A couple of weeks ago, Brian and I harvested the last, or nearly the last, of our cucumber crop, and we thought a good way to use it up would be to make the Summer Pasta Salad he invented last August. He mixed everything together, just as he did the first time, and it looked nice and colorful, but it tasted...wrong. The flavors of basil and garlic and tomato were all still there, but there was this unpleasant, bitter undertone to everything. After cautiously trying another bite or two, I managed to pinpoint the source of the problem as the cucumbers, which were so bitter I couldn't even eat a whole piece. I thought perhaps it might be possible to just eat around them, but when Brian tried his, he declared, "I'm not eating this," and he dumped the whole bowlful straight into the compost bin. It was a bit of a wrench to my ecofrugal soul to see a cheap, healthy, home-cooked meal go to waste, but I had to admit, he was right: what we had in front of us wasn't truly food. He said the taste was so bad that it seemed like it had to be bad for us—like our bodies were trying to tell us, "Warning! Don't eat this!"
Fortunately, there happened to be a new diner in town that we'd been planning to try anyway, so we didn't go hungry that night (although we had to scrounge a bit for lunch in the absence of the leftovers we'd expected to have). But the experience still left us with a bit of a puzzle on our hands. The recipe had tasted fine the first time we made it; what had gone wrong the second time?
A quick Google search on "bitter cucumber" pointed Brian toward the answer. According to a site called Gardening Know-How, cucumber plants, along with melons and squash, naturally produce bitter compounds called cucurbitans. Most of the time, these compounds are confined to the leaves and stem of the plant (which I guess explains why our groundhogs have never touched them). However, certain conditions, such as uneven watering or fluctuations in temperature, can cause these chemicals to work their way into the fruit. Eating too much of these bitter compounds can actually cause illness, so Brian was apparently right in thinking that the salad tasted like something unsafe to eat. (The amount of cucumber in it probably wasn't enough to harm us, but considering that we wouldn't have enjoyed it either, there was no good reason to take the risk.)
This answer, however, led us to another question: would other cucumbers from the same plant have this same unpleasant taste? Gardening Know-How warns that once a cucumber vine has started to produce bitter fruit, it will most likely continue to do so, and it's best to discard the whole plant. Since the cukes we'd just picked were pretty much the last of the season, that wasn't a problem, but what about the ones we'd picked earlier? Would the multiple jars of pickles in our fridge have that same nasty, bitter taste? Fortunately, Brian was able to assure me that all those cucumbers had gone into the jars long before this bitter-tasting one showed up, so there was no reason to worry about them.
Nonetheless, I wanted to relate our experience as a warning to all you other gardeners out there. If you grow cucumbers, make sure you always taste them before using them in a recipe—particularly if it's late in the season, or if the weather has been unpredictable during the summer. Otherwise, you risk ruining a whole dish with one bad cucumber.
Incidentally, in the course of researching this problem I also found a few suggestions on ways to deal with it. About Home says that the bitter compounds tend to concentrate in the skin of the cucumber and toward the stem end, so if you peel the cucumbers and use the parts toward the middle, you "should be able to salvage more than enough for a salad." A video on YouTube offers a far odder solution: cut a thin slice off each end of the cucumber, then press the cut slice against the cuke and rub vigorously in a circle for a minute or so. This will cause the cuke to release a white, foamy substance, which supposedly draws out the cucurbitans with it. I personally haven't tried any of these methods, so I can't vouch for their effectiveness, but if you've picked a cucumber that's essentially inedible, I guess you have nothing to lose by trying them.
As for us, we'll do our best to raise our cucumbers right in future years to keep them from developing this problem again. We can't do much about the weather, but we can take extra care to keep the plants evenly watered. And next time we buy new cucumber seeds, we might consider switching out our highly productive Marketmore 76 or Calypso cukes for a "sweet" variety, since these are less prone to cucur-bitterness.