Why does toast turn brown?
Show answer & explanation
Answer: Sugars and proteins react to form brown compounds
The bread burns from the oven heat — Toast is not burning. Burning would char the bread black and bitter, while browning stops at golden to deep brown and tastes sweet and nutty. The color comes from a chemical reaction between sugars and proteins in the bread's surface, the Maillard reaction, which happens well below the point where bread catches fire.
Sugars and proteins react to form brown compounds ✓ — Correct! Heat drives the Maillard reaction, in which amino acids from the bread's proteins combine with its sugars to form hundreds of new brown-colored and flavorful compounds. The reaction can creep along slowly at lower temperatures, which is why a crust darkens over a long bake, but it speeds up sharply once the dry surface passes roughly 140°C, so a toaster browns the outside in minutes while the inside stays pale. The same reaction gives seared steak and roasted coffee their color and aroma.
Moisture evaporates and leaves color behind — Moisture does leave the surface, and that is what makes toast crisp, but drying by itself does not create color: dried bread stays pale. Browning needs a chemical change, and the loss of surface water actually helps it along by letting the surface get hot enough for the reaction to run quickly.