Context: The informant was born in Pakistan (her parents are from Afghanistan originally), and the only variety of cucumber (almost a luxury item) available at the time was a large, hard, bitter kind that had to be prepared a certain way in order to be palatable. In her family, each end of the cucumber (about a half-inch on each side) had to be sliced off, then the two inner surfaces had to be rubbed very hard and fast against each other to “draw out the bitterness”. Sometimes a cut is made into the “body” end of the cucumber before rubbing the end slice against it. A thin film of pale green foamy substance would appear at the edge of the cucumber end, which was then wiped or washed off, and the cucumber was ready to prepare and eat.
Analysis: From a couple different anecdotal and semi-scientific accounts, it appears that this practice is not unique to Pakistan or even the Asian subcontinent, but is practiced by people of Czech and German descent, in the US, in Canada, and in Britain (and probably in many more regions, as it likely that only in the past couple of centuries have we been able to produce consistently non-bitter cukes through genetic modification). The practice is often called “rubbing” or “wicking“, and though it doesn’t seem to have a strong scientific basis (bitterness level is often a result of many growing factors such as temperature, moisture content, etc.), those who use the method (the informant included) have always found it to work. The belief seems to be that the foamy substance that is created from friction between the two ends is what contains the bitterness (in scientific terms, in contains the cucurbitacin compounds which are toxic to some animals and very bitter to humans). So although serious scientific research hasn’t been done on the subject, it doesn’t necessarily mean that this particular folk food practice has any less validity in the kitchen.