History of Olives
The olive, known by the botanical name Olea europaea, meaning “European olive”, is a species of small tree in the family Oleaceae, found traditionally in the Mediterranean Basin. The species is cultivated in all the countries of the Mediterranean, as well as in Australia, New Zealand, North and South America and South Africa. Olea europaea is the type species for the genus Olea.
The olive’s fruit, also called an “olive”, is of major agricultural importance in the Mediterranean region as the source of olive oil; it is one of the core ingredients in Mediterranean cuisine. The tree and its fruit give their name to the plant family, which also includes species such as lilacs, jasmine, Forsythia, and the true ash trees (Fraxinus).
The olive tree, Olea europaea, has been cultivated for olive oil, fine wood, olive leaf, ornamental reasons, and olive fruit. About 90% of all harvested olives are turned into oil, while about 10% are used as table olives. The olive is one of the “trinity” or “triad” of basic ingredients in Mediterranean cuisine, the other two being wheat for bread, pasta, and couscous, and the grape for wine.
Table olives are classified by the IOC into three groups according to the degree of ripeness achieved before harvesting:
Green olives are picked when they have obtained full size, while unripe; they are usually shades of green to yellow, and contain the bitter phytochemical, oleuropein.
Semi-ripe or turning-colour olives are picked at the beginning of the ripening cycle, when the colour has begun to change from green to multicolour shades of red to brown. Only the skin is coloured, as the flesh of the fruit lacks pigmentation at this stage, unlike that of ripe olives.
Black olives or ripe olives are picked at full maturity when fully ripe, displaying colours of purple, brown or black. To leach the oleuropein from olives, commercial producers use lye, which neutralizes the bitterness of oleuropein, producing a mild flavour and soft texture characteristic of California black olives sold in cans. Such olives are typically preserved in brine and sterilized under high heat during the canning process.
Traditional fermentation and curing
An olive vat room used for curing at Graber Olive House.
Raw or fresh olives are naturally very bitter; to make them palatable, olives must be cured and fermented, thereby removing oleuropein, a bitter phenolic compound that can reach levels of 14% of dry matter in young olives. In addition to oleuropein, other phenolic compounds render freshly picked olives unpalatable and must also be removed or lowered in quantity through curing and fermentation. Generally speaking, phenolics reach their peak in young fruit and are converted as the fruit matures. Once ripening occurs, the levels of phenolics sharply decline through their conversion to other organic products which render some cultivars edible immediately. One example of an edible olive native to the island of Thasos is the throubes black olive, which when allowed to ripen in the sun, shrivel, and fall from the tree, is then edible.
The curing process may take from a few days, with lye, to a few months with brine or salt packing. With the exception of California style and salt-cured olives, all methods of curing involve a major fermentation involving bacteria and yeast that is of equal importance to the final table olive product. Traditional cures, using the natural microflora on the fruit to induce fermentation, lead to two important outcomes: the leaching out and breakdown of oleuropein and other unpalatable phenolic compounds, and the generation of favourable metabolites from bacteria and yeast, such as organic acids, probiotics, glycerol, and esters, which affect the sensory properties of the final table olives. Mixed bacterial/yeast olive fermentations may have probiotic qualities. Lactic acid is the most important metabolite, as it lowers the pH, acting as a natural preservative against the growth of unwanted pathogenic species. The result is table olives which can be stored without refrigeration. Fermentations dominated by lactic acid bacteria are, therefore, the most suitable method of curing olives. Yeast-dominated fermentations produce a different suite of metabolites which provide poorer preservation, so they are corrected with an acid such as citric acid in the final processing stage to provide microbial stability.
The many types of preparations for table olives depend on local tastes and traditions. The most important commercial examples are listed below.
Lebanese or Phenician Type (olives with fermentation): Applied to green, semiripe, or ripe olives. Olives are soaked in salt water for 24-48 hours. Then, they are slightly crushed with a rock to hasten the fermentation process. The olives are stored for a period of up to a year in a container with salt water, fresh lemon juice, lemon peels, laurel and olive leaves, and rosemary. Some recipes may contain white vinegar or olive oil.
Spanish or Sevillian type (olives with fermentation): Most commonly applied to green olive preparation, around 60% of all the world’s table olives are produced with this method. Olives are soaked in lye (dilute NaOH, 2–4%) for 8–10 hours to hydrolyse the oleuropein. They are usually considered “treated” when the lye has penetrated two-thirds of the way into the fruit. They are then washed once or several times in water to remove the caustic solution and transferred to fermenting vessels full of brine at typical concentrations of 8–12% NaCl. The brine is changed on a regular basis to help remove the phenolic compounds. Fermentation is carried out by the natural microbiota present on the olives that survive the lye treatment process. Many organisms are involved, usually reflecting the local conditions or “Terroir” of the olives. During a typical fermentation gram-negative enterobacteria flourish in small numbers at first, but are rapidly outgrown by lactic acid bacteria species such as Leuconostoc mesenteroides, Lactobacillus plantarum, Lactobacillus brevis and Pediococcus damnosus. These bacteria produce lactic acid to help lower the pH of the brine and therefore stabilize the product against unwanted pathogenic species. A diversity of yeasts then accumulate in sufficient numbers to help complete the fermentation alongside the lactic acid bacteria. Yeasts commonly mentioned include the teleomorphs Pichia anomala, Pichia membranifaciens, Debaryomyces hansenii and Kluyveromyces marxianus. Once fermented, the olives are placed in fresh brine and acid corrected, to be ready for market.
