Olive Oil Production
How is olive oil made?
Making olive oil is a simple process. The oil inside a well cared for, undamaged, just-right olive on the tree is perfect; all the olive oil mill needs to do is get the oil out of the olive without messing it up. It’s not as easy as it sounds – like most simple things there is a lot of finesse involved—but the fundamentals are not complicated.
Great olive oil starts with great fruit
Time is of the essence when you are making high quality olive oil. The minute an olive leaves the tree, it begins to deteriorate. The best olive oil is made within hours of harvest. As olives sit, they begin to undergo anaerobic fermentation—that is fermentation in the absence of oxygen. They will start to heat up, just like a compost pile, and the resulting breakdown of the fruit leads to higher free acidity in the oil and defective fusty flavors.
Olives can be harvested in many ways, and all of them can yield excellent olive oil if the fruit is processed immediately. Hand harvest is the oldest and most expensive method. The olives can be picked into buckets, raked or beaten onto tarps, or rattled and shaken with various handheld machines to get them off the tree onto collecting nets. Larger machines that shake the trunks of the trees, or the canopy, are used extensively all over the world. There are also over-the-row machines that use a row of bars to beat the sides of the trees and dislodge the fruit into a collector.
Despite occasional romantic advertising copy to the contrary, first rate olive oil can be made from mechanically harvested fruit. In fact, there is an advantage to machines since they are so much faster and allow the fruit to be processed within a very short time of harvest. The down side of machines, beater-bar types in particular, is that some of them can bruise the fruit. This is not a major concern if the fruit is going to be processed within hours of harvest; the processing involves considerably more trauma to the olives than just bruising (it grinds them to a pulp, to be specific).
Once the olives are harvested they must be rushed to the mill. Really rushed. Do not pass Go, do not collect $200. Speedy processing is essential for making good olive oil; the sooner you mill, the less the fruit will deteriorate. At the mill, leaves and sticks are removed from the olives, and the fruit is washed. From the washer it goes to a grinder that creates a paste.
There are different methods of grinding. The most common by far is the hammer mill. It is fast, continuous, easy to clean and produces oil with a lot of character. Stone mills, the standard for most of history, are still used by some producers. Stone milling is less efficient because the olives must be milled in batches, there is more oxidation because the paste is exposed to the air for an extended period and stone mills are harder to clean thoroughly. Stone grinding results in a coarser paste and less bitterness in the oil. This can be helpful when milling certain varieties, but most producers opt for the greater flavor extraction and efficiency of hammer mills. Another option that is growing in popularity is the disk mill. A disk mill can produce paste that is more similar to a stone mill’s in texture, but it is continuous, easy to clean and results in less oxidation.
From the grinder, the paste goes into a tank for malaxation. In malaxation, the olive paste is stirred slowly with paddles to help reverse the homogenization that takes place during grinding. The small droplets of oil are coaxed together into larger droplets to help the separation process. Malaxation usually takes from 30 to 45 minutes, depending on the character of the paste.
From the malaxation tank, the olive paste is pumped to the decanter. A centrifugal decanter is the standard method of separating the olive oil from everything else—fruit water, pit fragments and pulp. The traditional method of separation was a press that used mats (imagine a round placemat with a hole in the middle), spread with olive paste and squeezed to release the oil and water. The solids stayed behind on the mats. The problem with this picturesque method is that a mat, even one made from a man-made material, is about as easy to clean as an oil and olive paste-saturated carpet, which often times leads to contamination of the oil. Cleanliness is everything in olive oil processing, so the use of mats introduces an unwelcome opportunity for defects to get into the oil.
In a decanter, separation that takes hours with just gravity is done in seconds. The olive paste is spun at high speeds to separate the different parts: solids (heaviest), fruit water and olive oil (lightest). The olive oil is pulled out of the decanter and sent for a final cleaning in the vertical separator. The solids and fruit water are sent for disposal, composting, etc.
The final cleaning of the olive oil is sometimes called polishing. It involves another centrifuging, this time in a modified cream separator. The last traces of fruit water and particles are removed and the fresh olive oil is ready for settling or filtration. Or you can just grab a piece of bread and enjoy it straight out of the mill as olio nuovo!