Maillard reaction
Japanese: メイラード反応（アミノカルボニル反応）
A chemical reaction between amino acids and reducing sugars that produces brown pigments and aroma compounds. The main reason miso darkens over time.
The Maillard reaction produces brown pigments (melanoidins) and complex aromas when amino acids combine with sugars. It was reported in 1912 by French chemist Louis-Camille Maillard.
The golden crust of bread, the fragrance of coffee, the appetizing smell of seared meat—all are products of the Maillard reaction. In fermented foods, this reaction proceeds slowly over time even without heating.
Miso darkening from white to beige to reddish-brown is evidence of the Maillard reaction in progress. Amino acids generated by protease and sugars generated by amylase meet and bind non-enzymatically. Higher temperatures accelerate the reaction, which is why winter-prepared miso ages slowly, maintaining a lighter color.