Zucchini specimens that contain less or no bitter-tasting substance called cucurbitacin are usually bred. Rarely, but on a genetic basis, this substance is still present in larger quantities in some specimens, taken from the genetics of "ancestors." Zucchini can be bitter even if it is too old or has stood too much. Unfortunately, zucchini can also react to environmental influences (temperature) by becoming bitter. It is more resistant than cucumbers, which is why it is less likely to be bitter because of it, yet it does occur.