The Maillard reaction is a chemical reaction in which amino acids and sugars in food combine under heat, browning the surface and producing hundreds of new aroma compounds. The browned crust on meat and bread and the aroma of roasted coffee all come from this reaction. Much of what we describe as flavor in cooking comes from its products, and one key factor can start or stop it in the kitchen: the amount of moisture left on the food’s surface.
The reaction can occur at lower temperatures in theory, but for it to become visible in the kitchen, the surface must reach 140-165°C / 285-330°F. Since water boils at 100°C / 212°F, food with a wet surface cannot reach this range: all the incoming heat goes toward evaporating the water, and the surface temperature stays at 100°C / 212°F. As long as meat in a pan is sizzling loudly, it is drying rather than browning; that sound comes from water vapor escaping the surface. Browning begins only after the sizzling subsides and the surface dries, once the water is gone. So if the food is not browning, the first thing to consider is how wet the surface is, not how high the heat is.
Three practical steps follow from this. First, pat the surface of meat or vegetables dry with a paper towel before placing them in the pan; meat taken straight from the refrigerator has an invisible film of water on its surface. Second, heat the pan before adding the food. Meat placed in a cold pan releases its juices before the pan gets hot and ends up simmering in them. Third, consider the oil’s smoke point: the milk solids in butter begin to darken at around 150°C / 300°F, so butter alone is not suitable for searing. Sunflower oil or refined olive oil can handle this temperature. If you want the aroma of butter, add it at the end, after lowering the heat.
The Maillard reaction is often confused with caramelization, but they are two separate reactions. Caramelization is the breakdown of sugar by heat and does not require protein; it begins at around 160°C / 320°F for table sugar. The Maillard reaction requires protein, so it occurs in meat, milk, eggs, and doughs made with flour. Even a piece of meat that appears to contain no sugar has enough naturally occurring sugar for the reaction. When onions darken in a pan, the two reactions occur together: the sugar and small amount of protein in the onions contribute at the same time. That is why caramelizing onions can take half an hour; onions that darken in ten minutes have browned but are not caramelized—they are burned.
Two factors speed up the reaction. The first is a slightly alkaline environment: brushing a very small amount of baking soda onto the surface of dough noticeably darkens the crust, but too much leaves a soapy taste. The second is the presence of sugar; dough brushed with milk, egg, or honey browns much faster than dough brushed with plain water. The one factor that slows the reaction is moisture: in a covered pot, a steamy oven, or an overcrowded pan, the surface never dries, so browning does not occur. With the same ingredients, the food begins to brown when the lid is off and the pan is less crowded.
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