Cultivation and climate
The countries bordering the Mediterranean sea produce most of the world`s olives, nearly all of which are crushed for oil, and the rest - about 10 per cent - are preserved for eating. These countries possess an estimated 800 million trees, with 500 million in the European Community. The leading producer is Spain, followed closely by Italy, then Greece and Tunisia. The leading consumers per head of population are, in descending order, the Greeks (especially the Cretans), the Spaniards and Italians, the Libyans, the Syrians, the Portuguese, the Turks, the Tunisians and the French.
The olive tree will tolerate poor, rocky soil and so thrives in mountainous parts of Spain, Italy and Greece which are otherwise unsuitable for crop planting, even though it yields more fruit in undulating or lowland sites. It will not tolerate frosts below - 7°C (18°F), prolonged cold weather or excessively high annual rainfall but needs a stable cycle of hot, dry summers, short, wet springs and autumns and mild winters, with plenty of sunshine throughout the year. It will, however, withstand the high winds often experienced in this part of the world. When trees are apparently killed by cold, as happened in Tuscany in 1985, the base and roots will throw up fresh shoots, but it will be some years before these are mature enough to flower and fruit. The mean annual temperature range for olive cultivation is 16-23°C (61-74°F).
Olive trees flower at the end of the winter and in the spring and the fruit develops very slowly, turning from green to pink and purple and finally to black when fully ripe. The tree produces flowers and fruit on the previous year`s wood, and a good year tends to be followed by a less fruitful one. Black olives are usually harvested from November until February or March, while green olives are picked earlier, beginning in October. There is considerable variation in the times and techniques of harvesting. Growers in some countries allow the olives to fall of their own accord on to nets or on to cleared, prepared ground, while others beat or shake the fruit off the branches, with sticks and poles, rakes or mechanical shakers, or climb up ladders to pick them by hand before they are ripe enough to fall spontaneously. Whatever the method, it is usually exhausting and tedious work, undertaken in inclement weather, but it does bring communities together to perpetuate an unbroken chain linking them with their earliest ancestors.
The percentage of oil in the olive increases with ripeness, and oil pressed from ripe olives is golden, while less ripe olives tend to produce more peppery, pungent oils with a pronounced green colour. There are scores of different varieties - Spain alone has twenty-two - of which some of the most famous eating varieties are the Greek Kalamata, the Spanish Man-zanilla (also very successfully cultivated in California) and the Italian early fruiting Ascolona which also yields very good oil. The age of the tree and the variety determine the yield. Immature trees up to twenty years old will produce far less than trees in their prime, with an age of thirty to over 100 years. Really ancient trees, over 150 years old, also yield little and have begun their slow decline into death, although some trees attain a great age of some 200 years.
Olives are pressed in a variety of ways, using either methods that have barely changed over millennia, or right up-to-date technology. Small communal mills in remote villages will press an individual`s sackful of olives from a handful of privately owned trees, together with everyone else`s, while, at the other end of the scale, the large industrial producers press vast quantities with optimum efficiency, using the latest stainless steel mills and centrifuges. At one time even the smallest hamlets and villages had their own stone mills of an ancient design directly descended from man- or horse-driven Greek and Roman prototypes. In some Greek tavernas you may stumble across a vast, circular stone receptacle where once olives were crushed, more likely than not in current service as a storage bin for beer and Coca-Cola bottles. Ancient or modern, the process of oil extraction is essentially a simple one, involving up to five main stages: washing; crushing; grinding; pressing; and decanting or otherwise separating the oil from the water.
Olive oil mill
First the olives are brought in and laid out, usually in an outdoor receiving area where they can be checked for condition and sorted according to their maturation, which will influence the acidity level of the ensuing oil. Any leaves and twigs still adhering to them will be removed, either manually or with industrial blowers.
As olives must be processed very soon after delivery to the mill, before fermentation sets in, milling is often started only a few hours later, and certainly within two or three days: this short delay is thought to assist oil extraction by conditioning the olives. Next, the olives are washed in cold water, then drained, before being crushed to break up the tissues and release the oil. This can be done by traditional methods, with mechanical rollers, or in modern, stainless steel crushers which work by simultaneously cutting, shearing and rubbing. (This method is extremely quick and efficient but tiny metal particles do get into the oil.) Then they are ground into a smooth paste, stones (pits) and all, the purpose of which is to concentrate the small droplets released on crushing into larger drops of oil, while generating heat to encourage the oil to flow freely. (The stones harbour a lot of oil.) Traditionally, the paste is now spread out on to natural fibre mats which will be stacked layer upon layer in a vertical press to extract, with relatively little pressure, what is known as the first cold pressing of oil. Alternatively the oil can be extracted in a continuous centrifuge.
If a modern centrifuge is used for oil extraction, the paste produced by milling and crushing is fed into the machine, which spins at high velocity to separate the oil from the pulp. The oil emerging from press or centrifuge is actually a reddish mixture of oil, vegetable matter and water. This can be decanted manually or put into another centrifuge to separate the oil from the water.
The cloudy, unfiltered oil is then stored in large containers. The ratio of olives to oil is approximately five kilos of fruit to each litre of oil. This oil can be filtered, to remove the sediment, or, more primitively, left to rest so that the sediment can naturally fall with gravity to accumulate at the bottom of the storage containers as the temperature rises in the spring and the oil becomes less viscous.
Either way, this is `virgin` olive oil. `Pure` olive oil is made by blending oil of originally high acidity, from second or subsequent pressings which are then subjected to a process of refinement, with first-pressed, virgin oils. In the European Community the term `pure olive oil`, as just defined, was recently changed simply to `olive oil. To be labelled `extra virgin, oil must have an acidity of 0.2-1 per cent. `Olive oil` should not exceed 1.5 per cent of acidity. (The higher the acidity, the poorer the quality, leading eventually to a certain toxicity.) Other processes involving hot-water pressings of the residue cakes and use of solvents produce oil for industrial use and soap-making. The oily vegetable waste can be burned as fuel, converted to fertilizer and treated for use as cattle feed.