Unfiltered oil is the result of the olive pressing process. Suspended in the liquid are minuscule particles of olive (both pulp and stones), that give it its typical cloudy aspect. If the oil isn’t filtered, these particles will slowly sink to the bottom of the container.
So why is oil usually filtered? It is because these particles tend to reduce its shelf life. In order to make it last longer, the producers usually prefer to filter extra virgin olive oil so as to eliminate most of the suspended particles.
The resulting oil has the typical glossy aspect consumers are used to, and keeps for a longer period of time.
However, it is always best to buy enough oil to last until the next harvest because, as the old saying goes, one should go for “new oil and aged wine”. Unlike wine, which improves with age, oil is at its best when it is young and tasty. As the months go by, the flavour mellows down and becomes sweeter, and while all its nutritional values remain unchanged, its fruity bouquet becomes weaker.
In other words, the structure of oil slowly changes. After 18 months olive oil is still edible, but by then it will have lost much of its perfume, flavour and colour.