An unripe papaya tastes very bitter if you try to eat it. Actually, the juice that comes from it is so corrosive that it can burn your skin, causing severe pain. I've experienced this first hand when cutting raw papayas.
However, after boiling, these papayas taste quite bland. Very little of the original bitterness remains.
What chemical substances cause the original bitterness, and what happens to them when boiled?