By Harold McGee
The Chinese water chestnut and the tiger nut, or chufa, are both members of the sedge family, a group of water grasses that includes papyrus. The water chestnut is a swollen underwater stem tip of Eleocharis dulcis, a native of the Far East cultivated primarily in China and Japan. (Horned water chestnuts or caltrops are the seeds of species of Trapa, water plants native to Africa, central Europe, and Asia.) Tiger nuts are small tubers of Cyperus esculentus, a native of northern Africa and the Mediterranean that was cultivated in ancient Egypt. Both taste slightly sweet and nutty, and both are remarkable for retaining their crispness when cooked and even when canned, thanks to phenolic compounds in their cell walls that cross-link and strengthen them. The Spanish make the sweet drink horchata de chufa from dried tiger nuts by soaking them in water, grinding and resoaking, straining, and adding sugar.