The best time to pick the olives for oil quality ( not quantity) is when they are just starting to change colour from an green to slightly yellowish green.
At this stage the fruit softens a bit. Since the olive fruit do not all ripen at the same time – some fruit will have turned red / black or purple (less than 5% of the crop) most will be the slight yellowish-green colour with some still being green.
All the fruit contain chlorophyll. Olive oil is a “fruit juice” it is extracted from the fruit – not the seed or nut like sunflower, canola, coconut, grape seed etc.
During the oil extraction process some of the chlorophyll will be extracted with the oil. The chlorophyll is one of the components of Extra Virgin Olive Oil (EVOO) that gives the desired green notes like “fresh cut grass” sometimes also refered to as “tomato leaf”. Oleuropein another powerful antioxidant is also present in extra virgin olive oil.
Other oils do not contain chlorophyll or oleuropein. Both of these components impart a bitter taste the oleuropein more so than the chlorophyll.
The bitter being an indication of the antioxidant content of the oil.
I quote this paragraph from “The Olive Oil Handbook” – the online version was published on 21 March 2014 and is obtainable from Wiley Online Library.
page 11 of chapter three. “Colour and odour components of extra virgin olive oil”
“The colour of extra-virgin olive oil ranges from green to yellow due to the prevalence
of chlorophyll or carotenoids, respectively. The green colour of early-harvested olive
oil, which is particularly intense in oil from some cultivars (e.g. Correggiolo), is very
appealing to many consumers. Chlorophyll is a molecule very sensitive to light, and
careful storage to protect the oil from oxygen and light helps maintain the green
colour for a longer time. A rapid loss of the green colour is a sensitive indicator of
poor storage conditions.”
The different cultivars will differ in taste and composition. We have several cultivars here on the estate. Coratina being one of the cultivars with the highest anti-oxidant content. The oil that we pack in the PET bottles are kept in stainless steel tanks (no light) until they are packed. the Bottles are packed in cardboard boxes – again no light, so as to preserve the quality of the oil. This oil is a blend of two Cultivars Favalosa and Coratina. It can be regarded as a medium to intense oil. A Robust oil. Stronger flavours and sharper taste than most oils.
In short then a good quality Extra virgin Olive Oil should be green due to Chlorophyll and should be bitter to some extent.