So, why are some olives green and others black?
After olive trees have flowered the fruit starts growing. Initially the olives are green and hard, and gradually as they ripen they change colour to yellow-green and then to reddish purple and finally to black. These changes in appearance are matched by chemical changes within the fruit itself.
What causes the colour to change?
Unripe olives have high levels of chlorophyll which gives them their green colour. During the ripening process the levels of chlorophyll decrease and increasing amounts of carotenoid (the pigment which produces the darkening of the fruit) develop.
How does colour affect the flavour of the olive oil?
Oil yield is low when the fruit are unripe and increases as the fruit matures. The oil is initially very bitter but sweetens with ripening. In addition the ripening will be influenced by the position of the trees in the grove - those in more sheltered areas and olives at the top of the tree are likely to reach maturity first.
Can the olive become over-ripe if left unharvested for too long?
It is possible for the fruit to get too ripe. Most producers believe that once the crop is starting to fall off the tree the peak time for harvest is gone and the resultant oil is of poorer quality.
Is there an optimum time to harvest the olives for oil?
There is no ideal stage at which to harvest the olives. When deciding to create an oil the producer has to balance factors such as shelf life (greatest in less ripe fruits), yield and flavour. Leaving the harvest until later can be a major gamble for producers as the whole yield could be wiped out with frost.