What makes a Kirby cucumber burpless?

Kirby cucumbers, like other greenhouse cucumbers, are milder than garden grown slicing or pickling cucumbers. As a result, to some people they seem to be easier to digest, resulting in less burping. The greenhouse cucumbers now known as “Kirby-type” were developed to produce fruit parthenogenically, that is, without pollination of the flowers. Thus, Kirby-type would produce fruit in a greenhouse. The Kirby’s have no seeds, a very tender skin, and a distinctive long, slender shape. The selection process that resulted in the seedless, thin-skinned cucumbers also selected out some of the compounds that produced the digestion problems. Nowadays, the easy-to-digest qualities of the Kirby-type cucumbers have been re-selected by plant breeders. Home gardeners can choose from a number of seed sources for burpless cucumbers. Look for varieties that are long and slender with solid dark green skins.

From The Old Farmer’s Almanac.