4. Sugar Dissolves.

For many batters and doughs, sugar dissolves completely during mixing. However, when batters and doughs are high in sugar or low in moisture—as is the case with most cookie doughs and some cake batters— undissolved sugar crystals are present at the start of baking. These undissolved crystals help thicken and solidify batters and doughs.

When they heat up, however, sugar crystals dissolve in batters and doughs. As they dissolve, sugar crystals pull water from other molecules, such as starches and proteins, to form a sugar syrup that thins out batters and doughs. This thinning becomes significant as temperatures approach 160°F (70°C). As with melted fat, dissolved sugar increases cookie spread. Dissolving sugar also thins out cake batter in the oven, making it more susceptible to collapse or tunneling. To prevent cake batter from collapsing as it heats up, structure builders must begin to thicken and set up.