Filtered vs Unfiltered Olive Oil: Myths and Truths

Olive oil is a staple on our tables, yet we know very little about it. One of the things many people do not understand is why olive oil should be filtered.

Let’s start at the beginning.

To produce olive oil, the olives must go through several processing stages. After washing, the olives are crushed using a machine called a crusher. At this stage, all parts of the crushed fruit are turned into a paste, known as the olive paste. This paste is transferred to a machine called a malaxer, where it is mixed by stainless-steel blades for several minutes. It is then sent to a centrifugal separator, or decanter, where the solid parts of the fruit are separated from the liquid parts of the paste – olive oil and water. The next step is the vertical separator, which separates the olive oil from the water. Normally, after this stage, the olive oil should pass through a filter.

There are different types of filters, depending on production volume and the quality of olive oil the producer wishes to achieve. No matter how effectively separation is carried out by both separators, and even if natural settling has taken place for days, small solid particles of the fruit – pieces of flesh and skin, waxes, fragments of olive stone – as well as tiny water droplets, remain in the olive oil. These give it a cloudy appearance.

If the olive oil remains unfiltered, these small particles create a dark sediment, commonly known in Greek as mourga, which undergoes anaerobic fermentation. The remaining moisture causes oxidation, and these two processes together not only alter the oil’s sensory characteristics, such as its fruity aroma, but also create defects.

One such defect is muddy sediment, which can give olive oil aromas reminiscent of a dirty diaper, spoiled salami, a drain trap or pig slurry. The result is the gradual deterioration of its quality.

To avoid consuming olive oil with these unpleasant aromas, you must trust the producer, know the exact production date and consume the olive oil while it is still fresh – within two months of production at most, if it is unfiltered. Otherwise, always choose filtered, packaged olive oil.

The olive oil found in the unlabelled 17-litre tins of olive oil many people bring home from the village is, in 99.9% of cases, unfiltered. If unfiltered olive oil is extra virgin on the day it is produced, it has a very short shelf life as extra virgin olive oil. Its quality will deteriorate quickly due to the changes it undergoes, and it will be downgraded to virgin olive oil. This means that if an unfiltered olive oil was extra virgin when it was produced, for example in November, and a consumer buys it in June, they certainly will not be buying or consuming extra virgin olive oil – even if they paid for it as such because that category was written on the label.

Similarly, if you bought a tin from the village in December and we assume it was extra virgin – a uncommon case – that does not mean it will still be extra virgin when you consume the remaining quantity a few months later. Both the muddy sediment and the air inside the container, together with high temperature, degrade olive oil very quickly.

Many olive oil producers insist on not filtering their olive oil because they believe it loses aromas and valuable phenols – the antioxidant compounds in olive oil. This is true, but the percentage lost is very small, and what is gained with filtration over time is more important. In practice, the small particles of fruit and moisture that remain in olive oil do not allow us to perceive its fruity aroma clearly and may leave an unpleasant coating in the mouth.

Filtration may remove a small percentage of phenols, but by removing moisture and solid particles, it allows the oil's aromas to shine through – such as freshly cut grass, fresh green olives, tomato, almond, apple and banana (depending on the variety). Naturally, it also extends the olive oil’s shelf life in the bottle or storage tank.

Filtration is a technical process that has a cost. A quantity of olive oil is lost – for example, some remains in the filter papers – and it requires expertise. However, for producers who wish to bottle their olive oil and place it on the market, it is the only viable option for a high-quality product.

That's why the majority of premium, award-winning extra virgin olive oils available on the market, both Greek and international, are filtered.