oil for english episode 2 >>grinding the olives into a paste


The two main types of crushers are stone mills and hammermills. There are also metal tooth grinders, although these are much less common. One of the most common myths about making olive oil is that stone mills are superior to hammermills and other metal crushers. This notion must be based on the romantic idea that old-fashioned methods are better. This is not the case. Different equipment have different pros and cons, as you will see below, but the notion that stone mills are superior is mainly a marketing twist to sell olive oil.

Stone Mill

le macine in pietra del nostro frantoio, fino al 2010

Stone rollers or wheels roll in circles on a slab of granite to grind the olives into a paste. They are sold in different configurations, with two, three, or even four stones.



  • The stones do not cut the olive skin, so less chlorophyll is released.
  • Larger size drops of oil are formed, which minimizes mixing times.
  • The paste is not heated.
  • A lower level of polyphenols is extracted so the oil is less bitter.


  • They are bulky.
  • The stones are difficult to clean.
  • Green olives prolong grinding time.
  • They are expensive.
  • The process is discontinuous, hence time-consuming.
  • The labor costs are high.
  • A lower level of polyphenols is extracted so the shelf life of the oil is shorter (but as we saw above, the oil is less bitter.


A hammermill has swinging arms that push the olives into the sides of a rotating chamber through centrifugal action, similar to a home garbage disposer. The mill may have one to several hammers.


  • The process is continuous with a high throughput.
  • Cleaning is easier than stone mills.
  • They tolerate debris such as rocks and grit better than stone mills.
  • It is a well-known, perfected technology.
  • More polyphenols are extracted so the oil has a longer shelf life.


  • They may create an emulsion that impedes the separation between the oil and water, especially with very ripe, over-watered fruit.
  • More polyphenols are extracted so the oil is bitterer than oil extracted with stone mills. This may be an advantage with mild olives or with olives with naturally low levels of polyphenols, such as Arbequina.
  • There is some wear and tear of the metal parts.
  • The paste may heat up.

Double-hammer/double-grid mill setups allow a decrease in the bitterness sometimes found when using traditional hammermills. The mill rotates much more slowly than traditional hammermills and the size of the grids can be changed to optimize the crushing process based on the characteristics of the olives. The olive skin is not pulverized as in other hammermills but torn open, in a way that is more similar to stone mills.


Destoning the Olives First.
This is a fairly uncommon practice. Some studies show that destoning lowers olive oil yields by less than 1.5%. Gas chromatographic comparisons of oil made either way show that stones do not make any distinctive contribution to the flavor of the oil, since they are made of primarily lignin and other woody compounds. There is some debate on the usefulness of this technique.


  • Destoned olives are easier on the equipment.
  • Oils are subjected to less heating due to stone fracturing.
  • Destoned pomace is easier to use as animal feed.
  • The stones can be burned to create heat to dry a watery pomace for easier disposal.
  • The polyphenol level may be slightly higher, so the shelf life may be longer.
  • The acidity may be slightly lower.
  • The peroxide levels may be slightly lower. 



  • There is more chance of an emulsion forming which impedes the separation between the oil and water.
  • The paste requires longer mixing times, thereby increasing oxidation.
  • Destoning requires special machinery at additional expense.
  • The oil yield is slightly lower.
  • The polyphenol level may be slightly higher, so the oil would be bitterer. Again, this could be an advantage.



Leggi anche:


Leggi anche

oil for english episode 7 >> tasting

oil for english episode 6 >> quality factors

oil for english episode 5 >> final separation

oil for english episode 4 >> separating

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