Unfiltered oil is appealing for its rich, cloudy, raw colour, produced by tiny particles of olive left in suspension. It might seem superior to filtered oil, but that isn't always the case. Here's why.
Since antiquity, pressing olives has yielded a solid part (the pomace) and a liquid part composed of oil and vegetable water. The subsequent separation of oil from water, until about a century ago, took place by natural decantation, since these two elements are immiscible — that is, they cannot be mixed together. With the development of technology, today the separation of oil from water is achieved instantaneously by means of a centrifugal separator. Even after this process, today as in the past, the oil retains a cloudy appearance of varying intensity, due to the presence of residual water in the form of tiny droplets together with micro-particles of olive pulp in suspension. Filtration is carried out to remove precisely these small impurities from the oil — impurities that convey to the consumer a sense of wholeness and artisanal character, but which tend to settle on the bottom of the bottle, shortening the oil's shelf life. Filtration is therefore one of the essential steps in preserving the product properly until the moment of consumption.
The proper preservation of extra virgin olive oil depends on several factors: exposure to light and air is the primary cause of oil degradation; the light-oxygen combination triggers chain reactions that can rapidly degrade its quality, reducing its antioxidant content of vitamin E and polyphenols until it turns rancid. But there is a further threat to the oil's integrity: the presence of water and enzymes in the cell juice, which, over time, sets off degradation reactions affecting both the chemical characteristics (for example, it can cause acidity to rise) and the organoleptic characteristics of our health elixir. It is entirely intuitive — as well as scientifically proven — that removing these substances through filtration allows the oil to achieve greater stability and better resistance to ageing.
Setting aside methods that are now obsolete, today the filtration of oils — as with wines and fruit juices — is carried out using filters in which diatomaceous earth (diatomite) and virgin cellulose powders, both of which are completely inert, are layered inside a filter to form a filtering bed that, as the oil passes through, retains the water droplets and solid particles, leaving it bright in appearance. Filtration neither reduces nor alters the aroma and taste of the product; on the contrary, it allows them to be preserved intact for longer, while at the same time safeguarding the oil's nutritional value.
In recent years, the sale of unfiltered extra virgin oils has become widespread in supermarkets. In the presentation of these products and on the label, terms such as "unfiltered", "raw", "whole" and "100% natural" are emphasised, calling to mind the image of a product taken straight from the olive mill; these oils are not infrequently attributed a supposedly superior quality, both sensory and nutritional.
When choosing an extra virgin oil, however, one cannot overlook this fact: just as with wines there are a thousand nuances in the bouquet that distinguish an ordinary wine from a grand cru, so too with extra virgin oils — beyond the minimum legal requirements — the antioxidant and polyphenol content, and the intensity and elegance of the nutritional characteristics, can vary greatly from product to product, to the point that a filtered oil may have characteristics far superior to an unfiltered one, or vice versa, since quality depends on the raw material and not on whether the oil has been filtered or not; with the proviso, however, that a filtered oil can preserve its premium characteristics far better over time than an unfiltered oil of the same level.
For this reason, unfiltered oil should be enjoyed in the first months of the year — when, between October and November, at the start of the new olive harvest season, one wishes to taste the young, freshly pressed oil — provided it is consumed within a short time, just as one does with nouveau wine.