Bitterness is cucumber and other cucurbitaceae vegetable is due to the presence of compounds called cucurbitacins. Chemically these are tetra-cyclic triterpenes having high oxidative levels. They occur in nature as free glucosidesor as complicated mixtures, at high concentrations, in fruits and roots, for example in a wild variety of cucumber called Cucumis hardwikii.
High temperatures above 92 degrees have been implicated in the increase f bitterness in fruits, although there is no evidence to support this. Conversely more bitter cucumbers are seen growing during the cooler growing season.