The seemingly exponential browning of toast can be explained by the principles of heat transfer and the Maillard reaction, which is responsible for the browning of food during cooking.
When you put bread in a toaster, the heating element starts to radiate infrared radiation, which transfers heat to the bread. The initial stage of toasting is relatively slow because the heat needs to penetrate the surface of the bread and raise its temperature to a level where the Maillard reaction can take place.
The Maillard reaction is a chemical reaction between amino acids (proteins) and reducing sugars present in the bread. As the temperature of the bread increases, the Maillard reaction starts to occur more rapidly, leading to the browning of the outer layer of the bread. This is what gives toast its characteristic flavor, aroma, and golden-brown color.
The Maillard reaction is a complex process, and as the toasting continues, more and more regions of the bread undergo the reaction. This results in an exponential increase in browning as the toasting progresses. However, as the browning deepens and the surface of the toast becomes drier and browner, it also becomes more susceptible to burning.
The last 15% of the toasting process is crucial because the temperature at the surface of the toast rises rapidly, and if left unattended, it can quickly go from a desirable golden-brown to burnt. This is because the Maillard reaction continues to produce browning, but the high heat also starts to break down the carbohydrates and proteins in the bread, leading to the formation of undesirable burnt flavors and darkening of the toast.
To avoid burning, it's essential to monitor the toasting process closely and remove the toast from the toaster when it reaches the desired level of browning. The time required for toasting can vary based on factors such as the toaster's temperature settings, the type of bread used, and personal preferences for toast color and texture.