Overview
The edible olive, probably first collected wild in the eastern Mediterranean or south-central Asia region, is undoubtedly one of the world’s oldest cultivated crops. There is evidence that the olive was grown on the island of Crete about 3500 BC, and the Semitic peoples apparently cultivated it as early as 3000 BC. Olive oil was a highly prized luxury for anointing the body in Greece during the time of Homer, about 900 BC, and it was an important crop of the Romans around 600 BC. In subsequent times olive growing spread to all the countries bordering the Mediterranean.
Plant Characteristics
The olive plant is a relatively slow growing but long-lived evergreen tree, 10-40 feet or more high. Branches are numerous and the tree, young or old, is very bushy. Strong, old trunks are deep furrowed with ridges running from the branches to the roots, presenting a wide buttress at the base of the tree. The leaves, leathery and lance-shaped, are dark green above and silvery on the underside and are paired opposite each other on the twig. The wood is resistant to decay, and if the top dies back a new trunk will often arise from the roots.
The trees bloom in late spring. Small, inconspicuous whitish flowers are borne in loose clusters in the axils of the leaves. Flowers are of two types:
(1) perfect, containing both male and female parts, which are capable of developing into the olive fruits; and
(2) male, or staminate, which contain only the male, pollen-producing parts. The olive is wind‑pollinated, the presence of bees or other insects being unnecessary for fruit production.
Fruit-setting in the olive is often erratic; in some areas, especially where irrigation and fertilization are not practiced, alternative bearing is the rule. The trees may set a heavy crop one year and not even bloom the next.
The Fruit and its Composition
The olive fruit is classed botanically as a drupe, similar to the peach or plum. It consists of the skin, the flesh and the pit or stone. Within the stone is found the seed, usually one, but sometimes two. Olives tend to have maximum oil content (about 20-30% of fresh weight) and greatest weight six to eight months after the blossoms appear. At this time they are black, and will continue to cling to the tree for several weeks. Fruits for oil extraction are allowed to mature, but for processing as food, immature fruits are handpicked or shaken off the tree.
Varieties
There are literally hundreds of named varieties of both types of olives, table and oil, grown round the world in the warmer climates. In California, olives are grown almost exclusively for table use. The Mission
variety, which still ranks high in acreage planted, was selected from the seedlings first grown by the Franciscans in San Diego. Other table varieties introduced later from Europe include Ascolano (white olive from Ascoli) from Italy; Manzanillo and Sevillano from Spain; and Karydolin, Konservolin and Amygdalolia from Greece. In Europe, olives are grown primarily for oil.
Cultivation
The tree will tolerate drought periods of f ive or six months through the summer, provided winter rainfalls of at least 8‑10 inches occur. Olive trees will not survive temperatures below about 10 degrees F (‑12 degrees C), being injured at 15 degrees F (about ‑9 degrees C). To form flowers and fruit the olive seems to require an exposure of several months to winter‑chilling conditions. The tree itself will grow satisfactorily in tropical regions of the world but fails to bear fruit, probably because of a lack of winter chilling. Commercial olive production is generally found to occur in two belts around the world, one between 30 degrees and 45 degrees N latitude, the other between 30 and 45 degrees S latitude. In these areas the necessary climate requirements for vegetative growth and fruitfulness can be found.
Olive varieties do not come true from seed. Seedlings generally produce inferior fruit and must be budded or grafted to one of the normal varieties. Olives can also be propagated by cuttings, either by hardwood cuttings set in the nursery row in the spring or by small leafy cuttings rooted under mist sprays in a propagating f lame. Either type responds markedly to treatments with a root‑promoting hormone.
The nursery trees are planted 25-35 feet apart in irrigated orchards, or 40-75 feet apart in unirrigated groves. They start bearing in 4-8 years, but full production is not reached until 15-20 years, or when dry-farmed, until the trees are 40-50 years old.
In the Mediterranean countries, most olive groves are unirrigated. Although remarkably drought resistant, the olive will use just as much water as other trees if it is available. A general misconception is that olives require planting on poor rocky hillside land and should be unirrigated. Although the olive plant will endure adverse conditions that would kill most other tree fruits, it responds markedly to irrigation, good soil and nitrogen fertilizers.
In California, where many groves are planted on some of the best orchard soils and carefully irrigated through the dry summers, yields of five to ten times those in the unirrigated orchards of the Mediterranean countries are obtained. In Spain, however, the general belief is that irrigated olives do not pickle as well as those dry-farmed and often the trees are not irrigated even when water is available.
Harvesting
Harvesting of the fruit for pickling is done in the autumn before the fruit turns black. Harvesting usually continues for about two months, or until the fruit becomes damaged by frost. Temperatures of 27 degrees or 28 degrees F (about -2 to -3 degrees C) for several hours are likely to injure the tissues so the fruit cannot be used for pickling; such frozen fruits can be allowed to remain on the tree, however, to be harvested later for oil extraction. Table olive harvest is done by handpicking.
Olives to be harvested for oil extraction are allowed to remain on the tree until late winter when they turn black and reach their maximum oil content – 20-30% of the fresh weight of the fruit. A common harvest practice, although undesirable, is to beat the trees with long poles, knocking the olives onto sheets spread on the ground under the tree. In many orchards, however, the fruit is handpicked even for oil extraction.
