What is unrefined, extra virgin cold-pressed avocado oil?
By Marie Wong, Cecilia Requejo-Jackman, and Allan Woolf
The high oil content of the avocado fruit (Persea americana Mill.) has been known since Aztec times, with the fruit sometimes referred to as "vegetable butter" or "butter pear." The plant originated in Central America, and its cultivation has spread to warm subtropical and temperate climates worldwide. The flesh of an avocado can contain up to 30% oil (based on fresh weight), but there is very little in the seed (≈2%) or the skin (≈7%). Avocado oil was originally, and still is, extracted for cosmetic use because of its very high skin penetration and rapid absorption. Following pre-drying of the avocado flesh to remove as much water as possible (≈65% water in avocado flesh), avocado oil for cosmetics is traditionally extracted with solvents at elevated temperatures. After extraction, the oil for application in skin care products is usually refined, bleached, and deodorized, resulting in an odorless yellow oil.
Avocados are primarily grown for the fresh fruit market, either domestic or export. There are 12 major cultivars of avocado, but the main cultivars grown globally are ‘Hass' and ‘Fuerte.' The ‘Hass' cultivar constitutes more than 90% of the avocado crop in New Zealand (NZ) as this cultivar has excellent yield potential and also suffers less postharvest and handling disorders owing to their thicker skins. In 2000, two processing companies in NZ began production of extra virgin avocado oil; they pioneered the process for extracting cold-pressed avocado oil, which is sold as culinary oil for use on salads and for cooking (Eyres et al., 2001). Like extra virgin olive oil, cold-pressed avocado oil is unrefined and so retains the flavor and color characteristics of the fruit flesh.
Production levels in NZ vary year to year depending on the season (some trees bear fruit biennially), weather (wind and storms can damage fruit, which are then not suitable for fresh fruit export), and export markets. Over the 2008/2009 season, NZ 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 Chile, South Africa, and Kenya. Extra virgin avocado oil from NZ is exported to Australia, Japan, Southeast Asia, Europe, and North America by Grove Avocado Oil (Tauranga, NZ) and Olivado (Kerikeri, NZ). Avocados are primarily grown in NZ for fresh fruit export; any fruit not suitable for export is sold on the local market or to processors (for oil and other processed avocado products). Approximately 3% of the NZ avocado crop is processed for the oil. Windfall fruit not suitable for the local market is sometimes bought by oil processors. Fruits that are rotten or exhibit postharvest disorders and are unsuitable for consumption are not used for oil production.
The process developed in NZ for extraction of avocado oil is based on the mechanical extraction method used for olive oil. However, before we discuss the extraction process, we need first to consider the avocado fruit and its development. It is important to understand the pre- and postharvest physiology of avocados, particularly how their maturity and degree of ripeness impact the optimum time for oil extraction.
Cultivation of avocado
Avocados are grown in frost-free subtropical regions. Once the fruit has formed on the tree, it slowly matures (10 months), increasing in size and oil content. Most avocado-exporting countries have guidelines for when to harvest the fruit such that they are considered to be at optimum maturity to ensure they ripen and develop the desirable flavor and texture profile. The maturity of avocados is easily determined by measuring the dry matter content of the fruit (e.g., for NZ, the minimum dry matter for export is 24% by fresh weight; for the United States, it is 20.8%). The dry matter content is highly correlated with the total oil content in the fruit, hence harvesting at less than the minimum recommended dry matter content level will result in fruit with less than optimal oil content. The correlation between dry matter and oil content has been found to be valid across a number of countries (NZ, Australia, and United States), with different regions and orchards.
Avocado fruit do not ripen while they remain on the tree even once they have reached maximum maturity. If the fruits are not harvested, they can remain on the tree even when the next year's fruit is developing, and can remain on the tree for more than 18 months from flowering. Once harvested, the avocado will begin to ripen. This process involves the softening of the flesh due to endogenous pectolytic enzyme activity and, for some varieties, the coloring of the skin from green to purple-black. The degree of ripeness of the avocado is primarily determined by measuring the firmness of the fruit. Hence to ensure the oil content in the avocados is at the maximum for processing, the fruit should ideally be mature at harvest. This is not the case with immature fruit blown off the tree in a storm (windfall fruit); this fruit is still ripened but the oil content in the fruit is less than optimum. Ripening leads to tissue softening, which aids with the extraction of the oil due to the release of the oil from the parenchyma cells. Ripening can be promoted by treating the fruit with ethylene (a gaseous plant hormone that synchronizes ripening) in controlled-temperature rooms.
To have optimal oil quality, avocado fruit should not be overripe and also should have minimal rots or other postharvest disorders (such as flesh greying due to long storage). The amount of oil extracted from mature and ripe avocados earlier in the season has been found to be only approximately 75% of the maximum available oil in the flesh (15% oil by fresh weight) compared to later in the season when it is possible to extract more than 90% of the available oil, this being the maximum oil yield (≈25% oil by fresh weight).
