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Scientific Studies Related to Avocado Oil » Avocado Oil: A New Edible Oil from Australasia

Avocado Oil: A New Edible Oil from Australasia
Laurence Eyres, Nimma Sherpa, Glenn Hendriks

Oil from the flesh of the Avocado pear (Persea americana) has been an item of commerce for many years and it has been produced on a large scale in several countries notably Mexico, South Africa and Israel. This oil however has been initially extracted from poor quality and waste avocados using solvents then refined, bleached and deodorized. It has mainly been used as a first-class cosmetic raw material and it has not been viewed as edible oil. A new industry has sprung up in New Zealand whereby oil is produced from good quality avocados and is of such quality that it is termed extra virgin and is ideal as top quality culinary oil.

Introduction
The avocado originated in Mexico and was prized by the Aztecs as a health enhancing fruit for their nutrition and as an excellent emollient for their skin. For many years it has been processed for oil in Mexico and other countries as a method of utilizing waste fruit.

Although well-published data is not available for production and international trade of oil, the US volume is said to be around 1000 tonne. The main outlet for this trade is into the cosmetics industry where it is highly valued for its beneficial effects on skin. The relative production of avocados by country shows that Mexico is the largest producer (34%), followed by the USA (8%), Israel (4%) and South Africa (2%) with the total production being an estimated 250,000 tonne.

Avocado fruit production has increased significantly in New Zealand and projected avocado production figures suggest that there will be a large surplus of second grade (i.e. non-export quality) avocados in the near future. Recent agricultural data indicate a growth in exports of 20-25% per annum and an almost doubling of the land under cultivation for this crop. This will present a problem for avocado growers that will increase with their production volumes. The small population of New Zealand (3.8M) can only consume a finite amount of avocados, even with marketing campaigns encouraging the increase of avocado consumption. This over-supply is likely to result in a drop in avocado prices, making them less economical for growers.

Figure 1: Projected trend of avocado production in New Zealand (1).
Figure 1: Projected trend of avocado production in New Zealand (1).
An alternative to wasting these second grade fruit is to turn them into useful by-products. It is well known that avocados have relatively high oil content, which is comparable to olives. This oil can be extracted as a way of utilising surplus fruit as well as producing a value added product. The oil is of good quality because the processed fruit is still intrinsically sound and is only termed second grade due to its appearance.

Avocado oil is not recognised as significant on the world market due to the relatively high raw material cost and small production scale. For this reason, producing a 'cold-pressed' extra virgin avocado oil (analogous to extra virgin olive oil) would be more likely to recover the value of the fruit. Such a process has been commercialised at Olivado in New Zealand utilising modern Alfa Laval centrifugal extraction equipment. Currently there are two processing plants in New Zealand and both can produce olive oil as well as avocado oil.

Raw Material
In the main traditional producing countries avocado oil has been available only as solvent extracted, refined, bleached and deodorized oil. This is because the crude oil has been extracted from fruit of very poor quality and the oil component had already undergone significant degradation. This processed oil was available mainly to the cosmetics industry that valued it because of its beneficial effect on the skin. Since extra virgin avocado oil undergoes only minimal cold-press extraction, fruit that are processed must be of good quality. In New Zealand, this means that there is a consistent outlet available for fruit that are still of high quality, but do not make export grade for aesthetic reasons. Previously these had been sold locally where the value of the fruit has become low due to oversupply.

Fruit of the 'Hass' variety have been found to be the most compatible with cold-press oil extraction procedures due to their high content of flesh that contains high levels of oil. Depending on the location of the orchard, oil content in the flesh of these fruit can range from 16-17% in September to 25-30% in April (1). Not only is there more oil in fruit from late season, but also the oil is also easier to extract from the fat containing 'idioblast' cells.

Typical composition of a New Zealand Hass avocado
Figure 2: Typical composition of a New Zealand Hass avocado.

