Cucumbers have been cultivated for thousands of years, and like many food plants, they have been improved by cross-breeding to accentuate the better and eliminate the bitter.
Old recipes often include a de-bittering step, such as soaking the slices in salt water. (I doubt that that works anyway.) But modern varieties are rarely bitter except in the skin, which can be peeled off.
Part of the flavor of cucumbers is due to slightly bitter compounds called cucurbitacins. But when a cucumber has had a hard life back on the plant, such as a long spell of hot, dry weather or a battle with insects or disease, the amount of cucurbitacins builds up defensively in the flesh as well as in the skin.
The bitterness is Nature’s way of saying, “Don’t eat me or you’ll be sorry.” Alkaloids, for example, a class of mostly toxic chemicals found in plants, all have a bitter taste. But the amount of cucurbitacin you’re likely to find in a cucumber certainly won’t kill you. If you come upon a bitter cuke, chalk it up to the luck of the draw and move on to some others that may have had a less stressful youth back on the farm.
Today’s rarely bitter cucumbers are often sliced and salted, not to remove bitterness but to crisp them up. Sprinkle salt on sliced cucumbers in a bowl, top them with a layer of ice cubes, and put them in the refrigerator for an hour or so. The salt will draw water from between the fruit’s cells, firming up their structure. Wash the excess salt off before using.
While a coating of solid salt will draw water out of cucumber slices and crisp them, soaking them in salt water has the opposite effect: they will soak up water and become softer, or wilted. That’s because osmosis draws water from a less salty environment into a saltier one. When the cucumber cells are in contact with solid salt, some of its water will be drawn out. But when they are in contact with a rather dilute salt solution, some of the solution’s water will be drawn into the cells.
Cucumber skins aren’t completely impermeable to moisture, so the fruit will eventually dry out and shrivel if not protected by a moisture-proof coating. Cuke moguls therefore spray their product with an FDA-approved, edible wax to prolong their produce-counter lifetimes. The smaller, warty-skinned pickling cucumbers are not waxed because in pickling it is essential that the pickling liquor penetrate the vegetable.
So-called English cucumbers, being long and thin with consequently large surface areas, must be protected by more than a wax coating and are usually wrapped in plastic film.