Cucumbers contain a variable amount of a substance called cucurbitacin,
which gives them the bitter taste. Legend has it that one can "take the
bitterness out" of a cucumber by cutting off both its ends (or just one,
depending on whom you ask), rubbing the cut surfaces together for a
while, and washing away the thick white foam that is produced.
I've always been suspicious of this claim.
For one thing, if the bitter substance is evenly distributed throughout
the cucumber, how could rubbing the cut ends remove it? (The suggestion
that it "creates suction" seems patently absurd.) Or if the substance is
concentrated at the ends, why is it not sufficient to just discard them?
On the other hand, what is the white foam, which does sometimes (but not
always) taste bitter? And why does everyone seem to believe in the
efficacy of this method?
Speculation aside, I see no sensible way to test the proposition.
If you take a bite out of the middle of the cucumber and it turns out to
be bitter, it's because you "didn't rub the ends"; but if you do rub the
ends and it's still bitter, you "didn't rub the ends enough". If, on the
other hand, it doesn't taste bitter, how can you tell whether
it was sweet to begin with, or if the rubbing cured it? I can think of
many strange kitchen rituals, but none with the strangely ambiguous,
undecidable nature of the cucumber ritual.
The question remains unresolved, but until I learn the truth either way,
I will continue to eat my cucumbers without any voodoo preparation.