Don't open the oven early
Early in the bake, steam, air bubbles and leavening gas swell the batter. But the egg and starch network around those bubbles is still soft. Open the door now and the temperature drops while the cake can't yet hold its own height.
Check through the oven window
Use the oven light and window first. Once most of the baking time has passed and the top looks set, open the door briefly for a toothpick test. Close it fast to keep the heat in.
Test yourself
Questions from the interactive lesson, with the answer and the why.
Why can a cake sink if you open the oven door early?
Cooler air shrinks the gas before the structure has set. Gas bubbles contract as they cool, and the soft network can't hold the cake up.
True or false: A cake's structure is already firm the moment it starts to rise in the oven.
False. Structure sets gradually. Early on the cake is still soft and fragile.
What went wrong? You open the oven door ten minutes into a long bake to check on the cake. By the end, the center has sunk.
Cooler air hit a cake whose structure had not set. The expanded gas shrank and the soft network couldn't hold the height.
Fill the blank: A cake falls when its gas shrinks before the ___ has set.
A cake falls when its gas shrinks before the structure has set. Set protein and starch hold the bubbles open, even as they cool.
When is it safest to open the oven door for a toothpick test?
Near the end of the bake time. By then the egg and starch have mostly set, so a brief opening is far less risky.