Olive oil is increasingly competing with other "healthy" oils on the market shelf. Health food stores and upscale markets are offering expensive "expeller pressed" seed oils. Many web visitors to the Olive Oil Source are asking why olive oil isn't expeller pressed if that is a more healthy process.
First of all, olive oil is never made with an expeller press.
An expeller is actually a relatively boring and cheap piece of industrial equipment. The term "Expeller pressed" is less about the expeller and more about what wasn't used to make the oil. It implies that a seed oil is more healthful because it has been made without solvents and steam or toxic chemicals in a large industrial sized refining plant.
Continuous pressing by means of expellers (also known as screw presses) is a widely used process for the extraction of oil from oilseeds and nuts. It replaces the historical method of batch extraction by mechanical or hydraulic pressing. Typically the seeds or nuts (sesame, corn, safflower, rapeseed, peanuts, etc.) are first cleaned, de-hulled, flaked, cooked and dried, then put through the screw press.
If you want to know how it works read the fine print:
According to the Food and Agriculture Organization of the United Nations: "an expeller (consists of a screw (or worm), rotating inside a cylindrical cage (barrel). The material to be pressed is fed between the screw and the barrel and propelled by the rotating screw in a direction parallel to the axis. The configuration of the screw and its shaft is such that the material is progressively compressed as it moves on, towards the discharge end of the cylinder. The compression effect can be achieved, for example, by decreasing the clearance between the screw shaft and the cage ( progressive or step-wise increase of the shaft diameter ) or by reducing the length of the screw flight in the direction of the axial movement The gradually increasing pressure releases the oil which flows out of the press through the slots provided on the periphery of the barrel, while the press-cake continues to move in the direction of the shaft, towards a discharge gate installed at the other extremity of the machine. "
Expellers are popular because they are relatively crude inexpensive devices which can even be made in small cities in developing countries. They can be quite small to handle the harvest from a single farmer or large enough to handle a whole village. They are made by literally hundreds of manufacturers world-wide. The same expeller can be used for palm nuts one month and sunflower seeds the next.
The disadvantage of an expeller is that even the most sophisticated models cannot remove all of the oil. It is less of a problem with an oil rich seed such as sesame or peanuts but is a big problem with oil poor seeds such as soybeans where leaving a seedcake with 5% residual oil may represent half the oil in the bean. Expellers cannot be used with very low oil content agricultural products such as rice bran.
Solvents or steam can be injected into the press to increase yields or can be used afterward on the seedcake to recover any retained oils. If the seeds are not heated or cooked then the yield is lower but there may be health advantages to the resultant oil, hence the "expeller pressed" label.
In contrast to the expeller, most seed oils in the US are made in large plants costing millions of dollars which use heat, solvents and other technology to extract virtually all of the oil. There are only a few companies worldwide which manufacture such plants which are akin to a petroleum refinery in size and economies of scale. Trainloads of seeds go in one end and tankers of oil come out the other. Such large scale technology naturally arouses suspicions in some that the resultant product is somehow inferior.
A seed oil extracted with only the expeller has all of the oil soluble components of the original seeds. That can include valuable trace minerals, oils and vitamins. But it can also include undesirable flavors and toxic substances such as are found in rapeseed.
So how is this all relevant to olives? As stated at the beginning, olives are never pressed with an expeller. Their moisture content is too high for this type of press. Olive oil is usually separated from the olive pulp using a spinning centrifuge without friction or heat.
As far as the health aspects, 98% of seed oils are produced using heat, steam and solvents. In contrast, virgin and extra virgin olive oil is never produced using heat, steam and solvents.
The 2% of seed oils made with only an expeller are the exception and are considered a whole food product and more healthy. 100% of virgin olive oil is considered a healthy whole food product. While making seed oils without chemicals might be a new fad, by definition of the International Olive Oil Council, virgin olive oil has always been made without the use of chemicals.