Processing of olives isn’t for the faint-hearted. To make it in this market requires capital and expertise.

Olives can’t be eaten straight from the tree. They need either to be processed to make table olives or the oil needs to be extracted from the fresh fruit.

Most growers sell their fresh olives to commercial processors, since processing is a specialised business requiring high capital investment, high levels of management and marketing inputs, and is becoming increasingly competitive.
Both the fresh fruit and the finished product have to comply with industry standards, which is especially important when it comes to international trade.

EXTRACTING OIL

Specific olive cultivars are grown for oil extraction and these have been selected for high yield, high quality and ease of extraction.

• The olive fruit of typical commercial oil cultivars contains about 20% oil, 55% water and 25% solids.
• Oil content depends on the cultivar, ripeness and environmental conditions.
• The oil occurs mainly in the fruit flesh, although some oil is also found in the seed, so the entire fruit is usually milled for oil extraction.
• About 90% of the oil in the fruit can be extracted.
• To produce a high-quality edible oil with a fresh and delicate fruity flavour, it is essential to use good quality fruit and clean equipment.
• Don’t use fruit that has been infested with olive fly or infected by anthracnose fungal disease, or any rotten, damaged or badly bruised fruit.
• Metal equipment, which is in direct contact with olive paste or oil, should be made of stainless steel.
• All equipment should be washed regularly using high-pressure hot water hoses.
• Fruit should ideally be processed within 24 hours after harvest to limit the breakdown of fruit tissue and the resultant oxidation and contamination of the oil.
• Carefully picked, firm, green fruit can be cool-stored for a while, but bruised ripe fruit should be crushed immediately.

Olives are washed in water, then crushed in hammer mills, after which the pulp or paste is churned in a malaxer so that the droplets of oil coalesce into larger drops to facilitate separation. The resulting continuous oil phase can then be separated from the other phases by means of a horizontal centrifugal decanter.

Some natural flavour and health components in olive oil, such as phenols, are water-soluble and can be reduced by adding excessive quantities of water or by stirring the paste for a very long time during oil extraction.

Cold-pressed oil, in which the natural properties and flavours are maintained, has to be produced at below 30°C.
Clarifying oil is usually done by a vertical centrifugal separator, after which oil is run into stainless steel settling tanks where sediment is removed and the clear oil is regularly drawn off.

• Oil should not be in contact with sediment longer than is absolutely necessary, since the sediment is highly fermentable and imparts off-flavours to the oil.
• Oil is stored in stainless steel tanks under nitrogen in cool (15°C), dark conditions and – if necessary – can be filtered before bottling.
• Light, air, moisture and heat will promote oxidation and rancidity.
• The pulp remaining after extraction (pomace) can be used in the orchard as lignin-rich organic material, preferably after an aerobic composting treatment.

QUALITY ASPECTS

The quality of olive oil depends on a number of factors, during the various production stages.

These include the cultivar, climatic conditions where grown, condition of the fruit when harvested, the absence of pesticide residues, the harvesting method and fruit storage, the time that elapses between harvesting and processing, cleanliness of fruit and equipment, production techniques and storage conditions of the final product.

• Cooler growing areas generally provide more intense, desirable fruity flavours, compared to hot areas.
• After reaching a fruit maturity level where oil content and oil quality are high, the quality of the oil and stability to oxidation deteriorate as ripeness increases.
• Olive oil fresh from the press normally has a strong, throat-catching, pungent, astringent or peppery taste.
• Fresh oil is usually more intensely flavoured with grassy and green fruit notes.
• After a few months, however, depending on the cultivar, these flavours tend to soften.
• The oil of some cultivars (for example, Coratina and Manzanilla) is characteristically bitter compared to Arbequina or Kalamata, which is sweeter.
• Oil from Picual and Barouni has a pronounced banana or subtropical fruit note, while oil from some ripe table olive cultivars such as Gordal have a wild berry aroma.
• Mouth-feel of some oils is heavy and fatty, while the more desirable oils are less limpid.
• The colour of the oil is also influenced by the stage of ripeness and the cultivar.
• Greener-fleshed fruit produces greener-coloured oils.
• The cultivar Koroneiki can give a pronounced green colour to oil.
• Oil from riper fruit is generally mild, compared to the fruitier, aromatic but harsher green fruit.

