Everyone knows what bread is, and everyone knows what toast is. And everyone knows that the first can be transformed into the second. Less clear: when, exactly, does the transformation take place? When does bread become toast?
The browning process we call toasting is an example of the Maillard reaction, in which amino acids and sugars interact to produce the characteristic brown color, texture, and flavor we know as toast. When heat encounters amino acids (many are present in wheat and flour) and sugars, the two rearrange and produce brown polymers (called melanoidins). The Maillard reaction is also responsible for the deep flavors of browned barley in beer, roasted coffee, seared meats, and French fries.
Toast pedants will stress that the Maillard reaction is not the same as caramelization, which is a type of thermal decomposition that chars—that’s what can happens when you toast your toast too long. Too much charring and you’re carbonizing bread, not toasting it.
That’s all well and good, but when, precisely, does bread become toast?