When you burn toast in a toaster oven, the charring is not caused by tiny fires on the surface. Instead, it is the result of the Maillard reaction and caramelization.
The Maillard reaction is a chemical reaction that occurs when proteins and sugars in the bread undergo a non-enzymatic browning process due to heat. As the temperature increases during toasting, the amino acids (proteins) and reducing sugars in the bread react, leading to the formation of new flavor compounds and brown pigments. This reaction is responsible for the delicious aroma and golden-brown color of toasted bread.
Caramelization, on the other hand, is a separate process that occurs when sugars are heated to high temperatures. As the sugars in the bread are subjected to heat during toasting, they break down into simpler compounds, producing a sweet and browned surface.
Both the Maillard reaction and caramelization are responsible for the charring effect on the surface of the bread when it is over-toasted. The degree of charring depends on the intensity and duration of the toasting process. It's important to find the right balance when toasting to achieve the desired level of browning without burning the toast.