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EXTRACTION OF AVOCADO OIL
The process for recovering oil from ripe avocados is a mechanical extraction, similar to olive oil extraction, with the additional step of removing the skin and stone (seed). After this, the flesh is ground to a paste and then malaxed for 40-60 minutes at 45-50°C. This is a higher malaxing temperature than used for olive oil extraction, but it is still considered to be cold-pressed extraction for avocado oil. The slightly higher temperature aids the extraction of the oil from the oil-containing cells and does not affect the quality of the oil. The oil and water phases are separated from the pulp using a high-speed decanting centrifuge, and then the oil is separated from the water in final polishing centrifuges. The pulp from the decanting centrifuge and waste skin/seeds are returned to orchards for soil conditioning and mulch, or used as animal feed.
Avocado oil, if extracted from sound fruit (no rots, physiological disorders, or damage), will result in oil with a very low percentage of free fatty acids (%FFA) (<0.5% as oleic acid). Also, the peroxide values (PV) can be very low (<2 meq/kg). Recommended standards for extra virgin avocado oil have proposed a maximum PV of 4 meq/kg (Table 1).
In sound, ripened fruit, the level of lipolysis that occurs is low, resulting in low %FFA. The fruit does not need to be processed immediately after ripening, but long delays should be avoided. Generally a higher %FFA is due to poor-quality fruit, delays in processing ripened fruit, or poor manufacturing practices.
PROPERTIES OF AVOCADO OIL
Extra virgin avocado oil from the ‘Hass' cultivar has a characteristic flavor, is high in monounsaturated fatty acids, and has a high smoke point (≥250°C), making it a good oil for frying. ‘Hass' cold-pressed avocado oil is a brilliant emerald green when extracted; the color is attributed to high levels of chlorophylls and carotenoids extracted into the oil.
Cold-pressed ‘Hass' avocado oil has been described as having an avocado flavor, with grassy and butter/mushroom-like flavors. Other varieties may produce oils of slightly different flavor profile as has been seen with ‘Fuerte,' which has been described as having more mushroom and less avocado flavor.
The fatty acid profile is very similar to olive oil, in that it is very high in oleic acid. A typical avocado oil has 76% monounsaturates (oleic and palmitoleic acids), 12% polyunsaturates (linoleic and linolenic acids), and 12% saturates (palmitic and stearic acids); these values are given as percentage of fatty acid/total fatty acids. The main antioxidant in the oil is a-tocopherol, which is present at levels of 70-190 mg/kg oil. b-, g-, and d-tocopherols are only present in minor amounts (<10 mg/kg oil). Other nonlipid components present in the oil include chlorophylls (11-19 mg/kg oil) and carotenoids (1.0-3.5 mg/kg oil).
The chlorophylls from the flesh and the skin contribute to the characteristic emerald green color of the oil. Depending on the location in the mesocarp, the chlorophyll content varies, but the majority of chlorophyll and carotenoids are present in the greener layers of flesh next to the skin. If avocado skin is included in the pulp during malaxing, then the likelihood of extracting more pigments is greater. Chlorophyll does not contribute to oil stability but can be a problem, as chlorophyll can act as a sensitizer for photo-oxidation to occur. Therefore, it is important to store the oil away from light.
Carotenoids in avocado fruit have long attracted attention for their potential anti-carcinogenic effect; these same carotenoids are subsequently extracted into the oil. The most significant carotenoid present in the oil is lutein (0.5-3.3 mg/kg oil). Lutein is beneficial for eye health by reducing the progression of age-related macular degeneration. The cold-pressed avocado oil also contains high levels of phytosterols (b-sitosterol being the main sterol present), at 2.23-4.48 mg/g oil. Based on its fatty acid makeup and the presence of these phytochemicals, extra virgin cold-pressed avocado oil is considered to be a healthful oil.
STANDARDS FOR AVOCADO OIL
The impacts of postharvest procedures, preprocessing treatments, extraction, and storage on the composition, quality, and sensory characteristics of avocado oil have been investigated over the last 10 years in NZ in collaboration with Australian and Californian research groups. Standards have been proposed for avocado oil, including extra virgin, virgin, and pure grades of oil (Table 1). These standards have been recommended to ensure that avocado oil sold is of good quality in terms of standard quality indices, composition, and sensory properties. The standards are unique to avocado oil, where cold-pressed avocado oil is recovered by mechanical extraction at temperatures less than 50°C, without solvents; water and enzymes can be used. These standards are important, as the production and culinary consumption of cold-pressed avocado oil, with its light, distinctive flavor, is increasing worldwide.
Marie Wong is senior lecturer at the Institute of Food, Nutrition & Human Health, Massey University (Auckland, New Zealand). She can be contacted via email at M.Wong@massey.ac.nz . Allan Woolf and Cecilia Requejo-Jackman are with the New Zealand Institute for Plant & Food Research Limited (Auckland, New Zealand).
For further reading:
Eyres, L., L. Sherpa, and G. Hendriks, Avocado oil: A new edible oil from Australasia, Lipid Technology 13:84-88 (2001).
Woolf, A., M. Wong, L. Eyres, T. McGhie, C. Lund, S. Olsson, Y. Wang, C. Bulley, M. Wang, E. Friel, and C. Requejo-Jackman, Avocado oil. From cosmetic to culinary oil, in Gourmet and Health-Promoting Specialty Oils, R. Moreau and A. Kamal-Eldin, eds., AOCS Press, Urbana, Illinois, USA, 2009, pp. 73-125.
For more information about the Gourmet and Health-Promoting Specialty Oils monograph, visit http://tinyurl.com/gourmet-oils-aocs . For a review of the book, see inform 21:164, 2010.