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There are several processes used in the production of virgin olive oil. From the most traditional in which a stone mill is used, the press with straining and natural decanting in pools, to the most modern, where the two-phases centrifugation process is used. All of them have in common that they use only mechanical and physical procedures.
Below we will explain the steps for the production of virgin olive oil through the extraction in two phases, which is the most modern process to date, and is that we use in Las Valdesas.
Olives are received in the mill. The first process is the removal of leaves mixed with the olives during the harvesting. Olives are dropped a small height in which a fan generates a stream of air that makes the leaves fly away. These leaves will be used for preparing a compost.
In cases in which the olives are covered with mud or dirt, they are washed through an industrial washing machine for disposal. Finally, they are weighed and taken to the mill.
The milling is the crushing of the olives to make it a paste. This is achieved through a metallic hammer. They turn to high revs and crushed olives on a mesh of a caliber which is usually between 3 and 5 mm. With this you can get thick or thin olive pulp according to the preferences of the miller.
We believe that this is the most unknown process in the production of olive oil. Once obtained the pulp in the grinding process, it contains between 15 and 20% of olive oil. The rest is composed of bone, water and vegetable solid remains.
The problem is that olive oil is in the pulp in tiny drops, which makes very difficult to remove from the pulp. To get larger droplets that may be extracted, it is necessary to knead the pulp. This is achieved by using kneading machines, consisting of a bowl of stainless steel with a blade that knead and turn the mass slowly.
On the other hand, the bowl is surrounded by a pipe with hot water to control the temperature of the pulp. The higher the temperature, the lower the viscosity of the olive oil and therefore it will ease get large droplets needed for the extraction. However, at high temperatures the olive oils loose aromas and flavors, therefore, high quality extra virgin olive oils are produced in "cold extraction" (what are the oils of cold extraction?). The temperature often used during the process is between 25º and 35º C.
Therefore the kneading is a key stage in the production of olive oil. The miller can play with the temperature and the time of kneading of the mass. The kneading time is usually between 30 and 60 minutes.
Once the pulp is prepared, the centrifugation process begins. This is done in a horizontal axis centrifuge commonly called "decanters". Centrifugation process uses the different densities of the components of the pulp to separate them. Olive oil is the least dense of all of them, followed by water and vegetable remains and bones.
The centrifuge consist of a cylindrical body with a horizontal axis that rotates at about 4000 rpm. This gets that the dense paste concentrate on the outside of the cylinder and the less dense concentrate in the central part. Once the pulp has advanced and the separation has been made, a diaphragm or plate retains the olive oil and let the rest of the components of the pulp pass through
As we have seen above the olive oil is retained by a diaphragm. This mechanism is not very precise and olive oil may even contain in its interior solid remains. A vertical centrifuge is used to remove them.
To remove these traces the olive oil passes by centrifuge, where water is added in the center of the disk of oil. Due to the difference in density, water is moved to the perimeter of the disc, dragging all the solids that have been left in the oil. After this last process, the olive oil is sent to stainless steel tanks for its storage.
Oil is stored in inerted atmosphere stainless steel tanks. In all cases the olive oils contains very small vegetable solids remains and water. These will slowly decanting and stood at the bottom of the tanks.
Most mills filter their olives oils just before packaging. This get that oil has a clear and bright appearance and is guaranteed to not appear remains at the bottom of the bottles.
Other mills filter their olive oils just after vertical centrifuge process or a few days later. Other mills, like Las Valdesas, prefer not to filter olive oils to let them decant naturally not only in tanks but also in packagings.