Cucurbitacins are the bitter-tasting components present in all cucumbers. These cucurbitacins can cause people to burp after eating cucumbers. Cucurbitacin compounds are usually confined to the leaves, stems and roots of the plants, which humans don't eat. It is only when these bitter tannins move into the fruits that we start detecting an unfavorable bitter taste. Although these cucurbitacins are present in all varieties of cucumber plants, not all cucumbers will have a bitter taste. Favorable growing conditions can help prevent bitter tasting fruits, while plant stress in the form of temperature, drought, or nutritional imbalances can make fruits more taste more bitter.
Many excellent varieties and selections of cucumbers are available with reduced amount of these bitter-tasting compounds. These are known as "Burpless" cucumbers. You'll find some of the best in this category.

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