How do cooks in Southern Louisiana imbue the "burnt" flavor in gumbos and gravies?

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The mouthfeel and flavor of gumbo comes from excessive application of a French cooking technique, the roux. Flour is fried in fat until the desired darkness and richness is achieved.

The roux is the focused application of the principal technique of all of cooking, the Maillard reaction. The Maillard reaction happens any time an organic compound is exposed to high heat. The molecules break down and, since there’s a lot of different compounds in organic tissue, lots of different products are produced from the reaction.

If you apply too much heat, then too many of the organic materials are reduced down to parts and the tissue fast approaches ash. But small amounts are quite tasty.

The Cajun roux is intended to bring to a stew almost too many of these compounds. The richness is provided by the fat that the flour is fried in, and the flavor, the just-shy-of-burnt taste, is provided by the flour getting almost too much of the Maillard reaction.

French chefs usually make roux with butter. Cajun chefs usually make roux with oil. Butter has a lower smoke point and so it can’t stand too much heat before it itself burns. “Burning a roux” is when too much of the flour breaks down, but it can also mean a butter roux getting heated past the smoke point.

Cajun chefs will make a butter roux in etoufee. An etoufee is flour fried in butter to which the Cajun trinity is added, onion, bell pepper, and celery. This makes a delicate sauce that is flavored with shellfish, usually shrimp or crawfish. A shrimp etoufee is usually called shrimp creole and tomato paste is added. Small amounts of tomato paste might be added to a crawfish etoufee.

Outside of Cajun food, the roux is used in the classic French mother sauce, the bechamel. A bechamel sauce is made by frying flour in butter for a short period of time, then milk and seasonings are added. This is also called a white sauce and is a common technique in homemade mac and cheese.

Bechamel plus cheese is called a Mornay sauce by the French. They have lots of rules on what kinds of seasonings you add to the roux for a bechamel and what kind of cheese you add for Mornay. There are five French mother sauces.

We’ve already discussed bechamel, which is light roux with milk added. There is also sauce veloute, which is light roux with broth or stock added, sauce espagnole, dark roux with stock added, sauce tomat, which is tomatoes cooked down.

Hollandaise is a bit different. Egg yolks and lemon juice are whisked to create the other great chemical reaction that underlies cooking, the emulsion. An emulsion is how you mix fat, which is an organic polymer, or a chain of similar molecules with loads of useful chemical properties, with water, an abundant substance that also has loads of useful chemical properties. Emulsions create richness not through the Mailliard reaction, but by making something that would normally be solid, into a silky-smooth liquid.

Chemistry is ultimately what makes things taste so awesomely yummy, and judicious use of the Mailliard reaction and emulsions will make you a dynamite chef.