Compounds with peptide linkages produce a blue-violet color with copper ions in alkaline solution. This is named biuret reaction after the chemical compound biuret, first known to produce the color. The color is due to a complex bond formed between cupric ion and four peptide nitrogen atoms, two from each of two adjacent peptide chains. This method is applicable to all cereal grains, such as wheat, corn, barley, rye, and oats.