HOW OLIVE OIL IS MADE

Olive oil extraction by centrifuges

The old way of producing olive oil

The process of extracting olive oil during ancient times was a very different and labor-intensive process. The type of equipment used to extract oil was an old stone mill, a series of mats, and a donkey.

To separate the oil from the pulp, pits, and other solids (collectively referred to as “pomace”) the paste was “placed into woven bags or baskets, and the baskets themselves were then pressed. After that hot water would be poured over the pressed bags to wash out the remaining oil.

As the olives were pressed, the liquid was typically drained into a stone settling tank where the oil would rise to the top and separate from any remaining water and particles. The olive oil would then be skimmed off the top and stored in terracotta pots for later use in cooking, medicines, religious ceremonies, and more.

Today’s new way of producing olive oil

Today, olive oil making has certainly been modernized. But the basic process is still quite similar—especially for the highest quality olive oil: extra virgin olive oil. Olive oil falls into two main categories: virgin olive oils and refined olive oils. A third category is olive pomace oil, also a refined product.

The top olive oil-producing countries are (unsurprisingly) all in the Mediterranean region: Spain, Greece, Italy, Tunisia, Portugal. And the state of California, with a similar climate to the Mediterranean, produces around 4 million gallons per year. In each of these places, most modern virgin olive oil making involves the following steps:

1. Picking and cleaning the olives.

When growers decide they are ready, olives may be picked mechanically by shakers or over-the-row harvesters, or by hand with the help of various tools that rake or shake olives onto nets underneath the olive tree. Once picked, olives are loaded onto a truck and transported quickly to a production facility. The delay between harvest and processing results in lower-quality olive oil as the olives—like any fresh fruit—deteriorate after picking. At the mill, they go through a defoliator to remove any leaves, twigs or stems in the batch of olives. Then, the olives are washed to remove dirt, pesticides, and other debris. Next, they're ready for processing.

2. Extract the olive oil from the olive fruit.

After the olives are washed, they undergo an extraction process that grinds them up and separates the olive oil from the olive pomace.

Step 1: Grinding

Grinding is the crushing of the olive fruit—pits and all—into a thick paste. This process can be done by several different sorts of machines. Some olive oil makers still use traditional millstones powered by motors, but these are being phased out in favor of machines that crush and grind the olives with stainless steel hammers, knives, or disks. These newer milling machines cause less oxidation of the paste and are easier to clean, which means a healthier, higher quality olive oil in the end.

Step 2: Malaxation

After grinding, the olive paste goes into a malaxer—basically, a stainless steel trough with a corkscrew-shaped mixer turning along the bottom. This machine slowly stirs the olive paste, which allows the tiny microdroplets of oil to coalesce into larger drops of oil that are easier to extract.

Step 3: Centrifugation

The olive paste is then run through a horizontal centrifuge (called a decanter) to separate the olive oil from the water and solids. Decanters have largely replaced presses. After the olive paste is fed into the bowl by a screw pump, the conveyor and the bowl begin to rotate at a high speed in the same direction but at a different speed. The centrifugal force generated by high-speed rotation causes solids to quickly deposit on the wall of the bowl. At the same time, two liquid phases of different densities (olive oil and water) will be separated by layers, and the conveyor of the decanter centrifuge will continuously transport the solids to the cone’s end. After dehydration, the dry solids are finally discharged out of the centrifuge through the solids discharge port. The two clarified liquid phases overflow into the casing of the centrifuge through different outlets at the cylindrical end of the bowl. After the decanter, the oil passes through a second higher-speed centrifuge, a disc-stack centrifuge, to remove the last remaining water and particles. Exiting this vertical centrifuge is extra virgin olive oil—ready to use as "olio nuovo" ("new oil") if you so desire!

3. Racking or filtering the olive oil.

After processing, the extra virgin olive oil can be finished by either filtration or racking. Although still, a topic of debate among producers, filtration is becoming the preferred method for high-quality extra virgin olive oils.

4. Testing and bottling the olive oil.

While not technically part of the olive oil-making process, olive oil needs to pass several tests before it can be called extra virgin olive oil—considered the gold standard. For example, the oil must undergo a chemical analysis to make sure it meets numerous chemical parameters including a free fatty acid content (a.k.a. free acidity level) of 0.8% or less. There is also a battery of tests for authenticity/purity that olive oils need to be able to pass. Then, it must undergo a sensory analysis in which a panel of trained tasters looks for any flaws in flavor and aroma. If there are no defects, and some fruitiness, it can be bottled and sold as extra virgin olive oil; if there are very minor flavor defects or slightly increased acidity, it may be bottled as virgin-grade olive oil.

Extra virgin and virgin olive oils pack the most health benefits since they retain a very high percentage of their antioxidants due to their minimal processing compared to other olive oils. And extra virgin olive oil is the flavor star: fresh and fabulous tasting.

Conclusion

The answer to “How is olive oil made?” is relatively nuanced, but at its core, the production of high-quality olive oil involves: picking and washing the olives, grinding and mixing them into a uniform paste, separating the olive fruit solids and water from the oil in a centrifuge, removing the last bits of sediment and water in a vertical centrifuge, and finally, filtering or racking, testing and bottling. Any processing and refinement beyond these basic steps typically result in a lower quality, less flavorful oil with fewer health benefits.