Production And Harvesting
Production – In the 20th century, Spain became, by far, the leader in commercial olive production accounting for about 38% of the world’s olive production, followed by Italy with about 20% and Greece with about 13%. Other important olive-producing countries are Portugal, Turkey, Tunisia, France, Morocco, Algeria, Syria, Yugoslavia, Jordan, The United States, Cyprus, Israel and Argentina. Europe, with nearly 500,000,000 olive trees, has more than three-quarters of the world’s cultivated olives, followed by Asia (about 13%), Africa (8%) and America
Olives are grown mainly for the production of olive oil. This is well illustrated in Spain where more than 90% of the crop is crushed annually for oil extraction.
The two leading producers of table olives are Greece and Spain, the former producing 35,000 tons in a good year and the latter 25,000 tons. Next in order of production are the United States (average 17,000 tons), Algeria (15,000), Argentina (12,600) and Italy (10,000).
Processing
Fresh, unprocessed olives are inedible, as they are extremely bitter. This bitterness, due to a glucoside, can be neutralized by treatments with a dilute alkali such as lye. Salt applications also dispel some of the bitterness.
Pickled Green Olives
In the Mediterranean basin, the table olive crop is utilized for pickled green olives. For this the fruits are harvested before they begin to soften or to show any colour change. They are placed in vats immediately after picking and are covered with 1 ¼ – 2% lye solution; this solution is allowed to penetrate about two-thirds the distance to the pit (as determined
by sampling and applying a chemical solution as an indicator, to the cut surface of an olive). Leaving some bitterness around the pit imparts a more pleasing flavour to the product.
The lye solution is removed and the olives are covered with water, which is changed several times a day f or one to two days until the lye is nearly removed. During treatment and washing, undue exposure to the air is avoided as this causes an undesirable darkening of the fruit.
The olives are next transferred to 180-gallon vats (in Spain) or to 50-gallon oak barrels (in California). These are filled with a salt solution to produce 7% to 8% Salt brine. The olives are sealed in these containers for one to six months while lactic acid fermentation takes place. Sugar is often added after several weeks to maintain the fermentation process. Finally, the olives are packed in sealed glass jars filled with a 7½ % salt brine.
Canned Ripe Olives – In California most of the table olive crop is processed as canned black ripe olives. This method is used little or not at all in other olive‑producing countries.
Pests and Diseases Pests
The olive is not subject to many insect or disease pests. The most important, by far, is the olive fly, which occurs only in the Mediterranean countries, not being found in the olive-producing regions of the western hemisphere nor in Australia. So serious is this pest that in Spain, Italy and Greece it reduces the olive crop by half in some years. There are three to five generations during the growing season, the first starting in late spring. The larva burrows in the flesh of the fruit, forming cavities, making the fruit unfit for pickling and reducing the yield and quality of the oil. Trees grown under irrigation seem to be more susceptible to attack than those dry-farmed. No completely satisfactory control measures have been developed. Other less troublesome insect pests are black scale, olive scale and the olive kernel borer.
Diseases
Two diseases attack olives. One is a bacterial disease, called olive knot in California that is caused by an organism. It causes gall-like growths to develop on the branches and twigs. There is no satisfactory control for this pest other than pruning out and destroying diseased areas as soon as they appear. The second important disease is sometimes termed peacock spot, because of the variegated spots appearing on the leaves in late winter and early spring. This is caused by fungus that infects leaves during the rainy months of the year causing them to turn yellow and drop in late spring. Sprays of lime sulphur applied just before the rainy season starts will generally give control.
Olive Oil
Olive oils can be grouped according to the production method:
- Oils obtained from the first mechanical pressing without further treatment
- Oils from the second pressing, which is made with hot water
- Oils extracted with solvents
First – pressing oils are called virgin and their quality depends on the state of the fruit. Only oils from the very best fruit are fit f or consumption without further treatment. Only rarely are they exported without being treated, more often they are used as the basis for export types or for local consumption. The crushing apparatus used in expressing the oil varies from the most primitive Roman presses, consisting of conical stones operated by mule or by hand, to the most modern types of hydraulic presses.
The international grades of olive oil are as follows:
- Virgin (first pressing that meets defined standards)
- Pure, or edible (mixture of refined with virgin oil)
- Refined, or commercial (made from refinement of ‘lampante’)
- Sulphide (made from extraction with solvents and repeatedly refined).
To obtain the best-quality edible oil it is essential that the oil be removed from contact with the residual and putrescible pulp as soon as possible, for the consequent formation of free fatty acids may cause rancidity. In practice, the residual pulp is pressed again with hot water and from this pressing is obtained an oil with higher acid content, which, together with inferior virgin oils, constitutes the oil called ‘lampante’ because of its primitive use as a fuel for lamps. This is further refined to remove acid, colour and odour. It is sold as refined oil, which is used largely for mixing with first‑extraction oils to edible varieties. Still another type, of inferior quality, is obtained from extraction with a volatile solvent, usually carbon disulfide. This oil’s commercial name is sulphur oil and it is used both for food and for industry.
Edible olive oil should be practically devoid of free fatty acids. The ‘US Pharmacopeia’ requires that the fatty acid content of olive oil not exceed 1.41%; however, most export types do not exceed 1%. It varies in colour from clear yellow to golden yellow. Some varieties obtained from unripe fruit have a greenish tinge. Almost every producing country produces oils of varying characteristics and qualities; these variances depend on the districts in which the olive is grown and the degree of ripeness of the fruit.
Olive oil is sometimes mixed with other vegetable oils. Mixing, however, is not permitted in all countries; in some it is treated as fraud. Adulterations can be detected by chemical analysis.
Pure olive oil is used largely for culinary purposes and in the preservation of foods, particularly canned fish. It is also used in the textile industry (wool combing), in the manufacture of toilet preparations and cosmetics, in the pharmaceutical industry for medicinal purposes, in the manufacture of high-quality castile soap and as a lubricant.