The encyclopaedia of gravy
The Maillard reaction is the browning reaction between amino acids and sugars that creates the roasted flavours in fond, browned onions and a dark roux — and therefore most of gravy's flavour.
What it is¶
The Maillard reaction is not one reaction but a cascade. Amino acids (from proteins) react with reducing sugars, and the products react further, producing hundreds of new molecules — many of them intensely aromatic, and collectively responsible for the smell and taste of roasted, baked, fried and grilled food.
It was described by the French chemist Louis-Camille Maillard in 1912, though cooks had been exploiting it for millennia.
Why it needs a dry surface¶
Water is the limiting factor. As long as a surface is wet, it cannot rise much above 100 °C, and the Maillard reaction proceeds meaningfully only above roughly 140 °C.
This single fact explains a great deal of gravy practice:
- Pat meat dry before roasting.
- Do not crowd the pan when browning mushrooms or meat — crowded food steams in its own moisture and never gets hot enough to brown.
- Do not add water to a roasting tin mid-cook.
- Let mushrooms release and evaporate their water before expecting them to brown. See mushroom gravy.
Where it happens in gravy-making¶
| Stage | Reaction |
|---|---|
| Meat roasting | Surface browning, and juices caramelising onto the tin as fond |
| Bones roasting for stock | The difference between brown stock and white |
| Onions caramelising | Maillard plus sugar caramelisation |
| A dark roux | Browning of flour proteins and starch |
| Tomato purée cooked out | Browning plus caramelisation |
Maillard vs caramelisation¶
They are different reactions, often confused because they happen together.
- Caramelisation is sugar alone breaking down under heat. It needs no protein and happens at higher temperatures.
- Maillard needs both an amino acid and a sugar, and starts lower.
Browning onions involves both: caramelisation of their sugars and Maillard reactions with their amino acids. Gravy browning is pure caramelisation, which is precisely why it adds colour but almost no savoury flavour.
The limit¶
Past a point the reaction products turn bitter and then acrid. Dark brown and glossy is the target; black and matte is bitter. This is the difference between good fond and a burnt tin — see deglazing a roasting tin.