October 22, 2005
Nothing gets more discussion among olive growers than when to pick. It can make all the difference as far as yield, oil flavor, oil keeping ability and color.
Deciding on maturity desired
Harvesting the next year at the same maturity level
How olive maturity affects flavor profile
The main determinants of olive oil flavor are maturity, variety, cleanliness and time to milling.. Olives picked a week apart for the same tree can have strikingly different flavor profiles. Like any tree fruit, more mature olives will have more sweet and fruity notes through the development of alcohols, esters and aldehydes. Oil picked from mature fruit can often be consumed immediately without racking. Some varieties such as Mission can produce a bland oil if picked too late.
Greener fruit may produce oil with more antioxidants in the form of polyphenols so can have a longer shelf life. Green fruit tends to make a more bitter, peppery oil which takes several months of storage before it is palatable.
Fruit maturation will depend on temperature, sunlight and irrigation. A hot Fall can cause fruit to ripen quickly, resulting in a narrow window for optimum picking. A cool Fall may result in green fruit hanging on the tree well into winter. Some farmers are forced to pick green fruit to hedge against frost damage.
How olive maturity affects oil yield
Generally, the oil content of olives goes up as the fruit grows then plateaus when the fruit has reached its maximum size. Per ton oil yields may go up as the fruit ripens due to dehydration of the olives.
Using the Olive Maturity Index
The maturity index depends on the color of the skin and flesh to assess maturity. Olives are picked from representative trees in a random manner to fill a large container. 100 olives are picked out at random. An olive is picked from the 100 to represent each of the 8 maturity levels below. The rest of the 100 are now compared to the reference olives and sorted accordingly. The number of olives in each group is counted. The olive Maturity index is the sum of the number of olives in each category multiplied by the score, with the sum then divided by 100. The olives in a grove may reach this maturity index sooner or later in the year depending on weather but olives picked year after year at the same maturity index should produce similarly flavored oil.
|color over < 1/2 of the skin||2|
|color over > 1/2 of the skin||3|
|full color with white flesh||4|
|full color with < 1/2 flesh purple||5|
|full color with > 1/2 flesh purple||6|
|full color with purple flesh to the core||7|
Olive Fruit removal force
Fruit removal force" (FRF) has been commonly used in research, primarily evaluating effects of various chemical materials on loosening olives for mechanized harvest (i.e. shaking - it is generally believed a FRF of less than 150g is desired for adequate machine removal with a shaker on "willowy" varieties such as Manzanillo). FRF declines as physiological maturity is reached (usually about 3-4 months after the horticultural maturity desired for the "California style" olive) and further declines after that until fruit naturally fall. However, I know of no work establishing any FRF level as an index of physiological maturity. Steve Sibbett U.C. Farm Advisor Emeritus
Now that the harvest time in Oz has started or drawing near I see much of the discussion relating to indicators as to the appropriate time to pick, especially for oil. Pigmentation, finger-squeeze test, even fruit fall have been mentioned on earlier postings. Last season I saw demonstrated a homemade 'Tension meter' which measured the 'grip' of the olive to the tree - the theory being that the lower the tension the riper the fruit or the nearer it was to falling off the tree. No doubt different cultivars would have differing release measurements and one would have to establish appropriate benchmarks for each variety. The handheld device consisted of a short thread which looped the olive and a small spring which once gently pulled registered the pressure on a circular dial. As the increasing pressure eventually dislodged the fruit the dial needle remained in a stationary position indicating the force that it took to remove the fruit. A few sample tests around the tree or grove gave an average indication of the ripeness or otherwise of the fruit. - Alan Watt, Tanja Olives