The basic steps in making olive oil are always the same, no matter what kind of equipment is used, from The Olive Oil Source's First Press to very large commercial mills built to process many tons of olives per hour.
CLEANING THE OLIVES
The first step in the oil extraction process is cleaning the olives and removing the stems, leaves, twigs, and other debris left with the olives. The olives should be washed with water to remove pesticides, dirt, etc. Rocks and sand will damage a hammermill and quickly wear out a centrifugal decanter or oil separator, reducing life span from 25 to as little as 5 years. It is amazing, and sometimes entertaining, to see what can be found in the bins with the olives. We have heard millers talk not only about rocks and branches, but broken glass, rings, bracelets, pieces of metal, knives, and even razor blades. Light contaminants are removed by a heavy air flow (blower) and heavy objects sink in the water bath.
GRINDING OLIVES INTO THE PASTE
The second step is crushing the olives into a paste. The purpose of crushing is to tear the flesh cells to facilitate the release of the oil from the vacuoles. This step can be done with stone mills, metal tooth grinders, or various kinds of hammermills.
MALAXING THE PASTE
Malaxing (mixing) the paste for 20 to 45 minutes allows small oil droplets to combine into bigger ones. It is an indispensible step. The paste can be heated or water added during this process to increase the yield, although this generally results in lowering the quality of the oil. The most common mixer is a horizontal trough with spiral mixing blades. Longer mixing times increase oil yield but allows a longer oxidation period that decreases shelf life.
SEPARATING THE OIL
How is oil separated from vegetable water and solids?The next step consists in separating the oil from the rest of the olive components. This used to be done with presses (hence the now somewhat obsolete terms first press and cold press), but is now done by centrifugation, except in old facilities. Some centrifuges are called three-phase because they separate the oil, the water, and the solids separately. The two-phase centrifuges separate the oil from a wet paste. In most cases, the oil coming out of the first centrifuge is further processed to eliminate any remaining water and solids by a second centrifuge that rotates faster. The oil is then left in tanks or barrels where a final separation, if needed, happens through gravity. This is called racking the oil. Finally the oil can be filtered, if desired.
OPTIONAL STEP (DEPITTING, ADDITIVES, ADDITIONAL PROCESSING)
Although first described hundreds of year ago, there has been recent interest in producing oil from destoned olives. This is a fairly uncommon practice and there is some debate about its usefulness.
Talc is sometimes used to process difficult fruit or to increase yield with some types of fruit.
Other co-adjuvants can be used to increase the yield such as enzyme, steam, hexane and other solvents. These are not used in better quality oils.
Finally, possible additional processing steps include refining the oil to reduce its acidity and improve flavor (in defective oils) by alkali (chemical reaction with an alkali – caustic soda) or steam processing; bleaching the oil to reduce chlorophyll, carotenoids, residual fatty acids, and pesticides using diatomaceous earth, activated carbon, or synthetic silica treatment, and deodorization to reduce odors with the use of activated carbon. Needless to say, these steps are only used for low quality oil.