What happens to bread when toasted?

Toast is bread that has been browned by radiant heat. The browning is the result of a Maillard reaction, altering the flavor of the bread and making it firmer so that it is easier to spread toppings on it. Toasting is a common method of making stale bread more palatable.

What happens to bread when it is heated?

Answer: The heat, not unexpectedly dries the bread out. The heat causes carbohydrates and amino acids to react together in a reaction called the Maillard reaction. This produces the classic browning and flavour changes to produce toast.