What actually happens, step by step, inside the Maillard reaction?

Inside the Maillard reaction, a sugar and an amino acid hook together, rearrange, and fall apart into a huge family of flavour molecules. It starts when the reactive carbonyl group of the sugar reacts with the amino group of the amino acid and forms a complex mixture of poorly characterized molecules responsible for a range of aromas and flavors.

The first steps are known. The carbonyl group of the sugar reacts with the amino group of the amino acid, producing an N-substituted glycosylamine and water. That unstable glycosylamine then undergoes Amadori rearrangement, forming ketosamines. The ketosamines can go several ways, including brown nitrogenous polymers and melanoidins, which are brown, high molecular weight polymers formed at high temperatures and low water activity.

The reaction can produce hundreds of flavor compounds depending on the food's constituents, the temperature, the cooking time and the presence of air, and these compounds often break down to form yet more flavor compounds. In 1953 chemist John E. Hodge of the US Department of Agriculture established a mechanism for the reaction.

The same Amadori step turns up in medicine: glycated hemoglobin, the Amadori product, is used in the HbA1c diagnostic test for diabetes.