My daughter likes olives on her pizza. Somehow we talked about why you hardly ever hear about eating them raw. Since I didn't know, I decided to look into it.
Olives comes in many varieties. They can be harvested at different stages of ripeness. Many olives have an enzyme called catechol oxidase that causes them to change from a green to a red-brown to a black, but some just stay green or black.
But at all of these stages, they are too bitter though not toxic to eat, because of a chemical called oleuropein, which also has anti-microbial properties. I wonder if this has anything to do with olive trees living so long.
Humans have been cultivating olives for thousands of years, and have figured out various ways to get rid of the bitterness. The main ones are water, brine, and lye.
Oleuropein is water soluble. When you leave them in water, the oleuropein gradually leaches out of the olives and into the water through osmosis. Breaking the olives increases the surface area so it happens faster. You keep changing the water to maintain the concentration gradient. This takes several weeks and doesn't remove all the oleuropein, so they still have some bitterness and you have to eat them soon because they won't last.
Using a salt solution also allows oleuropein to leach out. This can take months. Olives an also ferment, using certain microbes like lactobacillus to break down sugars in the fruit into lactic acid. These usually green olives tend to have a stronger taste. Increasing the salinity to more than 8% stops the bacteria and increases the shelf life.
Using lye (sodium hydroxide) might sound harsh since it's used to unclog drains, but if it makes you feel any better, think of it as a traditional byproduct of wood ash that can be used to make soap. Of course, you still have to be careful.
The high pH of this alkali breaks down the oleuropein and helps break down cell walls so they become softer. This is a much faster process (about a day) and used commercially. Exposed to oxygen, this also makes green olives go black, like the ones on my daughter's pizza.
UC Davis has a free publication on safe methods for curing them at home. With so many variables, sometimes things can go wrong but I guess that is what makes things interesting. I have not applied any of these methods, but in researching this post, I did taste a variety of olives, because curious minds want to gnaw.