The precise moment when bread ceases to be bread and becomes toast is not a well-defined scientific point but rather a subjective and culinary distinction. Toast is essentially bread that has been subjected to dry heat, causing its surface to turn brown and become crispy.
The transformation from bread to toast occurs gradually as the bread is exposed to heat. Initially, the heat begins to evaporate the moisture from the bread, making it dry and warm. As the process continues, the sugars and amino acids in the bread undergo the Maillard reaction, which is a series of chemical reactions that create new flavor compounds and brown the surface of the bread. This browning is what gives toast its characteristic color and flavor.
The degree of toasting can vary depending on personal preference, from lightly toasted with a slight golden color to more deeply toasted with a darker brown appearance. Some people even prefer their toast with a few charred spots.
In essence, toast is still bread that has undergone a transformation due to heat. However, pinpointing the exact moment it becomes toast is challenging because it's a gradual process with no clear boundary. The distinction is more of a culinary concept, and the point at which bread turns into toast is subjective and based on individual taste preferences.