Proteins are macromolecules made up of the twenty existing L-amino acids connected in long chains. They are important components of organic life, and, with carbohydrates and fats, are held to be among the three most important nutrient groups.
Brown rice (genmai) consists of 7-10% protein, and this is concentrated in the outer portion and the surface. In the sake making process, proteins are broken down by enzymes from koji to provide nutrients for yeast, and by decomposition into amino acids, peptides, aromatics and so on, contribute greatly to the taste, aroma, appearance (colour and clarity), and maturation of sake.
However, excess proteins can cause zatsu-mi off-flavours or excessive colouring, so it is necessary to remove them by rice polishing (sei-mai) . For instance, in the case of a rice polishing ratio of 70% (30% of the brown outer portion removed), protein levels are reduced by 30%.