Green olives are unripe olives, black olives are ripe olives. Unripe olives contain very high levels of oleuropein, which is especially bitter and thus green olives cannot readily be eaten. Ripe olives have little, if any, of this compound.
To make green olives edible, they have to be cured (typically using a lye-based process for speed, but can be done with plain water over a longer period of time) to remove the oleuropein, and thus making them edible. After curing, they are typically kept in a salty brine to extend their shelf life.
Black olives do not need the curing process, but can be brined in either a salty water brine, or oil, or no brine at all via pasteurization.
I realized I left a few questions unanswered here, and for completeness' sake:
* Green olives can also be fermented (i.e. w/ yeast and bacteria, instead of using a lye- or pure-water 'cure'), to remove the bitter compound, and would thus be "pickled" in the traditional sense. These olives are not as green or firm as lye-cured olives.
* Why not pasteurize green olives, if you can do it with black olives? You can, but the heat would cause the green olives to become mushier, people expect green olives to be firm, and ripe (black olives) to be a bit soft. Soft green olives often are less desirable, outside of some specific preparations. (I.e. 'Greek-style' "crushed" olives.)
* Can you ferment, and thus pickle, black olives? Sure, why the heck not? Many Greek styles of black olives are done this way.
* Yes, you can pack cured green olives in oil too, like black olives. Anything to reduce the bacterial and oxygen activity in the olives will help to preserve them.
What makes Castelvetrano olives so distinct and delicious? They are almost always a vibrant green with an apple-like crunchiness. To me this is like a completely different fruit. Is this just the genetic stock or are they prepared differently?
I put my due diligence into internet research, without satisfaction. Would love to hear from the experts.
Castelvetrano is one of those interesting olives for me, personally, in that I'm not a big fan since I prefer the richer, earthier flavor of the slow-cured (dry-cured are my favorite!) olives, but my wife loves them most of all b/c she doesn't like the earthier olive flavor.
So, it's one part plant, one part process that gives them their unique flavor. The olive its self has a bright, crisp flavor with a good buttery profile. The process used for these is an extreme example of the lye-cured process, where they are lye-cured for just a few hours, and then soaked in fresh, cold water for at most a couple of days (changing the water every few hours). Basically, one of the most minimal processing of the olive world. After this, they are generally kept in a lower-salinity brine than other olives, to avoid the "salty bite" of a standard olive.
So, the Castelvetrano olives are really an exemplar of how lye can be used to great advantage in curing olives, to create a unique flavor profile.
Green and black olives are one and the same species, but green olives are unripe so they are still growing on a tree, but black ones are ripe so they start falling down by themselves and you can pick them up from the ground, which is much easier.
