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February 2018

Green Olives

Green olives are obtained from olives harvested during the ripening cycle when they have reached normal size, but prior to colour change. They are usually hand picked when there is a slight change in hue from leaf-green to a slightly yellowish green and when the flesh begins to change consistency but before it turns soft. Colour change should not have begun. Trials have been run to machine harvest table olives, but owing to the high percentage of bruised fruit they had to be immersed in a diluted alkaline solution while still in the orchard. Recently harvested, the olives are taken to the plant for processing on the same day if possible.

Green olives are processed in two principal ways: with fermentation (Spanish type) and without fermentation (Picholine or American type).

The olives are treated in a diluted lye solution (sodium hydroxide) to eliminate and transform the oleuropein and sugars, to form organic acids that aid in subsequent fermentation, and to increase the permeability of the fruit. The lye concentrations vary from 2% to 3.5%, depending on the ripeness of the olives, the temperature, the variety and the quality of the water. The treatment is performed in containers of varying sizes in which the solution completely covers the fruit. The olives remain in this solution until the lye has penetrated two thirds of the way through the flesh. The lye is then replaced by water, which removes any remaining residue and the process is repeated. Lengthy washing properly eliminates soda particles but also washes away soluble sugars which are necessary for subsequent fermentation.

Fermentation is carried out in suitable containers in which the olives are covered with brine. Traditionally, this was done in wooden casks. More recently, larger containers have come into use that are inert on the inside. The brine causes the release of the fruit cell juices, forming a culture medium suitable for fermentation. Brine concentrations are 9-10% to begin with, but rapidly drop to 5% owing to the olive's higher content of interchangeable water.

At first Gram-negative bacteria multiply, but after a week and a half they disappear. They are a consequence of contamination produced in the plant installations, and in the atmosphere and brine and can be avoided by stepping up hygiene measures. At a pH level of 6 and upwards, lactobacilli develop massively until the Gram-negatives disappear and the brine attains a pH of 4.5. There is a predominance of Lactobacillus plantarum which produces lactic acid from glucose almost by itself. When the fermentable matter is spent, acid formation ceases. Yeasts appear together with the lactobacilli. Fermentative yeasts do not cause deterioration but oxidant yeasts consume lactic acid and raise the pH level and may therefore jeopardise the process.

Under certain conditions normal fermentation processes can be altered by the presence of undesirable microorganisms which can transmit poor organoleptic properties to the olives or impair their keeping properties. Gas pocket fermentation is caused by the Gram-negative bacilli in the first stage of fermentation, but can be controlled by intensifying hygiene precautions when the olives are delivered to the plant, as well as during lye treatment and washing. If gas pockets still appear in spite of these measures, the pH level can be lowered to 4 by adding an acid. Butyric fermentation is well controlled by ensuring the proper pH level. Putrid fermentation is caused by poorly-kept containers and bad water. Lastly, there is a type of deterioration known by its Spanish name of "zapatería" (cobbler's) which produces an unpleasant taste and odour at the end of the fermentation process, often coinciding with rising temperatures in the spring or early summer. It is produced by bacteria belonging to the Clostridium and Propioni-bacterium genus. The right combination of brine concentration and pH level (5% salt and 4.5 pH) helps to control fermentation processes.

When properly fermented, olives keep for a long time. If they are in casks, the brine level must be topped up. At the time of shipment, the olives have to be classified for the first or second time as the case may be. The original brine is replaced and the olives are packed in barrels and tin or glass containers. Sometimes they are stoned (pitted) or stuffed with anchovies, pimento, etc.

The most commonly used varieties are Manzanillo, Gordal and Moroccan Picholine.


Olives belonging to the Picholine variety from Languedoc and Lucques in southern France are prepared in this manner, as are other varieties from Morocco and Algeria.

The bitterness of the olives is removed by treating them in a 3-3.5º B lye solution in which they are left for 8 to 72 hours until the lye has penetrated three-quarters of the way through the flesh. They are rinsed several times over the next day or two, and then placed in a 5/6% brine solution for two days. A second 7% brine solution is prepared, and acidity is corrected with citric acid (pH 4.5). After 8-10 days they are ready to be eaten and retain their intense green colour. Sometimes the consignment has to be postponed, and it is necessary to store the olives. This is easy, as long as temperatures do not rise. The olives can stay in an 8% brine solution until spring, but then it has to be raised to 10%. In larger installations they can be kept in cold storage, at a temperature of between 5º and 7º C, in a 3% brine solution.

Before shipment, the olives are washed repeatedly, sorted and packed in suitable containers in 5º or 6ºB brine.