Publication
from The American Oil Chemists' Society
by Marie Wong, Cecilia Requejo-Jackman, and Allan Woolf
What is unrefined, extra virgin cold-pressed avocado oil?
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).
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.
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