Extraction & Clarification of Olive Oil

According to Paul Vossen at the University of California, Davis, the quality of olive oil depends not only on the quality of the fruit, but also on the methods by which it is harvested, transported, cleaned, extracted and processed. Most of the oil in an olive is stored in its pulp, but the skin and even the pit contain small amounts as well. There are many methods of extracting oil--some centuries old and others the product of modern technology.

Readying the Fruit for Extraction

Bulk olives

Before producers can extract oil from the olive fruit, they must clean, weigh, sort and sample the fruit. Vossen cautions that cleaning that goes beyond removing leaves, stems and other foreign matter negatively affects the quality of the oil. He suggests washing only to remove excessive dirt or pesticide residue.

Milling the Pulp

There are many milling processes, all of which are capable of producing top-quality oil. The oldest and slowest of these is stone milling. Producers choose milling methods based on cost as well as on the variety of olives they plan to mill.

Quality Goals of the Producer

Ripe olives on the tree

The best quality olive oil has fruity flavor and green color, and is not overly pungent or bitter. These qualities are influenced by the type of milling used as well as the variety and maturity of the olives. Sometimes the processor must prioritize the qualities he hopes to achieve since most mills have both advantages and disadvantages.

Extracting Oil From the Pulp

Once the olives have been reduced to a pulp that is neither too chunky nor too fine, it must be stirred slowly to extract the maximum amount of oil. This is because the milling process initially causes oil and water to homogenize. After a while though, these elements break down. At this point a number of extraction systems remove the oil from the pulp. These include simple gravity systems that allow oil to run off. Pressing pulp between layers of filtering material also separates a percentage of the oil from the pulp.

Continuing Clarification

After the extraction process has already clarified the oil to a certain degree, further steps are necessary for producers who want their olive oil to be completely clear. Many producers use a vertical centrifuge. They then add warm water to the oil, which passes through as the oil spins in the centrifuge, taking solids with it. Although additional filtering removes more solids, many producers prefer to offer unfiltered olive oil. Once bottled, the solids in unfiltered olive oil normally sink to the bottom of the container.

References

• University of California: Olive Oil Processing Technology Influences on Quality