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Considerable variation exists between olive oils due to varieties, environments and growing areas but perhaps the greatest variation comes from the maturity of the olive at the time of harvest. As olives mature the oil passes through a range of quality levels in which many of the components are changing. At the same time the oil content is increasing but in many cases the ability to extract the oil will pass through an optimum period before the oil becomes difficult to recover.

This is the first scientific study of its type to relate harvest timing to olive oil quality and stability in Australia. The results provide growers with valuable information on how to achieve specific quality in their olives by targeting fruit maturity. These findings show that the greatest variation in oil quality and oil yield between the cultivars studied is due to fruit maturity. As a result a grower may choose to produce pungent and peppery oil or a mellow to bland oil. The opportunity is there to select oils with increased shelf life stability when antioxidants are at a high level. The oil may be processed when it is green or left to golden yellow. Even the level of polyunsaturated fatty acids can be influenced by monitoring fruit quality and selecting the optimum harvest date.

This study has identified the changes in many of the oil components during the stages of change.

The information gained will provide growers with the tools to improve their oil yield by optimising the harvest time and water application. Olive oil sensory characteristics such as pungency, bitterness and fruitiness, may also be determined by harvest timing. Shelf life stability is also sensitive to fruit maturity and is of critical importance to the product to ensure it retains quality long after the oil is bottled.

An olive grove based in Southern NSW was utilised for this study. The grove with 100,000 four year old trees included three common cultivars, Mission, Corregiola and Paragon. Furrow irrigation was established to allow a comparison of irrigation treatments. Environmental conditions during the three years of this project were less than ideal for an irrigation study. Drought conditions persisted over the entire period and water application was limited. All of the trees suffered different levels of water stress. The majority of parameters measured therefore showed no significant differences between high irrigation, moderate irrigation and severe deficit irrigation. Despite this, the study provided considerable information on soil water movements and the effects of harvest timing on some of the numerous parameters tested.

A major aim of the project was to relate physical and easy to measure parameters with oil quality and oil recovery. It is necessary to define for the grower, precise times when the fruit are at their peak and before the quality begins to decline. The tests traditionally used to identify quality in the field include fruit colour, weight, detachment force and hardness. All of these characteristics were monitored over the three years of this study, at regular dates throughout the maturation period.

Maturity index (MI), the most common European method to determine fruit condition, was shown here to provide little reliable information. Fruit of different cultivars changed colour at different times. In fact, the variability of fruit colour within cultivars, and even within individual trees ranged from green to black (MI 3 to MI7). While fruit were still green oil quality and yield were changing.

Although the skin of Corregiola turned black, the flesh often remained green throughout. Reliance on this character under Australian conditions would lead to late harvesting and loss of fruit and oil quality. Variations in MI from year to year, despite consistent oil accumulation over the same time, show that MI would not produce consistent product.

It is known that the growth of the fruit follows a similar pattern to oil accumulation and has been discussed in the report (4.2.1). As such, it should be possible to design harvest timing on fruit weight. Fresh fruit weight in olives was shown to be variable, increasing and decreasing with rainfall or irrigation events. The fruit take up water when it is available and release it when it is limited.

Fresh fruit weights were useless in determining fruit development, rising and falling across the growth period. A more useful tool was found to be the dry weight, which was calculated from fruit moisture contents. Fruit dry weight followed oil production closely, producing similar curves and levelling out as fruit weight reached its maximum. The limitation was that fruit weight continues to increase for a period after optimum oil content has been reached. As a result, reliance totally on this method would result in late harvests with reduced fruit yield. Fruit firmness also showed useful trends with similarities to oil accumulation. Firmness dropped rapidly at point when oil content had reached a maximum. This test has potential for further application.

It is more difficult to apply chemical tests to determine harvest timing as they are often expensive and time consuming. By the time chemical analysis has been carried out, the fruit may have missed the optimum time. This analysis is however necessary to help growers relate physical testing to oil quality under their own conditions. Chemical testing is recommended for individual conditions, at least in the initial years of generating a crop. Moisture is variable over the growing period but generally the total moisture will drop as the fruit reaches maturity. This was not always the case however and Mission tended to hold moisture levels throughout the growing period. Paragon and Corregiola also showed variation from year to year. It is not likely therefore to be a reliable indicator for harvest timing.

