We know it’s important to wait for the oven to fully preheat before we start baking, but sometimes we're guilty of jumping the gun and sliding food in early. We wondered: How much of a difference does it really make?
To find out, we baked cake layers and chocolate chip cookies, starting one set in ovens that were fully preheated and the other set in ovens that were only halfway to their target temperatures. The results were surprising: The type of oven that was used, and specifically how it preheated, had a major impact on the final results. Ovens that preheated using only the bottom element produced acceptable food, though barely so. Even in those ovens, cookies spread too much because the oven wasn’t hot enough for the dough to set when it should have. The results were much more problematic in ovens that preheated using both the top and bottom heating elements. (They are designed this way so that they preheat quickly; the top element turns off when the target temperature is reached). Food placed in the oven early was exposed to a blast of direct heat from above. Cookies set before they’d spread enough, and the heat from above aggressively kick-started browning so both cakes and cookies were charred on top by the time their centers cooked through.
The takeaway? Since the results often suffer when you add food too early, we’ll wait until the oven is properly heated before placing food inside.