Sicilian or Greek type (olives with fermentation): Applied to green, semiripe and ripe olives, they are almost identical to the Spanish type fermentation process, but the lye treatment process is skipped and the olives are placed directly in fermentation vessels full of brine (8–12% NaCl). The brine is changed on a regular basis to help remove the phenolic compounds. As the caustic treatment is avoided, lactic acid bacteria are only present in similar numbers to yeast and appear to be outdone by the abundant yeasts found on untreated olives. As very little acid is produced by the yeast fermentation, lactic, acetic, or citric acid is often added to the fermentation stage to stabilize the process.
Picholine or directly-brined type (olives with fermentation): Applied to green, semi-ripe, or ripe olives, they are soaked in lye typically for longer periods than Spanish style (e.g. 10–72 hours) until the solution has penetrated three-quarters of the way into the fruit. They are then washed and immediately brined and acid corrected with citric acid to achieve microbial stability. Fermentation still occurs carried out by acidogenic yeast and bacteria but is more subdued than other methods. The brine is changed on a regular basis to help remove the phenolic compounds and a series of progressively stronger concentrations of salt are added until the product is fully stabilized and ready to be eaten.
Water-cured type (olives with fermentation): Applied to green, semi-ripe, or ripe olives, these are soaked in water or weak brine and this solution is changed on a daily basis for 10–14 days. The oleuropein is naturally dissolved and leached into the water and removed during a continual soak-wash cycle. Fermentation takes place during the water treatment stage and involves a mixed yeast/bacteria ecosystem. Sometimes, the olives are lightly cracked with a hammer or a stone to trigger fermentation and speed up the fermentation process. Once debittered, the olives are brined to concentrations of 8–12% NaCl and acid corrected, and are then ready to eat.
Salt-cured type (olives with minor fermentation): Applied only to ripe olives, they are usually produced in Morocco, Turkey, and other eastern Mediterranean countries. Once picked, the olives are vigorously washed and packed in alternating layers with salt. The high concentrations of salt draw the moisture out of olives, dehydrating and shriveling them until they look somewhat analogous to a raisin. Once packed in salt, fermentation is minimal and only initiated by the most halophilic yeast species such as Debaryomyces hansenii. Once cured, they are sold in their natural state without any additives. So-called oil-cured olives are cured in salt, and then soaked in oil.
California or “artificial ripening” type (olives without fermentation): Applied to green and semi-ripe olives, they are placed in lye and soaked. Upon their removal, they are washed in water injected with compressed air. This process is repeated several times until both oxygen and lye have soaked through to the pit. The repeated, saturated exposure to air oxidises the skin and flesh of the fruit, turning it black in an artificial process that mimics natural ripening. Once fully oxidised or “blackened”, they are brined and acid corrected and are then ready for eating.
Olive wood is very hard and is prized for its durability, color, high combustion temperature, and interesting grain patterns. Because of the commercial importance of the fruit, and the slow growth and relatively small size of the tree, olive wood and its products are relatively expensive. Common uses of the wood include: kitchen utensils, carved wooden bowls, cutting boards, fine furniture, and decorative items.
The yellow or light greenish-brown wood is often finely veined with a darker tint; being very hard and close-grained, it is valued by woodworkers.
In modern landscape design olive trees are frequently used as ornamental features for their distinctively gnarled trunks and “evergreen” silvery gray foliage.
The earliest evidence for the domestication of olives comes from the Chalcolithic period archaeological site of Teleilat el Ghassul in what is today modern Jordan. Farmers in ancient times believed that olive trees would not grow well if planted more than a certain distance from the sea; Theophrastus gives 300 stadia (55.6 km or 34.5 mi) as the limit. Modern experience does not always confirm this, and, though showing a preference for the coast, they have long been grown further inland in some areas with suitable climates, particularly in the southwestern Mediterranean (Iberia, northwest Africa) where winters are mild. An article on Olive tree cultivation in Spain is brought down in Ibn al-‘Awwam’s 12th-century agricultural work, Book on Agriculture.
Olive plantation in Andalucía, Spain
Olives are cultivated in many regions of the world with Mediterranean climates, such as South Africa, Chile, Peru, Australia, Oregon, and California, and in areas with temperate climates such as New Zealand. They are also grown in the Córdoba Province, Argentina, which has a temperate climate with rainy summers and dry winters.