Solvent extraction of the oil from olives is the required method to compare oil contents between harvests, treatments or cultivars. It removes 100% of the oil and can show when the oil production has reached a maximum. This is also useful when comparing the efficiency of cold-pressed systems and determining how much oil has been lost in the process. It does not indicate however what the cold-press extraction will yield. Cold-press extraction is the best indicator of when to harvest as it imitates the oil extraction process in commercial situations. It was found that cold-press extraction reached a plateau well before the olive had ceased producing oil. This is due to the changing properties of the olives which make the oil harder to extract. It is therefore necessary to harvest before the solvent extraction would indicate to get the maximum oil recovery.

Harvest timing may be targeted toward producing particular quality traits in the oil rather than just trying to increase yield. This report describes the major quality components and the changes which take place over the growing period. The major contributor to oil quality is the polyphenol content.

This plays a strong role in determining the stability of olive oil. The close relationship with shelf life stability and induction time has been discussed. Organoleptic quality is also dependent on polyphenols which determines pungency and bitterness. The high polyphenols shown to be present in young olives rapidly decreases as the fruit mature. The oil concurrently changes from strong pungent oil to a light mellow product. The relationship between induction time and several other components in the oil, including fatty acids and chlorophyll has also been discussed.

The fatty acid profile is important in determining oil stability but also in the oils nutritional quality.

Oleic acid is considered beneficial and high levels are encouraged. Although oleic acid changed little over the maturity period, linoleic acid (polyunsaturated) was found to increase while palmitic acid (saturated) decreased. Higher levels of polyunsaturated fatty acids are nutritionally beneficial but reduce oil stability. Harvest timing can be used to select for stability or nutritive value. Linolenic acid also decreased with maturity. This component is unstable and can increase the rate of oxidation.

However, in Australia it has been found that linolenic acid levels often exceed international standards, thus making it a possible problem in international trade. The linolenic acid needs to be below 1.0% which is usually achieved early in the maturity process. Free fatty acids must be less than 0.8% in extra virgin olive oil. We have shown that overripe olives can exceed this limit while still on the tree. It is necessary to harvest, particular cultivars such as Paragon, before free fatty acids increase. Peroxide value was not found to be a problem and did not change during maturation.

Chlorophyll, which provides the green colour to oil, has some benefits. However, when exposed to light, chlorophyll becomes a pro-oxidant and can contribute to oil instability. The rate of chlorophyll reduction with maturity is discussed and decisions can be made to select for early harvested green oil or late oil of lighter colour but better stability. Tocopherol, which decreases with maturity, also acts as an antioxidant and provides stability to the oil.

As olive production increases as a result of the massive plantings that have taken place in recent years, olive oil sales will be more competitive. Although yield is of major importance, oil character and quality are gaining recognition. Already, olive oil competitions are in abundance in ever state in Australia with many commercial bottles now carrying medals to indicate the quality standards. Fruit stability is also gaining importance and oil that oxidises quickly will be unacceptable to consumers.

Also of major importance are the nutritional benefits of monounsaturated oil with reduced levels of saturated fatty acids. Growers and processors can have significant controls over all of these factors through monitoring crop maturity and adapting harvesting and processing management to suit their requirements.

Despite having achieved most of the goals of this study, it was apparent that there are more factors which need to be better understood. The irrigation system at this grove was of limited value to the study as it was furrow irrigation and difficult to monitor and apply at regulated levels. As a result, comparisons between deficit and adequate irrigation were not achieved. More needs to be understood about the polyphenolic compounds in the oil which provide the majority of resistance to oxidation. The individual responses of each cultivar to the rancimat test indicate that each was under the control of variable antioxidants. It is likely that the sensory characteristics are also controlled by these individual polyphenol profiles. The stability of the oil and the maintenance of good organoleptic characteristics, both with significant relationship to harvest timing, are probably the most important aspects of olive oil. This study has shown that despite the differences between olive cultivars and growing conditions, the greatest changes between olive oil quality characteristics are a result of the fruit maturity and harvest timing.

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Last updated: February 2005
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