Extraction
Modern 'cold-press' equipment does not involve pressing oil from the fruit. Instead it involves maceration of the fruit flesh by a high-speed grinder, followed by mixing of the pulp in malaxers. After this process, oil, water and solids are separated by a three-phase decanter and then polished using multi-cone centrifuges. These tasks are performed by modern Alfa Laval olive oil processing units in New Zealand that have been modified to suit the parameters of the avocado fruit. Seminal work in this area was performed by Werman and Neeman (2), who studied the effects of centrifugation rate, pH, salt and mixing temperature on the centrifugal extraction efficiency of avocado oil. They also characterized in detail the compositional analyses of Israeli avocado oil. The analyses do not appear to differ significantly from New Zealand results.

Extraction rates obviously vary during the season because of the change in absolute oil content and typically vary from 10-18% of the whole fruit. The remaining portion is then disposed of as vegetable water and two solid phases - the husks (seed and stones expelled from the de-stoner) and pomace -exhausted fruit flesh expelled from the three-phase decanter.

Economics
The purchase price of the fruit at the factory door is the main determinant of the manufacturing cost and resultant selling price of the oil. The skin and seed are discarded so the ultimate yield of oil can range from ten to fifteen percent of the original weight of the fruit. Currently the oil retails at approximately $5USD per bottle (250ml) on the New Zealand market and at a comparable price in Australia. Extra virgin olive oil sells anywhere between $2USD and $10USD for 250 ml depending on whether it is imported or New Zealand produced. The latter oil has only a two percent market share of the total bottled vegetable oil market. Bulk avocado oil sales are at $10USD per litre and are increasing as overseas users become aware of the reliable supply in New Zealand.

After one season the avocado growers have already seen the positive effects of extracting oil as an alternative to supplying the domestic market with low-grade fruit. The season 2000-2001 saw approximately 1200 tonne of fruit processed yielding around 160 tonne of oil.

Oil quality
Since minimal, low temperature processing is used to extract oil from avocado, many natural constituents of the fruit are retained. These can have both positive and negative effects on the stability of the oil. Recent research performed at Massey University showed that unlike virgin olive oil, FFA values were consistently very low indicating little lipase activity. Thus oxidative deterioration may be considered the main route of quality loss in this oil. The analyses of avocado oil are compared with recent work carried out on New Zealand olive oil carried out by Reed et al (3).

The oil is composed primarily of triacylglycerols with minor amounts of free fatty acids and up to 1.5% unsaponifiable matter. It is similar to olive oil in many respects but has a higher beta-sitosterol content and lower levels of squalene and polyphenols.

Table 1:Typical analyses of extra virgin avocado and olive oil

Analytical result Avocado oil Olive oil
Colour (chlorophyll)(ppm) 40-60 4-6
FFA (as oleic)(%) 0.08-0.17 0.15-0.25
PV (fresh - mEq/kg fat) 0.1-0.2 1.0-2.0
Specific gravity (25C) 0.915-0.916 0.914-.918
Iodine value (from GLC) 82-84 75-82
Beta-sitosterol(%) 0.45-1.0 0.1-0.2
Total vitamin E (mg/kg) 130-200 100-150
Alpha-tocopherol (mg/kg) 130 100
Beta/Gamma-tocopherol 15 10
Delta-tocopherol 5 10
Copper (ppm) <0.05 0.05-0.1
Pesticides Not detected ND

High levels of chlorophyll in avocado oil (40-60ppm) can adversely affect the oxidative stability of the oil, causing rapid formation of oxidation products via the photosensitised singlet oxygen pathway (4) when stored under light. However, the emerald green colour of the oil (originating from high chlorophyll levels) has been identified by consumers as desirable, thus a novel approach to increasing the oxidative stability of this unique oil is currently being studied. This will include minimising oxygen and light exposure during handling of the oil (including shelf storage in dark glass bottles).

There is a growing trend worldwide toward the production of organically grown produce and a significant proportion of the avocado crop meets this specification. Samples of oil were screened for pesticide and elemental copper analysis and the results were favourable as seen in Table1.

