What does the underline sentence mean?

"No cucumbers burp, but compounds called cucurbitacins produced in the skin of the fruit can have an adverse effect on the digestive system of those who eat them. Cucurbitacins also taste bitter, ruining the best cucumber sandwich. Due to genetic differences, one person in five can’t taste cucurbitacins at all, which explains why some people think others are crazy when they complain about bitter-tasting cucumbers. But two in five people have an acute sensitivity to cucurbitacins, which makes it understandable if they think the rest of us are crazy for eating cucumbers at all. The standard solution to the problem used to be simply peeling the offending skin. But people are not the only ones affected by cucurbitacins. Insect pests are attracted to the compounds and focus on cucumber plants that produce them, either naturally or through stress. So when plant breeders developed “burpless” varieties, with little to no cucurbitacin in the skins, everyone was happy but the bugs."

Hi teachers. This is from the book I study now. Firstly I don't get the meaning of the underlined sentence. Second I wonder what "at all" means in a positive sentence.

Thank you very much.