Growth and Propegation
Olive trees show a marked preference for calcareous soils, flourishing best on limestone slopes and crags, and coastal climate conditions. They grow in any light soil, even on clay if well-drained, but in rich soils, they are predisposed to disease and produce poorer oil than in poorer soil. (This was noted by Pliny the Elder.) Olives like hot weather and sunny positions without any shade, while temperatures below −10 °C (14 °F) may injure even a mature tree. They tolerate drought well, due to their sturdy and extensive root systems. Olive trees can live for several centuries and can remain productive for as long if they are pruned correctly and regularly.
Only a handful of olive varieties can be used to cross-pollinate. ‘Pendolino’ olive trees are partially self-fertile, but pollenizers are needed for a large fruit crop. Other compatible olive tree pollinators include ‘Leccino’ and ‘Maurino’. ‘Pendolino’ olive trees are used extensively as pollinizers in large olive tree groves.
Phenological development of olive flowering, following BBCH standard scale. a-50, b-51, c-54, d-57, (<15% open flowers); f-65, (>15% open flowers); g-67, (<15% open flowers); h-68.
Olives are propagated by various methods. The preferred ways are cuttings and layers; the tree roots easily in favorable soil and throws up suckers from the stump when cut down. However, yields from trees grown from suckers or seeds are poor; they must be budded or grafted onto other specimens to do well. Branches of various thickness cut into lengths around 1 m (3.3 ft) planted deeply in the manured ground soon vegetate. Shorter pieces are sometimes laid horizontally in shallow trenches and, when covered with a few centimeters of the soil, rapidly throw up sucker-like shoots. In Greece, grafting the cultivated tree on the wild tree is a common practice. In Italy, embryonic buds, which form small swellings on the stems, are carefully excised and planted under the soil surface, where they soon form a vigorous shoot.
The olive is also sometimes grown from seed. To facilitate germination, the oily pericarp is first softened by slight rotting or soaked in hot water or in an alkaline solution.
In situations where the extreme cold has damaged or killed the olive tree, the rootstock can survive and produce new shoots which in turn become new trees. In this way, olive trees can regenerate themselves. In Tuscany in 1985, a very severe frost destroyed many productive, and aged, olive trees and ruined many farmers’ livelihoods. However, new shoots appeared in the spring and, once the deadwood was removed, became the basis for new fruit-producing trees. In this way, an olive tree can live for centuries or even millennia.
Olives grow very slowly, and over many years, the trunk can attain a considerable diameter. A. P. de Candolle recorded one exceeding 10 m (33 ft) in girth. The trees rarely exceed 15 m (49 ft) in height, and are generally confined to much more limited dimensions by frequent pruning.
Olea europaea is very hardy: drought-, disease- and fire-resistant, it can live to a great age. Its root system is robust and capable of regenerating the tree even if the above-ground structure is destroyed. The older the olive tree, the broader and more gnarled the trunk becomes. Many olive trees in the groves around the Mediterranean are said to be hundreds of years old, while an age of 2,000 years is claimed for a number of individual trees; in some cases, this has been scientifically verified. See paragraph dealing with the topic.
The crop from old trees is sometimes enormous, but they seldom bear well two years in succession, and in many cases, a large harvest occurs every sixth or seventh season.
Where the olive is carefully cultivated, as in Languedoc and Provence, the trees are regularly pruned. The pruning preserves the flower-bearing shoots of the preceding year while keeping the tree low enough to allow the easy gathering of the fruit.
The spaces between the trees are regularly fertilised.
Pests, diseases, and weather
Various pathologies can affect olives. The most serious pest is the olive fruit fly (Dacus oleae or Bactrocera oleae) which lays its eggs in the olive most commonly just before it becomes ripe in the autumn. The region surrounding the puncture rots becomes brown, and takes a bitter taste, making the olive unfit for eating or for oil. For controlling the pest, the practice has been to spray with insecticides (organophosphates, e.g. dimethoate). Classic organic methods have now been applied such as trapping, applying the bacterium Bacillus thuringiensis, and spraying with kaolin. Such methods are obligatory for organic olives.
A fungus, Cycloconium oleaginum, can infect the trees for several successive seasons, causing great damage to plantations. A species of bacterium, Pseudomonas savastanoi pv. oleae, induces tumour growth in the shoots. Certain lepidopterous caterpillars feed on the leaves and flowers.
Xylella fastidiosa bacteria, which can also infect citrus fruit and vines, has attacked olive trees in Apulia (Puglia), southern Italy causing the olive quick decline syndrome (OQDS). The main vector is Philaenus spumarius (meadow spittlebug).
A pest which spreads through olive trees is the black scale bug, a small black scale insect that resembles a small black spot. They attach themselves firmly to olive trees and reduce the quality of the fruit; their main predators are wasps. The curculio beetle eats the edges of leaves, leaving sawtooth damage.
Rabbits eat the bark of olive trees and can do considerable damage, especially to young trees. If the bark is removed around the entire circumference of a tree, it is likely to die. Voles and mice also do damage by eating the roots of olives.
At the northern edge of their cultivation zone, for instance in southern France and north-central Italy, olive trees suffer occasionally from frost. Gales and long-continued rains during the gathering season also cause damage.