Raw olives are not poisonous. However, they have a bitter taste before they are processed. They are typically cured using one of several methods that remove the bitter substance oleuropein. Curing methods include the use of liquid brine and drying between salt layers.
Olives also can be cured using lye, which draws out more of the bitter substance but also takes away more flavor than other methods. Water also removes the bitterness from olives, but does not work as quickly as other methods.
Olives are a fruit that grows on small trees throughout Africa, the Mediterranean Basin and southern Asia. They also are cultivated in France, parts of the Middle East, Bermuda and California. Olives are important in the Mediterranean area because they are used to produce olive oil. Olive trees typically do not grow taller than about 49 feet. Olives are usually less than 1 inch long and are harvested when they are green or purple. Canned black olives sometimes include chemicals that artificially turn the fruit black.
Olives have been cultivated for as long as 6,000 years, with early evidence found in Palestine, Crete and Syria. Olives were commercially grown in Crete as early as 3,000 B.C. and may have contributed to the wealth of the Minoan people.