What kind of change occurs on the surface bread when it is toasted?

Physical Changes:

* Color: The bread turns brown due to the Maillard reaction, a chemical reaction between sugars and amino acids. This reaction produces hundreds of different compounds, contributing to the toasted flavor and aroma.

* Texture: The surface becomes crisp and firm due to the evaporation of water and the gelatinization of starch. The heat causes the starch molecules to expand and trap air, making the bread more porous.

* Surface area: The toasting process increases the surface area of the bread, allowing for a greater interaction with butter, jam, or other toppings.

Chemical Changes:

* Maillard reaction: As mentioned above, the Maillard reaction is a key chemical change responsible for the color, flavor, and aroma of toasted bread. It also forms melanoidins, which contribute to the browning.

* Starch gelatinization: The heat causes the starch granules in the bread to absorb water and swell, resulting in a softer and more digestible texture.

* Caramelization: At high temperatures, the sugars in the bread undergo caramelization, forming complex sugars and contributing to the browning and sweetness.

* Volatilization: Some compounds, such as water and volatile organic compounds, are evaporated during the toasting process, which contributes to the changes in aroma and flavor.

Overall, toasting bread involves both physical and chemical transformations that create a delicious and desirable product.