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Just about every recipe advises to preheat the oven before using it. I often forget this, but luckily this doesn't seem to matter all that much. Hence my question: why would it be necessary to preheat the oven?

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Just to mention it, the reverse is sometimes true of broiling. I actually like to broil toast, sandwich melts, and even steaks or pork chops in a non-preheated oven. –  zanlok Dec 17 '10 at 22:07

7 Answers 7

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When you don't preheat, you cook your food at a lower temperature as your oven heats up for the first 5-15 minutes, depending on the target temperature and your oven's strength.

For forgiving foods, like a casserole, this may not affect you much - you'll just have to bake longer than the recipe says to. As long as you're careful, you'll be fine.

But if you're baking something that should be baked for a short time at a relatively high temperature, your results are going to be very different. For example, take traditional Southern biscuits: they're baked very hot for less than ten minutes. This cooks them all the way through, and browns the top and bottom. If your oven starts out cold, they're going to be done in the middle before they're brown! Beyond under-browning, you'll also run into problems like lack of rising in steam-leavened foods. Pastries, in particular, could probably be disastrous.

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Chocolate chip cookies aren't forgiving, either. Definitely pre-heat. This is so we scientifically duplicate the conditions outlined in a recipe. Otherwise, you introduce unknowns as per other comments. –  zanlok Dec 17 '10 at 21:32
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@zanlok: Yup, that wasn't intended to be a complete list of things that can go wrong. And of course, scientific duplication of conditions and control of all variables is not always important, like with a casserole that bakes for an hour. You just have to know when you can get away with being less precise. (Good general rule for reducing kitchen stress!) –  Jefromi Dec 17 '10 at 21:59
    
agreed on the casseroles :) –  zanlok Dec 17 '10 at 22:06

I have noticed that when I forget to do this, the elements on the bottom of the oven are running very hot. So, if I put a pan in the oven, then the bottom of the pan gets much hotter than the topside.

For example, quick dinner rolls - refrigerated croissants. (Dinner in 15 minutes - forget to buy or make rolls... Oven cold...) If I do not preheat, they come out burned on the bottom, but barely done on the top. Wait 3-4 minutes for oven to pre-heat and they come out fine.

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Nice point, while heating you are getting the direct radiant heat instead of the volume of hot air to do the cooking. –  Eric Dec 17 '10 at 18:21
    
Put a heavy pan (e.g., cast iron) under them (on the next lower rack), that'll block the direct heat. Or cook them on a cast iron (with foil on it, possibly) instead of a cookie sheet. –  derobert Dec 21 '10 at 21:29

As Jefromi said, some items, when baked, will have a very different reaction to warming slowly than to going into a hot oven. Consider a dough, like a biscuit or puff pastry dough that has butter layered or dotted in it. If the dough heats slowly, the butter will melt and run out, altering texture. If it heats quickly, the butter will do its job of shortening the dough or preserving layers.

The concerns about browning are also there, in that a pre-heated oven for the right amount of time will create the browning and bubbling and melting that is called for in certain recipes.

All that being said, it is easy to modify recipes and adjust for pre-heating times, but until you are very comfortable with this I'd be careful.

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The baking times in your recipes would be almost useless if you would not preheat your oven. Yet it could spare you energy, but every different oven takes a different time to heat up...

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The effect a particular recipe has on the ingredients is often dependent on applying a specific temperature for a specific amount of time. You have quite a bit of latitude when you're making a roast, a casserole, or something similar. However, you have almost none when you're baking. You should treat a baking recipe more like a chemical equation -- be as precise as possible with ingredient measurements and the amount of heat you apply. There are chemical reactions in the food that depend on this level of precision.

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In most cases, when packaged food advises you to preheat your oven (often giving a ridiculously long time), it's in the interest of having a “ready in 10 minutes” highlight on the packaging.

You can just put the food immediately, set the timer to the suggested time plus half your normal preheating time, and go spend your time on something more fulfilling like doing the laundry.

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Because it's important to follow directions, else we would live in chaos! "up would be down; left would be right; cats living with dogs..."

That and if you pre-heat you have a quicker 'bounce-back' to cooking temp because the walls of the oven are heated up and not just the air in the oven box.

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Eh, I'd have to say that it's more important to know what you're doing than it is to follow directions. Half of the time the directions are wrong anyway. –  Aaronut Dec 17 '10 at 15:26
    
Or the directions don't translate directly to your oven. Still, the point of directions is to have a good approximation, better than running amok without a rudder. –  zanlok Dec 17 '10 at 21:34
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To me, it's more important to understand than to follow directions (which is why I don't bake much...). But good point about bounce-back. –  RolandTumble Dec 17 '10 at 23:08

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