Olives are a naturally bitter fruit that is typically subjected to fermentation or cured with lye or brine to make it more palatable.
Green olives and black olives are typically washed thoroughly in water to remove oleuropein, a bitter carbohydrate. Sometimes they are also soaked in a solution of food grade sodium hydroxide to accelerate the process.
Green olives are allowed to ferment before being packed in a brine solution. American black ("California") olives are not fermented, which is why they taste milder than green olives.
Freshly picked olive fruit is not palatable because it contains phenolic compounds and oleuropein, a glycoside which makes the fruit too bitter, although not unhealthy.[35] (One exception is the throubes olive, which can be eaten fresh.)[37][citation needed] There are many ways of processing olives for eating. Traditional methods use the natural microflora on the fruit and procedures which select for those flora that ferment the fruit. This fermentation leads to three important outcomes: the leaching out and breakdown of oleuropein and phenolic compounds; the creation of lactic acid, which is a natural preservative; and a complex of flavoursome fermentation products. The result is a product which will store with or without refrigeration.