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Green (Immature) Fruit

Immature olives are green and quite firm. They produce oil that is bitter and grassy with unripe and vegetative characteristics. These oils are high in polyphenols (anti-oxidants) and other flavour components. As a result, they are quite bitter and pungent and have a long shelf life thanks to these natural preservatives. The chlorophyll content is high so the oils are often quite green. It is more difficult to extract oil from unripe olives because the oil containing vacuoles within the cells are not easily ruptured. The olive paste has to be malaxed longer.

Veraison

As the olive fruit matures from green to yellow-green, it starts to soften and then the skin turns red-purple in colour. This is called veraison. The olives still have a high polyphenol content at this stage, and are starting to develop some ripe-fruity characteristics. Oils produced from fruit harvested at this stage have some bitterness and some pungency. They have close to a maximum amount of oil per dry weight. The olives are often considered to be at their peak for olive oil production.

Black (Mature) Fruit

As the fruit matures further, the skin turns from purple to black (although some varieties never turn completely black), and the flesh darkens all the way to the pit. At this stage, the polyphenol and the chlorophyll contents decline and the carotenoid content increase. Therefore, oils produced from late harvest fruit tend to be more golden in colour, less bitter, less pungent, and have a shorter shelf life. They are often described as sweet oils. The oil yield is high.
Given the factors described above, the best quality oils come from olives matured to at least the red-ripe stage. Fully mature, ripe fruit yield sweeter oils with no bitterness and pungency, but during harvest, they are soft and easily damaged. Immature olives that are green or straw colour are sometimes processed because of the unique flavour they impart to the oil.

Later harvest usually yields a better percentage of oil per ton of fruit, so processors and growers are often interested in harvesting as late as possible to augment oil quantity. The olive tree manufactures and stores oil in the fruit throughout the season, but the rate of oil storage flattens and stops just before maturity due to the low light intensity and cool temperatures, providing no real gain in oil content. Olives naturally loose moisture in the maturation process, so the perceived rise in oil content late in the season is actually a loss of fruit moisture.