The pressing of the olives takes place in several stages, some of which are decisive for obtaining a quality extra virgin olive oil. Among these, the extraction phase that takes place inside the kneading machines.
The oil present in the olive is extracted in several stages: a first small quantity is released during the mechanical break (crushing) by the crusher. Another part, the more consistent, is found in a sort of emulsion with the water of the pulp. It is not separated from it as in a glass of water but bound (another example of emulsion is soapy water). Depending on the cultivars, the ripeness and the temperature, this link is very different. There are varieties in which this link is weak while in others it is very strong. In addition, the droplets of emulsion oil are so small that it would not be possible to separate them with mechanical methods that provide for centrifugation. For this reason, the passage in the kneading machine is fundamental.
Inside the kneader the pasta is continuously stirred in a mild way (40 rpm). Furthermore, hot water is passed through the cavity between the external and internal wall, at a temperature that can be controlled by the operator. The temperature is fundamental to allow the breakdown of the emulsion and the separation of the oil from the water. The water temperature must be such as to allow processing and at the same time not to damage the oil inside the pasta. With a low degree of emulsification the kneading takes place in a short time and can be carried out at temperatures below 20 ° C while for more stable emulsions it is necessary to reach pasta temperatures above 22 ° C. This is because, inside the pasta, natural enzymatic reactions are triggered which interact on the bond of the emulsion, breaking it. Furthermore, the oil droplets join together reaching a sufficient mass to allow the separation by centrifugation which will take place with the decanter.
All the steps seen so far are common to all methods of processing olives for the production of oil. The temperature plays, in all the production phases, a determining role in the quality of an extra virgin olive oil as a key factor, together with the exposure to light, of the denaturation of the nutritive and organoleptic components of the oil. Here, how it becomes fundamental to remain below a certain temperature threshold value so as not to affect the product, in each of the processing phases. In cold pressing, this threshold is set at 27 ° C. If in each production phase, the temperature has remained below this value, the processing is considered optimal and the oil can be defined as “cold extracted”.