Olive oil is made by crushing olives to release the oil trapped inside. The olives are harvested and taken to an oil mill where they are cleaned, ground into a paste, mixed, and put under pressure to extract the oil. Virgin olive oil is made with a purely mechanical technique, while refined olive oil has been chemically treated to reduce acidity.
Olive oil has been a staple food in the Mediterranean for thousands of years and some olive trees that are hundreds of years old still bear fruit. The process of making this product is relatively simple, especially since it is practiced in rural areas of Greece and Italy. Commercial olive processing is somewhat more complex, but still relies on the same basic principle: crushing the olives to release the oil trapped inside. The process can also be used to extract oils from nuts and grains.
The olive harvest is the first step in producing olive oil. Traditionally, olive trees were pounded by hand to release the fruit, though many processors today use commercial machinery to strip the trees. Once the olives have been harvested, the clock starts ticking: they need to be taken to an oil mill as soon as possible, or the acidity level will rise too high, ultimately affecting the flavour. After being taken to the mill, the olives are picked through to remove large branches and leaves, and then sent through a cleaner which removes dirt, leaves and twigs, leaving the plain olives behind.
The cleaned olives are passed through an oil mill which transforms them into a paste. Traditionally produced olive oil still uses a grinder to grind the olives, although commercial processors have turned to high-volume metal grinders that can run continuously. The resulting paste undergoes the malaxing process, which means it is slowly mixed so that the oil droplets begin to adhere to other droplets in the mixture.
After being mixed, the olive paste is put under pressure to extract the liquid oil along with the water from the fruit, leaving the paste or pomace behind. The oil is separated from the water, usually in a centrifuge, and bottled while the water is discarded. The resulting olive oil is called virgin, because it is made with a purely mechanical technique. Olive oils with acid levels measured at less than 1% are considered “extra virgin” and usually have a particularly rich and intense flavor. Those with a higher acid level (1-3%) are labeled “virgin.”
Refined olive oil has been chemically treated to reduce acidity. Sometimes olives that have been left to rest too long before processing produce a very acidic oil, which needs to be refined. This reduces the value of the oil, so farmers try to avoid it whenever possible. The pomace can be further chemically processed to extract more olive oil which is usually blended with other oils or used for processes such as soap making.
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