The top layer of lasagna becomes crispy when it bakes due to a combination of factors:
Cheese and Sauce Interaction: The top layer of lasagna is typically covered with a generous amount of cheese, usually mozzarella, and sometimes combined with Parmesan or other types of cheese. As the lasagna bakes, the cheese on top melts and starts to caramelize and brown, creating a crispy texture.
Evaporation of Liquid: Lasagna typically contains a sauce made from tomatoes or other ingredients. As the lasagna bakes, some of the liquid in the sauce evaporates, leaving behind a more concentrated, thicker layer of sauce on top. This concentration of flavors and sugars also contributes to the crispy texture as it caramelizes.
Maillard Reaction: The Maillard reaction is a chemical reaction that occurs between amino acids and reducing sugars in the presence of heat. It is responsible for the browning and development of complex flavors in cooked food. During the baking process, the top layer of the lasagna is exposed to high heat, promoting the Maillard reaction and leading to the formation of a crispy, flavorful crust.
Baking Temperature: Cooking lasagna at a relatively high temperature allows for quicker browning and crisping of the top layer. This contrast in texture between the crispy top and the tender layers beneath adds to the overall appeal of the dish.
By combining cheese, sauce, heat, and the Maillard reaction, the top layer of lasagna becomes crispy and provides a delightful contrast to the softer layers underneath.