When choosing what extra virgin olive oil to purchase, consumers tend to give paramount importance to its appearance. Arguably, it is always important to see what one is buying, because everybody wants to make sure that the oil – but this is true for any type of food –appeals to the eye.
A green hue and veiled appearance of olive oil are usually associated with the idea of freshness and “genuineness”. In our subconscious, we link the verdant colour of an oil with the typical scent of freshly squeezed olives. However, all scientific evidence suggests that the colour of oil is completely independent of both its freshness and the ripeness of the olives from which it was made.
What emerges instead from these investigations is that the cultivar of the olives and the transformation process to which they are subjected are the sole factors responsible for the colour of the oil.
As regards the veiled appearance of unfiltered oils, it is common knowledge that this cloudiness is not stable, and in time, these oils will form a sediment at the bottom of the vessel, with negative effects on the overall quality of the oil.
In recent years, much research has been carried out to determine the nature of the suspended matter and how it affects the oil’s durability. All investigations indicate that cloudiness is the result of a variety of particles suspended in the oil. The nature of these particles is extremely variable, and depends on numerous factors, the most important of which is the production process. Despite many attempts, it has been impossible to determine the precise expiry date of unfiltered oils: durability depends on the compounds suspended in it, and therefore on the technique employed to extract it from the olive drupes.
Chemical analyses have revealed that the suspended particles mainly consist of pulp residues, lignin, and water droplets. The latter are extremely important, because they contain salts and small- to medium-sized organic polar compounds, which would otherwise be absent, because oil is an extremely lipophilic medium. The rate at which these particles settle forming a sediment differs extensively, but when this happens, the oil starts to spoil. Fermentation processes trigger the production of unpleasant smells (sensory defects) and as a consequence, the extra virgin olive oil is downgraded to a lower commercial grade (virgin or even lampante virgin oil, if the fault is very evident).
Filtered and fine filtered extra virgin olive oils never incur this type of spoilage. It must be noted however that these procedures too have their drawbacks, since they reduce the levels of useful compounds present in the oil. Even ten years ago it was evident from trials performed at the University of Bologna that the use of cotton and cardboard filters reduced the durability of the oil, which could be ascribed to an approximate 10% decrease in the level of its antioxidants. The exact amount was observed to depend on the type of filter employed and its ability to remove water droplets. These latter particles are responsible for solubilising and preserving the phenolic compounds in the oil, and phenols, as we all know, are the most important antioxidants present in extra virgin olive oil.
A problem oil-makers have to face is when to perform filtering: the oil should spend the least time possible in contact with the particles that have deposited to the bottom of the container, but it is always difficult to estimate how long it takes for the marc to settle. Experience tells us that it is best to filter, or at least decant the oil, one to no more than three months after pressing the olives.
Another piece of advice we should offer our customers is to consume unfiltered oil within three months of the production date and to store it at a temperature not lower than 10°C.