Vol. VIII · Browning Plate 1 of 9Maillard reaction, browning, amino acids, reducing sugars, pyrazines, melanoidins, flavour, crust, roastMF

The Maillard Reaction.

A seared steak, a toasted slice of bread, roasted coffee and the crust of a loaf share one chemistry. It is fast at 150 °C and almost stopped at 100 °C.

In 1912 the French chemist Louis-Camille Maillard heated sugars with amino acids and watched them turn brown. The reaction he described runs in almost every cooked food that contains both — which is nearly all of them. A reducing sugar (glucose, fructose, lactose) joins an amino acid; the product rearranges and breaks apart into a cascade of small, often strongly smelling molecules, which recombine into brown polymers called melanoidins. Hundreds of compounds are made, and which ones depends on the sugar, the amino acid, the water, the pH and above all the temperature.

Those compounds are the flavours cooks chase: pyrazines (roasted, nutty), furanones (caramel-like), sulphur compounds from cysteine (meaty), Strecker aldehydes (malty, chocolatey). The reaction is slow at the temperature of boiling water and fast above about 140 °C — which is why boiled meat is grey and a seared steak is brown. The workshop shows how quickly a surface browns at the temperature you set.

Below: browning (a teaching scale from 0, unbrowned, to 1) against minutes for surfaces held at four temperatures and yours, and the flavour families the reaction makes.

What you will learn
Workshop
Browning as a first-order approach to full colour with the course Maillard parameters (rate 0.30 per minute at 150 °C, activation energy 125 kJ/mol, within the 100–160 kJ/mol reported). "Golden" is taken as half-way on the scale.

Hot, dry and patient

A surface held at 150 °C reaches golden in this model in about 2.3 minutes; at 180 °C in about 13 seconds; at 120 °C it takes 35 minutes, and at 100 °C — the most a wet surface can reach — about 4 h 30 min, 117 times slower than at 150 °C. That is why steamed and boiled food stays pale however long it cooks, and why browning needs dry heat: a pan, an oven, a grill or hot oil.

Everything else that speeds the reaction is secondary but useful. More reducing sugar browns faster: milk solids, honey and egg glazes, which is why brioche and roast potatoes brushed with milk brown well. A higher pH speeds it (VIII.3). Less water speeds it, which is why the surface must dry first (VIII.2). And because so many compounds compete, the flavour of a crust depends on how it was made: a slow roast and a hard sear of the same meat taste different.

At the stove

Toast three slices of the same bread for one, two and three minutes. Smell each before eating it: pale bread smells of flour, golden of biscuit, brown of roast and nuts. Each step is more Maillard products.

Maillard thought his reaction might explain changes in the human body; it does — the same chemistry slowly browns proteins in ageing tissue — but food scientists took it up first, and by the 1950s John Hodge had mapped its main pathways.

Terms

Maillard reaction
The reaction of reducing sugars with amino acids that browns food and makes roast flavours.
Reducing sugar
A sugar with a reactive group, such as glucose, fructose or lactose; sucrose is not one until it splits.
Melanoidins
The brown polymers that give browned food its colour.
Pyrazines
Maillard products with roasted, nutty smells.
Sugars and amino acids react on a hot, dry surface into colour and hundreds of flavours; at 100 °C the reaction almost stops.

Sources

  1. Maillard browning: apparent activation energies of roughly 100–160 kJ/mol are reported for colour formation (M. A. J. S. van Boekel, Kinetic Modeling of Reactions in Foods, CRC Press, 2009). The course uses 125 kJ/mol and a teaching scale of browning.
  2. H. McGee, On Food and Cooking: The Science and Lore of the Kitchen, 2nd ed., Scribner, 2004.