Two compounds, cucurbitacins B and C, give rise to the bitter taste. Though often only the stem end is affected, at times the entire fruit is bitter. Also, most of the bitter taste is found in and just under the skin. Removing the stem end and the skin can often help salvage bitter fruit.
Bitter fruit is not the result of cucumbers cross-pollinating with squash or melons. These plants cannot cross-pollinate with one another.
Often newer varieties are less likely to become bitter than older ones. Proper cultural care is also often helpful. Make sure plants have the following:
– Well-drained soil with a pH between 6.0 and 6.5. Plenty of organic matter also helps.
– Mulch. Mulch helps conserve moisture and keeps roots cool during hot, dry weather.
– Adequate water especially during the fruiting season.
– Disease and insect control. (Ward Upham)