Olive oils are distinguished and classified by category, from pure to extra virgin, this being determined by the acidity of the oil.

The traditional methods of harvesting and production have not changed greatly since they first began, but the equipment used has become more sophisticated. The olives are generally collected in nets and pressed between heavy stone wheels or millstones to produce the oil from the flesh of the fruit. Some mills use stainless steel units and hydraulic presses in place of the screw presses.

This has affected both the quality of the oil and the yield. Older presses extracted around 40 per cent of the oil at the first pressing (or cold pressing). To this, hot water was added and it was pressed again for the second pressing (or hot pressing). The modern machinery used today extracts over 90 per cent of oil from the first pressing, the remaining pulp being sent on for further processing, thus eliminating the second pressing.

From Trees to Presses

Once harvested, the olives are separated from the leaves and washed thoroughly. They are then crushed between two millstones and formed into a paste. The process continues for 30 minutes, during which time the oil is released from the olives.

The paste is then spread over round woven mats, which are piled up and put on to a hydraulic press. The oil trickles through the mats to be collected at the base of the press. The oil produced is reddish brown, consisting of olive oil and olive vegetable water. The two are then separated in centrifuge and the oil is stored immediately in underground tanks.
The traditional method of decanting the mixture into troughs and skimming the oil from the surface is still used in some areas, and this oil can be recognized by the word affiorato on the label. Any oil produced that does not reach the standard required is sent to the refinery for further processing and cleaning.

Some smaller, single-estate farms extract the first-run oil before the paste is pressed. This oil is of the best quality and highly priced, having been bottled and sealed by hand and numbered; it can be recognised by the words, Yema Flor, or ‘Yolk Flower’, on the label. Another method commonly used to produce very high-quality oil is one where olive paste is passed over thousands of moving stainless steel blades; the oil sticks to the blades and is funneled off.

The Olive Oil Pecking Order

Only around 10 per cent of the world’s olive oil production is virgin oil; the rest is refined to remove impurities that would affect the final quality of the oil.

Oils are graded by their level of acidity, and this is done both by tasting and by chemical means. Generally speaking, oils with the lower acidity levels are the finer oils.
Extra virgin oil is top quality and is unprocessed oil with an acidity level of no more than 1 per cent and possesses a good flavour, aroma and colour. Fine virgin has an acidity level no higher than 1.5 per cent.

Virgin oil has an acidity level no higher than 2 per cent and a good colour, aroma and flavour. Olive oil is a blend of virgin and refined oils and has an acidity level no higher than 1.5 per cent. As refined oil has no flavour or colour, the virgin oil gives it these attributes and is added in varying quantities, altering its flavour.

Olive pomace oil is refined oil extracted from the pomace or paste left on the hydraulic presses. The oil will have an acidity level no greater than 1.5 per cent and is given its flavour by the addition of extra virgin olive oil.

-- Extracted from Ultimate Olive Oil For Healthy Living by Jenny Stacey (London: Caxton Street, 2001)