The Oxford Companion to Beer definition of
is not a grain, in spite of its name. Grains belong to the grass family, whereas buckwheat varieties belong to a family of herbs of Asian origin called Fagopyrum. However, because buckwheat has grain-like properties, it is often referred to as a pseudo-cereal. The buckwheat plant has small, off-white, triangular, edible, fruits called “achenes.” These are high in protein, and, like barley kernels, are mostly made of starch. Also like barley, they have endosperms and aleurone layers, and contain the diastatic enzymes alpha-amylase and beta-amylase.
The proportion of buckwheat in a mash may be as high as one-half, though experimental mashes with 100% buckwheat have been reported in brewing literature. However, because of the buckwheat’s relatively high protein content, a mash with buckwheat generally necessitates a grist-to-water ratio of 1:4 or thinner to avoid excessive mash viscosity, clumping, and low extract volumes. Buckwheat, unlike most cereal grains, is also gluten-free.