A VALUABLE FRUIT

Avocado is a highly nutritive fruit with a flesh rich in oil up to 30 percent (based on fresh weight). The fruit is primarily used for fresh consumption with recognised important health benefits and nutritional values thanks to content of antioxidant fat-soluble vitamins as well as protein, potassium and unsaturated fatty acids.

Avocado fat consists predominantly of mono-unsaturated oleic acid, which has been found to reduce harmful low-density lipoprotein (LDL) cholesterol while maintaining beneficial high-density lipoprotein (HDL) cholesterol, and performs better than typical low-fat diets. Although avocado is primarily consumed fresh, interest for avocado-based products (e.g. guacamole) and oil for cosmetics are growing.

TRADITIonALLY EXTRACTING AVOCADO OIL

The most commonly used method used to obtain oil from avocado is through organic solvent extraction. Fruit pulp is dried by warm air and then hexane solvent extraction is used with a yield of up to 95 percent. The resulting oil is brownish with a high pigment content, which needs to be refined for cosmetics or other applications.

In locations wher solvent extraction in not available, a mechanical method is used. The fruit is first peeled and destoned, and the pulp is then mashed before being dried. The resulting paste is heated with hot water and chalk and/or sodium chloride, and then pressed or skimmed off (by natural decantation) or centrifugation.

Centrifugation or pressing yield is reduced to 60-80 percent compared with the aforementioned solvent method. These traditional processes generally result in poor-quality oil that needs to be refined first before it can be used, thus comprise a poor cost/benefit ratio.

A NEW ALTERNATIVE: COLD PRESSED EXTRACTION

If avocado flesh is so rich of valuable fat, why should we not extract an oil suitable for direct human consumption and culinary-dressing purposes? An alternative is now available that answers this question.

Applying an extraction process adapted from the conventional one for olive oil with proper adjustments, a private New Zealand company and partner Alfa Laval obtained an unrefined avocado oil that is able to retain and exalt flavour with a fruity taste, and has a yield variable between 75 and 85 percent depending on fruit characteristics.

The process produces extra virgin unrefined avocado oil (termed extra virgin because of its similarity with olive oil). Whole fruits are washed in a two-stage washing system and then elevated into a destoning machine, wher pips and around 90-95 percent of skin are separated from the pulp. The pulp and its remaining skin is then pumped into a disk crusher for further refining if necessary, depending on ripe degree of fruit. The disc crusher is important in cutting the filaments that remain in the paste and simultaneously minimises the emulsion.

After this, the 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 the avocado paste is similar to what would happen with olive paste: small oil drops released during fruit milling merge into large drops (coalescence phenomena) that can be easily separated by centrifugal extraction. The definition of optimal malaxing time and temperature is important in order to reach the best compromise between quality and quantity of extracted avocado oil. In the case of the avocado mash, malaxing time should not exceed 90 minutes and temperature should be below 45 deg C.

Coalescence and oil extraction are not the only reasons for the crushing and malaxing processes. Like with extra virgin olive oil extraction, the total phenol content and aromatic fraction are strongly affected by parameter adjusting and design of the equipment.

After malaxation, the separation of oil from solid and liquid phases is done using a decanter centrifuge. The mash coming from the malaxer is fed into the machine wher oil, vegetation water and solids (exhausted pulp and residual skin) are separated.

Extraction should be carried out in two or three phases and 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 is pumped out to a disk stack purifier centrifuge, while the water phase (in case of extraction in three phases) is pumped out to a disk stack concentrator centrifuge.

The avocado oil flowing from the decanter would still have a certain amount of water and solids. Similarly, vegetation water from the decanter would still contain a small quantity of residual oil. The system therefore consists of a disk stack centrifuge for final avocado oil purification to remove residual water and solids. A second disk stack centrifuge should also be used to recover residual avocado oil from the vegetation water flowing from the decanter working in three phases.

The oil that comes out from the disk stack purifier centrifuge is an extra virgin high quality avocado oil and, 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

Since the first pioneering project in New Zealand in 2000,worldwide experience regarding this cold press method hasbeen accumulated crossed with deep historical knowledgeof the olive oil extraction process for avocado oil production.In order to produce top quality oil for a culinary-dressingpurpose, we must pay attention to multiple factors to ensurethe results. Of these, three are considered more important.

The first aspect to take in consideration is theagroecology and climate of the fruit’s area of cultivation thatis designated to high quality oil production. To obtain oilwith the highest ratio of monounsaturated fatty acids withrespect to saturated ones, the avocado needs to be grownin areas wher temperatures in the winter are not so high. Asimilar relation between fatty acids composition and climatehas been observed and investigated for olive oil. Of course,more research should be conducted to gain insight specificto avocado oil.

The second important factor to take into consideration isthe logistics of harvesting and identifying when would be theright moment to process oil. The dry matter content of fruitand firmness of pulp are important agronomical parametersthat affect the proper equilibrium between yield and level ofoil quality, so avocado should only be processed at the righttime. The third important factor to consider is the choice ofthe right process technology. only with the right equipmentand proper process knowledge can one transform the highquality of fruit into the high quality oil in the bottle.

AN IMPROVED COST/BENEFIT RATIO

The combination of proper cultivation climate, ripeness and process technology delivers high quality unrefined avocado oil with less than 0.5 percent of free fatty acids (as percent ofoleic acid) and less than 2 meq/kg peroxide value.

As described by Marie Wong and colleagues from Massey University of Auckland (New Zealand) who investigated the quality of avocado oil obtained by cold pressed method from "Hass" variety, oil has an avocado flavour, with grassy and butter/mushroom-like flavours. The fatty acid profile is verysimilar to olive oil with 76 percent of monounsaturates(oleic and palmitoleic acids), 12 percent polyunsaturates (linoleic and linolenic acids) and 12 percent saturates (palmitic and stearic acids). High percentage of monounsaturated fatty acids bring a consequent high smoke point (more than 250deg C) making it good oil for frying.

But positive attributes of extra virgin avocado oil are not limited to fatty acids composition. Researchers also report in literature that there is a high content of valuable antioxidants in the oil. The main one is alpha-tocopherol, which is presentat levels of 70–190 mg/kg oil. Beta-, gamma-, and deltatocopherols are only present in minor amounts (less than10 mg/kg oil). Other non-lipid components present in the oil include chlorophylls (11–19 mg/kg oil), phytosterols (2.23-4.48mg/g oil) and carotenoids (1.0–3.5 mg/kg oil) with lutein as the most representative.

High monounsaturated fatty acid, antioxidant andother non-lipid content make unrefined avocado oil a foodcomparable to high-quality extra virgin olive oil—seen as thebasis of the Mediterranean diet and considered one of themost important healthy foods that is anti-carcinogenic andhelps slow age-related molecular degeneration.

CONCLUSION

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 has yet to be fully explored.

Investigation in these areas to correlate agronomical and technological factors for extra virgin avocado oil is strongly recommended and could offer a wide range of potential flavours and characteristics for avocado products and an opportunistic market for manufacturers.