Rye is the grain of Secale cereale, a hardy cool-season cereal grass that ripens where wheat falters — on cold, acid, sandy and poor soils, through hard winters and short seasons. Its slate-grey, blue-tinged kernels mill to a dark, low-gluten flour whose sticky pentosans rather than strong gluten give the dense, moist, deeply tangy crumb of sourdough rye and black pumpernickel. The same grain is rolled or cracked for porridge, fermented into the mildly sour Slavic drink kvass, and distilled into spicy rye whiskey. Earthy, faintly sweet and assertively sour-bread-like, rye is the defining bread grain of Northern and Eastern Europe.