Further processing of extra virgin oil
Refined Bleached and Deodorized (RBD) oil has been produced very recently in New Zealand from cold pressed oil that does not meet extra virgin quality standards. This process effectively removes high levels of peroxides and free fatty acid in the oil by using alkali to remove free fatty acids (refining), bleaching earth to remove colour-producing substances (bleaching) and finally stripping objectionable flavours/odours by steam distillation (deodorizing). This RBD process removes almost all chlorophyll and other components of avocado oil that give extra virgin oil its unique properties. The resultant oil is pale yellow (instead of green) and has little remaining avocado odour or taste. This oil will be destined for cooking (since it still contains a desirable fatty acid profile) and cosmetic use. The target quality for the resultant bland oil is for a pale coloured, low acidity and highly stable oil. Typical results for the processed oil are seen in Table 2.

Table 2: Typical analyses of Refined Bleached and Deodorized Avocado Oil.

Colour (51/4 inch cell) 10Y, 1R
Free Fatty Acid (oleic) 0.1%
Peroxide Value (mEq/kg) 0.1
Flavour Bland

Like olive oil, avocado oil is high in monounsaturated fatty acids making it an excellent component of a healthy diet in accordance with the Mediterranean-style recommendations. Typical fatty acid composition in cold press avocado oil was determined using FattyAcid Methyl Ester (FAME) analysis on a Gas Chromatograph.

Table 3: Fatty acid composition of avocado oil/olive oil.

  %Fatty Acid NZ Avocado Oil NZ Olive Oil
Palmitic 16:0 12.5 - 14.0 8.6 - 12.9
Palmitoleic 16:1 4.0 - 5.0 0.3 - 0.7
Stearic 18:0 0.2 - 0.4 2.1 - 2.8
Oleic 18:1 70 - 74 * 77.0 - 82.6
Linoleic 18:2 9.0 - 10.0 4.6 - 7.5
Alpha-linolenic 18:3 0.3 - 0.6 0.5 - 0.7
Arachidic 20:0 0.1 0.0 - 0.6
Gadoleic 20:1 0.1 0.0 - 1.4

*Includes 18:1 isomer(5%)

HPLC of triacylglycerols provides additional data on the composition of the oil and table 4 shows the area% of triacylglycerol separation compared with olive oil.

Table 4: Triacylglycerol (TAG) composition of avocado oil by HPLC (area%)

Peak ECN TAG Avocado Olive Oil
44 OLL +P'OL 1.5 4.2
46 OOP' +OOL + POL 21 19.3
48 OOO +POO 50 71.0
48 OPP 26 -
50 SOO 1.0 5.2

ECN=Carbon no of the fatty acids in the TAG molecule-2n where n is the number of double bonds.
P' = Palmitoleic
O = Olive
L = Linoleic
Further work will be carried out on the fractions obtained by winterizing avocado oil.

Uses
The extra virgin oil has a mild flavour and the main use currently in New Zealand is as an excellent salad and cooking oil and an alternative to extra virgin olive oil which some consumers find too strong in flavour. Its low levels of free fatty acid and phosphatides give it a high smoke point (250 C) and the oil is proving popular in the shallow pan-frying of fish, poultry and venison. It makes a very attractive green dressing when mixed with lemon juice herbs and white wine vinegar. The use of it as a simple drizzle on potatoes and vegetables makes it a suitable replacement for common saturated animal fats, which is currently seen as desirable for Australasian consumers.

The refined oil has a high stability and smoke point and makes it a similar general-purpose oil for frying and baking, similar to pure olive oil. It is a most appropriate oil for barbecues since most common vegetable oils such as soya and canola tend to oxidize and polymerize on hot surfaces and open flames used in much of the southern hemisphere cuisine.

Full benefit is to be taken of the beneficial effect avocado oil has on the skin and the RBD oil makes an ideal constituent of hand creams and sunscreens. There have been many reports from workers in other countries who through handling avocado oil develop excellent skin texture. There seem to be particular beneficial properties in some of the unsaponifiable components, which have been linked to inhibition of lysyl oxidase. Lysyl oxidase initiates cross-linking in collagen by oxidatively deaminating the amino groups of lysine and hydroxylysine residues in collagen. This inhibition of enzyme activity may open up new areas in wound and burn treatment (5).

