Inspired by virgin olive oil extraction technology, cold pressed extraction could replace traditional ones for avocado oil, to produce a purpose-ready oils for the edible oil market.
The most commonly used method used to obtain oil from avocado is through organic solvent extraction. Fruit pulp is dried by warm air. Hexane solvent extraction is used with a yield of up to 95 per cent. The resulting oil is brownish with a high pigment content, which needs to be refined for application.
In locations where solvent extraction is not available, a mechanical method is used. Fruit is first peeled and destoned, and pulp is mashed before being dried. The resulting paste is heated with hot water and chalk and/or sodium chloride, and pressed or skimmed off (by natural decantation) or centrifugation.
Centrifugation or pressing yield, is reduced to 60 to 80 per cent compared with the solvent method. These traditional processes generally result in poor-quality oil that must be refined first before it can be used. It therefore comprises a poor cost/benefit ratio.
Cold pressed extraction
Applying an extraction process adapted from conventional treatment for olive oil with proper adjustments, resulted in a private New Zealand company and partner Alfa Laval obtaining unrefined avocado oil that retains its flavour and fruity taste. It offers a yield variable between 75 and 85 per cent depending on fruit characteristics.
Whole fruits are washed in a two-stage washing system and then elevated into a destoning machine, where pips and between 90 to 95 per cent of skin are separated from pulp.
Pulp and remaining skin are then pumped into a disc crusher for further refining if necessary, depending on the ripe degree of the fruit. The disc crusher is important in cutting filaments that remain in the paste and simultaneously minimises emulsion.
Avocado mash is pumped into the section equipped with malaxers (kneading machines). Each machine consists of a stainless-steel tank with a central screw stirring the mash slowly and continuously at a monitored temperature.
The effect of the kneading machine on avocado paste is similar to what would happen with olive paste: small oil drops released during fruit milling merge into large drops (coalescence phenomena). These are easily separated by centrifugal extraction. The definition of optimal malaxing time and temperature is important to reach the best compromise between quality and quantity of extracted avocado oil. In the case of avocado mash, malaxing time should not exceed 90 minutes and temperature should be below 45°C.
Coalescence and oil extraction are not the only reasons for crushing and malaxing processes. As with extra virgin olive oil extraction, total phenol content and aromatic fraction are strongly affected by parameter adjusting and design of equipment.
After malaxation, separation of oil from solid and liquid phases are done using a decanter centrifuge. Mash from the malaxer is fed into the machine where oil, vegetation water and solids (exhausted pulp and residual skin) are separated.
Extraction should be carried out in two or three phases in a continuous system. An automatic device that guarantees easy adjustable parameters during processing enables optimisation of yield and quality with water and energy consumption at minimum levels. The oil phase is collected separately under the decanter and pumped out to a disc stack purifier centrifuge. In the case of extraction in three phases, water is pumped to the disc stack concentrator centrifuge.
Avocado oil flowing from the decanter will still have a certain amount of water and solids. Similarly, vegetation water from the decanter will still contain a small quantity of residual oil. The system consists of a disc stack centrifuge for final avocado oil purification to remove residual water and solids. A second disc stack centrifuge should be used to recover residual avocado oil from vegetation water flowing from the decanter working in three phases.
The oil that comes out from the disc stack purifier centrifuge is an extra virgin high quality avocado oil. After simple filtration, is suitable to be bottled and consumed. In some cases, dewaxing by winterisation can also be required before bottling.
Key factors for top quality production
The production of top quality oil for a culinary-dressing purpose is a reality. Attention must be paid to multiple factors to ensure results. Amongst these, three are considered most important.
The first aspect is the agroecology and climate of the fruit’s area of cultivation, designated to high quality oil production. To obtain oil with the highest ratio of monounsaturated fatty acids with respect to saturated ones, avocadoes must be grown in areas where temperatures in the winter are not too high. A similar relation between fatty acids composition and climate has been observed and investigated for olive oil. Additional research should be conducted to gain insights specific to avocado oil.
The second important factor is logistics of harvesting and identifying when would be the right moment to process oil. The dry matter content of fruit and firmness of pulp are important agronomical parameters that affect the proper equilibrium between yield and level of oil quality. Avocado’s should only be processed at the right time.
The third important factor to consider is the choice of the right process technology. With the right equipment and proper process will turn high quality of fruit into the high-quality oil.
An improved cost/benefit ratio
A combination of proper cultivation climate, ripeness and process technology delivers high quality unrefined avocado oil with less than 0.5 per cent free fatty acids (as per cent of oleic acid) and less than two meq/kg peroxide value.
Marie Wong and colleagues from Massey University of Auckland in New Zealand investigated the quality of avocado oil obtained by cold pressed method from the Hass variety. The oil has an avocado flavour, with grassy and butter/mushroom-like flavours. The fatty acid profile is very similar to olive oil with 76 per cent of monounsaturated (oleic and palmitoleic acids), 12 per cent polyunsaturated (linoleic and linolenic acids) and 12 per cent saturates (palmitic and stearic acids). High percentage of monounsaturated fatty acid brings a high smoke point (more than 250°C) making it good oil for frying.
Positive attributes of extra virgin avocado oil are not limited to fatty acids composition. Researchers also report that there is a high content of valuable antioxidants in the oil. The main one is alpha-tocopherol, which is present at levels of 70 to 190mg/kg oil. Beta-, gamma-, and delta-tocopherols are only present in minor amounts (less than 10mg/kg oil). Other non-lipid components present in the oil include chlorophylls (11 to 19mg/kg oil), phytosterols (2.23 to 4.48mg/g oil) and carotenoids (1.0 to 3.5mg/kg oil) with lutein as the most representative.
High monounsaturated fatty acids, antioxidant and other non-lipid content make unrefined avocado oil a food comparable to high-quality extra virgin olive oil.
Avocado production is continuously growing as awareness of its healthy effects spreads among consumers, and consequently raises its consumption. The discovery and application of cold pressing as an extraction method has led to the introduction of a completely new food oil – avocado. Its role in cooking and its health-related benefits must still be fully explored.
Investigation in these areas to correlate agronomical and technological factors for extra virgin avocado oil is strongly recommended. It offers a wide range of potential flavours and characteristics for avocado products and an opportunistic market for manufacturers.