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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?

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The heat likely destroys the compounds responsible for the bitter taste. As for what the compounds are, I haven't been able to find any information other than that they have high polyphenol content, which are common astringents, but these are not susceptible to degradation by heat.

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I think that a biological process named OSMOSIS occurs when papayas are placed in boiling water. What happens is that:

The interior of the papayas has a more concentrated solution than the water surrounding it. The water molecules on the exterior move inwards through the semi-permeable membrane(the papayas outer covering) and lessen the bitter effect.

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  • $\begingroup$ 1) Osmosis is a physical process. 2) Osmosis may reduce perceived bitterness a tad, but not to the extent as presented in the question — and only if bitterness is due to something inside the papaya’s cells. $\endgroup$ – Jan Sep 2 '16 at 13:43
  • $\begingroup$ Thnx for the info @Jan :) $\endgroup$ – Devashish Joshi Sep 2 '16 at 13:45
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Unripe papaya contains a high concentration of Papain enzyme, which is an enzyme that breaks down peptide bonds and therefore "digests" proteins, including potentially human tissues. Enzymes themselves are proteins, and proteins tend to have a bitter taste.

Papain can self digest into amino acids over time, and this process could be accelerated by heating, as chemical digestion reactions will approximately double in rate for every 10K increase in temperature. So one mechanism is that Papain may break itself down quickly when heated. The other is that at a high enough temperature, Papain may denature.

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