Maillard reaction
A family of reactions between reducing sugars and amino groups that generates most of a crust’s colour and savoury complexity.
Established science
Measured and reproducible food science, described in textbooks or peer-reviewed work. The mechanism and the numbers are both supported.
The Maillard reaction is not one reaction but a large cascade beginning with a reducing sugar and an amino group from a protein or amino acid, and producing hundreds of compounds — brown pigments, and a very large number of aroma compounds.
It is the reason a crust tastes of far more than the dough it came from. Caramelisation, which involves sugar alone at higher temperatures, contributes as well, but Maillard chemistry accounts for most of the savoury depth in a bread crust.
It needs both partners present, needs sufficient temperature, and is inhibited by abundant water — which is why a crust browns and a crumb does not, despite being the same material. The crumb never exceeds the boiling point of its own water.
What is actually established
Required reactants
A reducing sugar and an amino group are both required. Sugar alone caramelises but does not undergo the Maillard reaction.
SPRINT 7: the requirement for both reactants is now passage-verified — the read source describes reducing-sugar carbonyls undergoing nucleophilic addition with amino groups, and treats caramelisation as a separate process competing for the same substrates. THE SECOND SENTENCE IS NOT QUOTED ANYWHERE: that sugar alone caramelises without browning by this route follows from the reactant requirement rather than being stated in the source, and it is recorded here as an inference so nobody later mistakes it for a verified passage.
RecordedY. Qi, W. Wang, T. Yang and W. Ding, Foods, Jan A. Delcour and R. Carl Hoseney, AACC International, Harold McGee, Scribner
Effect of pH
Alkaline conditions accelerate the reaction; acidic conditions slow it. Across the pH range 3 to 9 the rate generally rises with pH.
SPRINT 7: the read source gives the range explicitly — within pH 3 to 9 the rate generally increases with pH — which is more useful than the bare direction BakeHQ previously stated, so the claim was widened to include it. This is the mechanism behind the deeply coloured crust of an alkaline-treated bake such as a pretzel, and part of why a very sour dough can bake paler than expected.
RecordedY. Qi, W. Wang, T. Yang and W. Ding, Foods, Harold McGee, Scribner
What you can do with this
A pale crust on a fully baked loaf is usually a shortage of one of the reactants — often fermentable sugar consumed by an over-long fermentation — rather than a shortage of heat. Adding milk, egg or a little sugar supplies the missing partner, and steam early in the bake keeps the surface flexible for longer before browning begins.
What is not settled
The reaction produces a large and incompletely characterised set of compounds, and which of them dominate a given flavour is not fully mapped. Statements about a specific temperature at which the Maillard reaction 'starts’ are common and misleading: it proceeds slowly at low temperatures and faster at high ones, without a threshold.
Related
Ingredients this explains
Sugar supplies one of the two required reactants, which is why a low-sugar bake stays pale even when fully cooked.
Covered in
What happens when you reduce the sugar
Browning is the function people notice missing first.