Why turn on the oven at the start of a recipe?

Question

I have noticed that many recipes say to turn on the oven as one of the first instructions, even though there may be half an hour or more of preparation before the dish is actually placed in the oven.

This seems wasteful to me, and I usually wait to turn on the oven so that it will come up to temperature more or less at the same time the dish is ready to go in.

Am I doing it wrong? Is there some actual reasoning for turning on the oven so early?

(I'm aware that some people recommend a long preheat for pizza.)

Answer 1

For baking bread, cakes etc., it is important to properly preheat the oven.* For many other dishes it's not such a big deal; you may even be able to turn it on when you put the food in, though it will take a little longer to cook (still less than the total of preheating plus cooking, so dinner can be on the table sooner counting from when you enter the kitchen).

When writing recipes, a decent author will try to make the process easy to follow. This means turning on the oven when you're not occupied with other things and have clean hands. They also don't know how long your oven takes to heat up, and whether the indicator on it even reflects the air temperature (common, but apparently so is a timer; typical domestic ovens don't consider the temperature of the metal, which holds more heat than the air). So they'll err on the side of starting early. Maybe not by as much as you think, because once you've made the recipe a few times (as they will have done) the prep is a little quicker.

---

* "Properly" preheated is still variable, but one good rule of thumb, with a light that indicates when the element is on, is to wait for it to go off the second time: It first goes off when the air is up to temperature, comes back on as some of that heat soaks into the metalwork and the air temperature drops, then goes off again when the air is back up to temperature. That's still not enough if using things like pizza stones or preheated cast iron (common for sourdough and some other breads).

Answer 2

You want the interior of the oven, not just the air, to be nice and hot so little heat is lost when you open the oven door to put in the dish. That takes longer than just heating the air.

Answer 3

Case 1: Let's say you have a batter whose leavening agent is baking soda. Baking soda goes to work immediately producing CO2 gas in the presence of an acid (such as lemon juice, buttermilk, etc.). As soon as that batter is prepared it needs to go into the oven as soon as possible. The longer you have to first wait for the oven to completely heat, the less leavening power you will have. See this article and look for the quote "It is activated as soon as it gets wet so recipes with baking soda are best baked right after mixing ...".

Case 2: Once a batter is prepared and then sits, the flour becomes more hydrated over time. For some recipes that is a good thing and you are instructed to let the batter sit for some period of time before baking. See this article. But for other recipes, you may not want to wait. If, for example, you are baking bread, you might be directed to give the dough a second rising and to bake it when it has doubled in size. If the oven is not ready for baking, there is a danger that the dough could rise too much and change the structure of the final product. Since rising is generally a relatively slow process, waiting a few minutes will not make much of a difference. But you do not want to be starting to heat the oven at this point.

Answer 4

For most ovens, the heating element is either on or off, it doesn't not have a variable control. This means, if your oven supports a max temp of 500 degrees, the element will heat to 500 degrees when on.

Now consider your recipe -- suppose it calls for 300 degrees and you turn on your oven. The element turns on and begins to heat at 500 degrees to quickly bring the oven up to temperature. This means that anything that is close to the element may be way overheated in the process, resulting in burned food.

Once the oven reaches temperature the element turns off. As the temperature drops, the element will come on momentarily to restore the temperature, and then back off, cycling like this to maintain the set heat point. Since the element is only on momentarily to maintain heat, the risk of overheating is minimized or eliminated.

In a related note: this is typically why you place your food in the middle of the oven in order to ensure even cooking, rather than at the bottom near the element where you run the risk of burning the bottom.