By: Thomas Affatato
Certified Olive Oil Consultant, Italian Culinary Institute, New York, N.Y.
Milling and Pressing of the Oil
The Milling and Pressing
To ensure the high quality of the end product, it is essential that during the milling the olives (the stones in particular) be crushed sufficiently to produce paste of the right texture. Some producers still use the old method of two, three, or even four granite millstones-thus guaranteeing consistency in the milling while avoiding any increase in the temperature of the paste, a factor which would otherwise compromise the quality of the oil.
The olive paste, obtained through milling, is made u to three parts: water, pomace (often called sansa), and oil. The liquid is extracted and then put through a final separator, where centrifugation eliminates all remaining water and all of the impurities. The oil that emerges after final centrifugation appears amber in color, with an opaque quality-a characteristic feature of superior oils. The more acidic the oil, the clearer and brighter it appears.
Crushing used to be done by hand in spherical or conical stone basins. To improve the yield, the basin was made bigger and the olives were ground on top by revolving millstones driven by animal power. The stones have varied in shape over the years, ranging from cylindrical to truncated conical shapes with dual rotation around the vertical shaft of the base block and the shaft of the millstone which improves the characteristics of the resultant paste. Blades stir and remove the paste.
Just before being crushed, the olives need to be run through a washer to eliminate any remaining impurities. Generally, the olives are crushed whole, without prior stoning (pitting), in roller mills or modern hammer crushers.
To make sure the oil separates easily the paste has to be homogenized and prepared in mixers equipped with blades or propellers that rotate at a low speed to avoid creating emulsions. This operation has to be carried out at a moderate temperature and to do so the mixer is fitted with a chamber in which lukewarm water circulates. The oil becomes more fluid but temperatures should be kept within limits to ensure that the volatile substances responsible for the oil’s aroma are not released and that oxidation processes do not start.
The paste can also be run through an extractor that separates part of the oil on the basis of the different surface tensions of oil and water.
More recent developments include crushing rolls and mills. Crushing rolls have the advantage of being as fast at milling the olives as the hammer mills, while producing a better quality paste; their action imitates that of the traditional millstones.
With a hammer mechanism, whether fixed or floating, the olives are quickly reduced to a fine paste. This process tends to cause some of the paste to evaporate and fumes are produced which must be immediately drawn off with the aid of extractor fans or flues.
The traditional system corresponds to the age-old method using millstones and hydraulic presses. Roughly three centimeters of paste are spread on to what are called fiscoli in Italian-round mats that are made of natural or synthetic fibers. These are then piled, in groups of three, one on top of the other (a metal disk following every third mat), on to a perforated cyndrical steel shaft. A tower of approximately twenty such layers is formed, and placed upright on a trolley and gently pressed. The liquid produced by the pressing drains through the mats and cylinder and I collected ready for the final separation.
In the continuous system-the new method for extracting oil-the olive paste produced by the milling process, with or without the addition of water, is continuously fed into an extracting machine with a horizontal shaft. Centrifugation separates a mixture of water and oil from the pomace and a final separation cycle removes any water and other impurities from the oil.
Separating the Solids and Liquids
Initially, separation methods ranged were relatively basic although better operational capacities with higher oil recovery rates have developed over time. This was the case of the lever press, and then of the much later screw press and the more recent hydraulic presses. The paste was placed in cages but the juices were not readily or completely released, and so pulp mats or filter discs came into use. A layer of olive paste is spread onto each mat, which is than stacked onto others to form a cylindrical load held fast by a needle or central guide. The pressure exerted on the stack causes the liquids to run out while the solids are retained on the pulp mats. Modern hydraulic presses apply a pressure of 50-65 kg. /sq. cm. to the paste. The vegetable water and oil gradually seep out, running down into the press trolley from where they are run into a set of decanters. The oily musts obtained at pressures of up to 40 kg. /sq. cm. can be separated from those that follow because they contain the more easily released, better quality oil.
The paste used to be spread onto the pulp mats by hand although mechanical spreaders have been in use for some time and work well, raising labor productivity. The mats must be washed frequently to stop them from fermenting which transmits undesirable properties to the oil.
Separating the Oil
In recent years, the pomace, oil and vegetable water have been commonly separated by continuously centrifuging the paste or mash. After being suitably thinned with lukewarm water, the paste is injected into the decanters or horizontal-shaft centrifuges which revolve at 3,000 or 4,000 r.p.m. Because of the different densities of the three substances, separation is virtually immediate. The moist pomace is run through one pipe while the vegetable water mixed with some oil runs through another and the oil containing some water is discharged through a third outlet. Both oily musts are put through an upright centrifuge to separate them properly.
The mixture of water and oil produced by a traditional pressing method can be separated by gravity in decanting vats where it makes a long journey through basins connected by siphons which make it possible to increase the proportion of oil recovered and to further deplete the vegetable water. Rapid separation can also be achieved in upright centrifuges, which revolve at 6,000 or 7,000 r.p.m. For the operation to be efficient, the liquids must be homogenized in tanks prior to being run into each centrifuge. Some centrifuges are prepared to handle liquids containing a high percentage of oil while others are for vegetable water with a small amount of recoverable oil.   Infinite Health Resources does not at any point, for any circumstances suggest that you do not follow or stop medical advice of your physician. We do not advocate any drugs that has not been prescribed by your physician, nor suggest that we are medical doctors nor are we giving medical advice. Infinite Health Resources is here purely as a resource. |
