harvest, leaves and shells are sorted out, olives are washed and prepared to be pressed. In old days, stone or granite wheels were used to press olives. Today, stainless steel cylinders press the olives and turns them into paste. This paste is later kneaded. At the same time, water is added to the olive paste. This process helps to bring oil molecules together.
The paste is kneaded around 20-40 minutes. If the process is longer, there would be less oil and the paste would release different tastes into the oil. In addition, any contact with air would release free radicals and affect the quality.
In modern systems, the kneading is done in tanks filled with anti-oxidation gases. This method increases the amount and taste, and protects the quality of oil. The mix can be heated up to 27-28 ℃ to increase the amount but this would cause some oxidation. This press method in this temperature is called “cold-pressed”.
After kneading, the paste can be pressed more or put into a centrifuge machine. As the machine turns at a great speed, water and oil are collected in the central chamber. Later, oil is separated from water.
Following the oil separation process, the pulp would still have some oil in it. Some producers use steam, hexane or other solvents to extract this oil. We must state that this low quality oil is residue oil.
Furthermore, the oil can be refined and go through decolorization and/or deodorization processes. Refining the oil decreases acidity levels and bitterness. Decolorization process cleans the chlorophyl and carotenoids (pigments that give their color to plants) and possible leftovers of agricultural chemicals so the result would be a light-colored oil which has lower nutrition values. At last, eliminating the smell takes down the sharp smell of olive oil.
In order to protect the oil before bottling and transportation, it is kept in stainless steel tanks around 18 ℃.