TABLE OLIVES

Many cultivars can be pickled or cured, although those grown primarily for oil production would not give a good quality table olive and are not normally used for pickling.

The following qualities are desirable in properly processed olives:

• Attractive appearance: An acceptably uniform colour, uniform size grade and being entirely free of blemishes, leaves, stalks and any foreign matter.
• Taste: Largely free from the bitter glucoside (oleuropein) found in freshly picked fruit, and no malodorous taste caused by incorrect processing techniques.
• Texture: The olives should be fairly firm to almost crisp, but not hard, fibrous or mushy.
• They should have an adequate shelf-life, without any risk of spoilage, and also meeting local health requirements and international standards (Codex Alimentarius).

To achieve these qualities, it is essential to start with high-quality fresh fruit and to work in clean conditions.

RECIPES

Table olives can be processed according to the American method, which involves heat treatment, or by any of several other methods.

Some of these recipes require the chemical removal of bitterness through an alkali-leaching process (so-called treated olives) while other recipes call for a more natural, slower removal of bitterness (eg through repeated water washes or controlled fermentation).

Most recipes depend on specific microbiological processes that have specific temperature, pH and saline requirements. If not met, the lack could result in spoilage, serious losses and disastrous health consequences. On the other hand, overheating, too-high acidity or a too-high salt content can impair eating quality.

Sometimes olives are sold which have undergone spoilage, such as zapateira, and which have the penetrating stench of decomposing organic material. Some consumers and processors actually consider this smell to be a quality attribute of this type of olive. Unfortunately, many potential consumers are put off eating olives forever after being exposed to such inferior products.

Commercial table olive processors have to ensure a safe product with a long shelf-life, so they tend to maintain high salt and/or acid levels in the final product. Home processors, on the other hand, have more latitude in preparing a product suited to their personal taste, usually adding various herbs, spices, and so on.

Many traditional commercial processing techniques involve the use of large quantities of water, salts and alkalis. They also produce waste water containing phenols and other organic matter which can be environmental pollution hazards. Take care to minimise effluents through reusing brines, using alternative processes and products (such as natural debittering and at least partial substitution of sodium with potassium in salts and lyes).

PREMIUM OIL QUALITY

Badly made or defective oils have various characteristic off-flavours, such as fusty (from olives stored in heaps, which have undergone fermentation), musty (from damp, mouldy olives), dregs (contamination by fermenting sediment), rancid (from oxidation), leafy (from not removing leaves), metallic or rubbery (from contamination by non-inert equipment or storage tanks) or derived from dirty press mats or infested olives.

• The production of olive oil on a large scale in the major olive-growing countries has a beneficial cost effect compared to that of new world countries.
• However, this massive scale of production, together with the approach of maximised oil yield harvested late by trunk shakers, often causes inferior quality.
• Lobby groups interested in selling this oil benefit by misinformation and legislation enforcing the use of confusing trade names for olive oil.

According to the International Olive Oil Council (IOOC) Trade Standard, the attributes that determine the quality of olive oil can be determined chemically and physically by such parameters as acidity, ultra-violet absorbance, peroxide value and the more subjective organoleptic rating by a trained and accredited tasting panel.

The legislation uses the level of free acidity as a norm. Free acidity is a measure of the degree of breakdown of the fat molecule, and there is a close relationship between degree of breakdown of fat and decomposition or rotting of fruit.

The IOOC defines the best grade as Extra Virgin Olive Oil, where free acidity is below 0.8% and, when scored by the panel, the median of the defects is equal to 0 and the median of the fruity attribute is greater than 0.

Also read:
An introduction to olive farming
How to manage olive trees

• This article was written by Carlo Costa and first appeared in Farming SA.