Potential health benefits
This edible oil is an ideal candidate for the Mediterranean diet as it is predominantly monounsaturated and is low in saturates. It has an added benefit in that it contains beta-sitosterol in significant amounts (0.5-1.0%) and the consumption of this cholesterol lowering sterol is being encouraged worldwide. Beta-Sitosterol may also have further benefits for most males over fifty in alleviating the symptoms of benign prostatic hypertrophy (BPH) (6). Saw palmetto extract finds a credible acceptance around the world for this treatment and the active ingredient in the extract is beta-sitosterol, which is at a concentration of 0.2% in the extract. The daily dosage for efficacy is between 60 and 100 mg per day when administered as a drug. This equates to taking 10-15 gram per day of avocado oil an intake totally in keeping with a healthy lifestyle. Research into the mechanism does not yet appear complete but there have been sufficient clinical trials to confirm the effect. The suggested mode of action involves inhibition of the steroid enzyme 5-alpha-reductase. This enzyme reduces testosterone in the prostate to dihydrotestosterone (DHT). This reaction produces a hormone imbalance, which causes a progressive thickening of the epithelial and fibromuscular structures within the prostate gland. The resultant hyperplasia causes obstruction resulting in abnormal urinary functions. In the ageing process levels of testosterone fall whereas DHT remains high. Beta- sitosterol is reported to reduce the uptake of both testosterone and DHT and also inhibits the conversion reaction. Daguet recently reviewed the exciting potential for phytosterols in Lipid technology (7).

An interesting observation has been made by some consumers about the satiety effects of taking approximately 10 ml of avocado oil as a smoothie in the morning with reports that this alleviated the desire for mid morning snacks. This is an interesting area for future research as we have seen that technically sophisticated and expensive ingredients such as Olibra have been slow to take off.

Future research
The recent introduction of extra virgin avocado oil into the New Zealand market has spawned much interest in this area of research. The following projects are currently under examination at Massey University in Auckland:

  • The oxidative stability of the extra virgin oil and kinetics of photo-oxidation
  • The effect of fruit quality on oil quality and yield
  • Extraction process optimisation utilizing natural enzymes
  • Full compositional analysis and seasonal variability
Future research may lie in other components of the avocado, specifically avocatins in the fruit skin that are reported to have antiviral and insecticidal properties (8), and the unsaponifiable components in the fruit seed as well as further characterisation of the non-lipid components of the extra virgin oil.

Acknowledgements
One of the authors of this paper is sponsored by a Technology in Industry Fellowship (TIF). Olivado NZ Ltd and Technology New Zealand fund current research at Massey University and HortResearch Ltd, the post harvest partner in this research. Agriquality Ltd carried out the analyses for tocopherols and phytosterols. Kiwi dairies carried out the HPLC of the triacylglycerols.

References

  1. Requejo-Tapia LC. (1999) Masters thesis in applied agribusiness. Massey University.
  2. Werman MJ, Neeman I. (1987) Journal of the American Oil Chemists' society. 62(2): 229-232.
  3. Reed A et al. (2001) Food New Zealand. 1(1):20-25
  4. Rawls RR. Van Santen PJ. (1970) Journal of the American Oil Chemists' Society. 47:121-125.
  5. Werman,M.J.et al.(1990) J.Agric.Food .Chem.38,2164-2168
  6. Berges RR et al. (1995) The Lancet 345:1529-1532
  7. Daguet D. (2000) Lipid Technology vol 12 no 4(July), 77-80
  8. Thomson WW et al.( 2000) US Patent 6133313
Associate Professor Laurence Eyres, Nimma Sherpa and Glenn Hendriks are at the Institute of Food, Nutrition and Human Health, Massey University, Albany Campus, Private Bag 102 904 North shore MSC, Auckland, New Zealand

Email l.eyres@massey.ac.nz Fax + 649 443 9640

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