Gone are the days when olive oil was made by a donkey pulling stones around and around to grind the olives. Today, olive oil sold commercially is produced in a modern mill, ensuring better quality taste and hygiene. Fewer than 24 hours from harvest to processing produces the highest-grade oil.
But the basic steps in making olive oil are the same, which ever method is used. So here is a short explanation of what happens at the mill describing how this elixir is created.
First off, once the olives have been delivered and poured over a grid, they are trundled up a conveyor to be cleaned. All the twigs, leaves, stones and other debris are removed. This ensures that only olives make it into the machinery, so that the oil made is free of contaminants and the machinery (for example, the hammermill) is not damaged.
The next phase is the actual crushing of the olives. This is where the flesh cells of the fruit are torn and ground allowing the oil to be released from the vacuoles.
Instead of using huge stones to crush the olives like in olden times, today the automated process consists of a stainless steel container and crusher that rotate at high speed. The fruit is thrown against the a hammer-shaped metal grating - hence the name, hammermill.
At this point you get a brownish paste which then has to be mixed for about 30 - 60 minutes.
This is called malaxing. During this part of the process, the small droplets combine into bigger ones and the flavour comes out. Large commercial mills will sometimes heat or add water at this stage to increase yield although this results in a lowering of the quality.
After malaxing, the paste is pumped through a large tube into a decanter which is in fact a large horizontal conical centrifugal cylinder. Here the oil is separated from the solids in the paste as well as from the vegetable water that formed part of the olive. The solid and water comes out as a wet pomace. This part of the process used to be done with presses, hence the term 'first press' and 'cold press'. Modern milling is very gentle during malaxing and avoids over-heating the paste. Cold pressed extra virgin oils must not exceed 27C (80F) at any stage of the milling process.
Finally, the unfiltered oil makes its way through a pipe to be bottled. In some mills, it is run through a filter. There is much discussion as to whether unfiltered oil is healthier than filtered oil. Personally, I prefer the taste of unfiltered extra-virgin oil, with its tiny fruit particle deposits floating about in the oil. After a few months, this natural sediment will collect at the bottom of the bottle. You don't get this in commercially mass-produced olive oil.