What does preheating do exactly?

I am glad you asked that question …it’s something I evaluate frequently in my mind when I am cooking …does it need a preheat or not….I guess I kink of knew the reason behind it …but a lot was just speculation.

My curiosity got me on the 350 temp a few years ago. Why so many things call for that magical temp. To which I found out that there is something called the “Maillard reaction” that happens between 300 and 350 which apparently causes a chemical process with the proteins and sugars in your food …to turn it brown. Additionally that temp makes hundreds of flavor compounds that make foods much tastier.

If you are following a recipe (depending on what it is) that requires a pre-heat …your success will be greater by doing so. But apparently  there has been some wavering within the “always pre-heat camp”, although it does depend on what you are cooking.

When it comes to baking, I think it is critical to preheat … as opposed to doing a roast, casserole, pan fries, things like that….. But, what do I know …. I rarely follow recipes …maybe the first time around…. but after that, I will be supplementing and changing things.

As far as the science behind it…

The baking I totally understand, logically …when you are working with anything dough, yeast, eggs, or anything pastry based …. that pre-heated blast to jumpstart the reactions is necessary.

According to my daughter who is an executive chef….. meats going in the oven should be seared first or put in a preheated oven, because it starts the caramelization and release the meaty flavors…

In an oven...........this is necessary especially for baking.  You need the oven temp to be at a certain degree before baking can begin or the item will never bake properly.

Your Favorite Answer is also mine. As for people yakking about 'bacteria' and other nonsense, it takes HOURS for most types of things to generate dangerous levels of bacteria, if held outside of "safe temperatures" for that type of product (e.g., meat and dairy).  The time between putting it into a "cold oven" and "hot oven" is irrelevant when the product rises to proper temperature in due course.

For instance, using their logic, you should die of salmonella every time you roast a chicken or turkey, since it may take well over an hour for the interior temperature to reach a safe level.  But THEN IT GETS COOKED at the higher temperature, making their point entirely moot.

Many types of cooking tells you the time it takes to cook.  However, if you have an oven that takes a long time to heat up(such as a wood stove) then the cooking time will take longer than the recipe stated.  Meat such like pork requires that it be cooked thoroughly and they say preheated meaning when you stuff it into the over that is that temperature it should only take the time they say.

No you don't understand at all. It's nothing to do with even cooking. It's so that bacteria don't have a chance to multiply in a warm oven.

Pe-heating is exactly that. You are getting the oven to cooking temperature before you put the food in the oven. This ensures a proper cook. As an example, a cake recipe says to preheat to 350 deg.  If you put the cake in before the oven is at the proper temperature, at let's say 250 deg, then the cake cannot cook properly throughout as the outside of the cake has begun to cook at a lower temp. Even if the oven gets to the proper 350 deg later, the inside and outside will be cooked incorrectly. This means that the cake may look tough and crusty on the outside and may NOT be cooked completely in the inside.  Recipe's are tested and confirmed for a reason: so you don't have to make mistakes.  Always get the oven to temp desired before putting the food in the oven.  

You don't preheat the oven so the food cooks evenly. You preheat it because some foods need to be exposed to full heat immediately in order to cook properly. That applies most to things that are leavened. And other foods should not be exposed to the full heat of the bottom element which is on the whole time you are preheating. They could end up burned on the bottom while the rest isn't done yet.

You can find out more by googling 'why we preheat an oven'. Many articles will explain the science part of it. No need for me to write a few paragraphs on it.

You pre-heat the oven so the cooking time is correct- EVERY oven preheats at different rates (over time). If the recipe says set oven to 350f pre heated there is an excellent chance that the cook times will be very accurate.

no, you preheat an over because it takes time for the over to get to the temperature required to cook something, if you put something you were cooking into a cold oven it would take much longer to cook and the thing you are cooking would end up being all dried out 

A hot oven kills bacteria instantly whereas they can multiply in a wam oven