History of Olive Oil Extraction

Extraction of oil from the olive fruit for use as food dates back to 2000 bc (Amouretti, 1996) when oil was extracted by mechanical means involving crushing, pressing, and decantation (separation of the oil and vegetable water phases). Olives were crushed by hand using simple stone implements, the paste obtained was further pressed to expel the oil and then hot water was added to aid the separation of the oil from the vegetable matter. It has been well reported that during the Bronze Age stone rollers/millstones were built and used for crushing the olives. After crushing, the olives were placed under large mechanical presses in order to use pressure to expel the oil from the flesh. To aid with the decantation process water was added to the extracted oil as the oil would float and could be separated from the top of the earthenware jars (Amouretti, 1996). These three steps are still the key principles used in olive oil extraction to this day. There have been continual improvements to each step of the process to ensure oil of high quality not necessarily greater oil yields, though this is still important (Amouretti, 1996; Kiritsakis, 1998). Examples of improvements include the implementation of rapid processing soon after harvesting or improvement of press designs. More recently, improvements to the pressing and oil separation step and decantation/centrifugation have been implemented due to new equipment and technology.

Crushing results in the formation of a paste containing plant and oil cells. The oil is released from the oil cells by mixing and the application of pressure. The resulting oil is then separated from the vegetable material by decantation and in more recent times with the application of centrifugation to separate the various phases, making the process continuous and less labor intensive (Petrakis, 2006). The extraction process must produce oil that is of the same composition and sensory attributes as found in the fruit. To achieve this, oil must be extracted at no higher than 27°C to avoid any thermal degradation, and no additives can be used (European Commission, 2002). The key objective during virgin olive oil processing is to extract the maximum oil yield from the fruit while maintaining oil quality and important sensory and composition characteristics.

The recovery of oil from the olive pomace produces oil of lesser quality compared to the first press in terms of sensory attributes and quality, with higher free fatty acids. Pomace oils can be distinguished from virgin oils by their chemical composition as the second extraction process results in the extraction of previously unrecovered minor components. This oil is either sold as olive pomace oil or is further refined using standard oil refining techniques (Fedeli, 1996; Kiritsakis, 1998; Petrakis, 2006).

Development of Avocado Oil

In contrast to olives, avocados are primarily grown for fresh fruit consumption. Avocado production worldwide has increased by 52% since 1999. With this growth in production there is a concurrent supply of reject grade fruit for which growers would like to find a market. There is growing competition for this reject fruit for culinary oil and for guacamole products made possible with the development of high pressure processing technologies. Over 90% of the world’s production of avocados is of the cultivar “Hass.” The flesh of a “Hass” avocado can contain as much as 32% oil (based on fresh flesh weight) depending on the time of harvest. This is a maximum, and the average at the peak of the processing season is approximately 18%.

Avocado production in New Zealand has increased from 1,000 tonnes in 1987 to 26,766 tonnes in 2007 (Requejo-Tapia, 1999; Plant & Food Research, 2008) with considerable potential for future increases in production. Most first grade avocados are exported overseas while the New Zealand market absorbs the export rejects, rejects only in the sense of cosmetic appeal (size, shape, and color). Production levels in New Zealand vary year to year depending on the season (some trees bear fruit biennially), weather (wind and storms can damage fruit, which are unsuitable for fresh fruit export), and export markets. Over the 2008/2009 season, New Zealand processors produced more than 150,000 liters of extra virgin avocado oil, with production expected to increase in the 2009/2010 season. Extra virgin avocado oil is also being produced in Mexico, Chile, South Africa, and Kenya (Wong et al., 2010).

The use of avocado oil for skin and cosmetic uses has been reported as early as the 16th century, but it was not until the 21st century that commercial operations began to extract avocado oil for culinary uses. Early attempts to extract avocado oil involved dehydration first of the flesh and also the use of solvents to maximize oil yields (Woolf et al., 2009). Early in the 21st century two commercial producers in New Zealand began extracting avocado oil using a process similar to the extraction of olive oil. Earlier, olive oil extraction involved crushing and pressing but the pressing step was made continuous with the introduction of the three-stage decanter in 1970 by Alfa Laval (Alba et al., 2011). For large operators, the use of horizontal decanting centrifuges is now standard and final polishing of the oil was achieved with vertical disc centrifuges. Avocado oil processors in New Zealand adapted the process for olive oil extraction by taking advantage of the continuous process afforded by horizontal decanters. The process requires minimal addition of water and no other additives are required. The goal of this extraction process for culinary avocado oil is the same as for olive oil: maximum oil yields are desired while maintaining oil quality, sensory and compositional characteristics with no subsequent processing or refining required.

Olives and avocado fruit contain high levels of water, 50 and 65% (by fresh weight), respectively (Kiritsakis, 1998; Woolf et al., 2009). Advances in the extraction process for olives led to the development of continuous processing with minimal energy usage and reductions in waste streams. Where oil extraction from other oil bearing fruits and seeds may require pre-drying or digestion steps prior to oil extraction, the oil extraction process from olives and avocados uses the presence of water to aid with separation during the decantation steps and does not require high temperatures to encourage the release of oil from the oil bearing cells. However, too much water can lead to emulsion problems, which will be discussed later in this chapter.

Standards for Virgin Oils

The International Olive Council (IOC) was set up in 1959 under the auspices of the United Nations. The IOC standards for olive oil, also adopted by the European Commission regulations and Codex Alimentarius for food, define the various grades for olive oil. Virgin olive oil is oil extracted solely from the olive fruit by physical or mechanical means, involving the use of washing, crushing, decanting, centrifugation, and filtration. The process does not alter the oil found in the fruit, and the process must be controlled thermally without the use of solvents, esterification, or other oils (International Olive Council, 2011). Virgin olive oils are fit for consumption immediately after extraction. There are various classes of olive oils ranging from extra virgin olive oil (the highest quality) to ordinary virgin olive oil. For oils to fall into each class, they must meet standards based on chemical quality, chemical composition, and sensory evaluation. After virgin olive oil, there are also standards for olive pomace oil, extracted from the olive pomace, both refined and unrefined (International Olive Council, 2011). Suggestions that the IOC standard does little to prevent old and stale oils being sold on export markets led to the development of improved standards by the USA in 2010 and by Australia in 2011 (USDA, 2010; Standards Australia, 2011).

Guided by the standards for virgin olive oil,...