Science Questions

Why should you pre-heat the oven?

Sun, 22nd Jul 2007

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Nigel, Northampton asked:

Why are we advised to pre-heat our oven before we cook food, surely it would be more efficient to use the pre-heating to warm the food first?


The main reason is that when cooking from a recipe, or following the instructions on pre-packaged food, there is a recommended time that the food needs to (c) TreblRebl@Wikipedia" alt="Roast Chicken" />be cooked at the right temperature in order to be cooked properly and safe to eat. If the oven hasn’t reached that temperature yet, you may be cooking it for the wrong length of time.

There are lots of chemical reactions occurring when you cook food, and different reactions occur at different rates depending on the temperature. This means that while your food is in the oven below the right temperature, you will not get the right balance of reactions occurring while you cook.

The reactions which give food it’s distinctive ‘cooked’ flavours and colours are Maillard reactions (between sugars and amino acids) and caramelisation (the oxidation of sugars). These reactions work better at higher temperatures, and so your food will look and taste better if you wait for the oven to heat up.

Also, while the food is pre-heating it will be losing water by evaporation, meaning that the cooked meal is likely to be drier and less succulent.


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Most of the pathogens in food arise from other people handling it, so they reside on the outside of whatever you are going to eat. If you shove it into a hot oven you will kill the bugs before they have time to replicate, but if you allow it to warm up gradually you may end up with, if not a viable population, a significant quantity of the toxins they exude as they multiply faster in warm conditions.

It is interesting to note that a lot of the cuisine of hot countries involves frying rather than baking - a much more efficient way of killing bugs.  alancalverd, Fri, 29th Nov 2013

And, quite simply, if the oven element is on because the oven is still heating up, it will quickly brown the surface of your chicken, making you think your chicken is done, while the center is still undercooked. cheryl j, Fri, 29th Nov 2013

I quite often don't pre-heat the oven.

If you are following a recipe which says "put in an oven at 200C for 20 mins" then the only way to replicate that recipe is to put the stuff in an oven that's already hot.

Some ovens take a long time to heat up, others heat up quickly.
So it's impossible to know (from a book) how long it will take to cook, if you put the food in an oven while it's heating.

However, I'm a science nerd and I'm often in a hurry when I'm hungry. I only use one cooker- mine- and so I can do the experiments and work out how long it takes to cook stuff starting with the oven cold.
It's quicker and it uses less electricity.
I haven't died from food poisoning, not would it be sensible to expect me to have.
Most bacteria can roughly double their numbers in half an hour- given ideal conditions.
As long as it takes less than half an hour for the oven to get to a temperature where it will suppress the bugs, the idea that they grow while it warms up is a non-starter.

re "It is interesting to note that a lot of the cuisine of hot countries involves frying rather than baking - a much more efficient way of killing bugs.  "
It's interesting to note that , if you live in a hot climate, you don't have an incentive to keep an oven burning all day, and it's sensible to cook food quickly in hot fat, so you don't need to keep the cooking fire going for longer than you need.
Even so, at least one method of cooking is named after the oven it's cooked in even though it developed in a fairly hot bit of the world..
Tandoori chicken anyone?

Bored chemist, Fri, 29th Nov 2013

Aren't these somewhat opposite ideas. 

So, if you heat up your oven using the broiler & bottom burners, you'll likely rapidly kill anything on the surface. 

The problem is getting a uniform temperature throughout the meal.

In fact, you may get more heat from having the burner directly above or below your meal than you would get from the slow transfer of heat from the air surrounding the meal.  I.E.  It may cook at least the surface too fast when preheating.

Much of cooking, at least in the modern world is for flavor and texture.  How many people have died from licking the spoon and beaters? CliffordK, Sat, 30th Nov 2013

A lot depends on the design of the oven. Fan-assisted ovens generally don't require preheat because the air that hits the food is already at a high temperature, but many electric ovens depend on the heating element bringing the walls up to temperature, then cooking by radiation and natural convection, so the surface temperature of the food rises more slowly. Hence the instruction to preheat at least guarantees that the food always starts in the same thermal environment regardless of type, though it may be unnecessary in the case of a fan oven.

"Even temperature throughout.." only occurs with pulsed microwave cooking. When roasting a large joint with a roasting thermometer, the end point is determined by the middle of the roast reaching a target (usually above 75 deg C) whilst the outside is in near-equilibrium with the oven (180 deg C or more). Unless, of course, you want to ruin a perfect piece of beef. And the target temperature for pork and poultry is higher, because the endemic bugs are different.

If cooking didn't kill bugs, nobody would suffer from salmonella from undercooked chicken. So is salmonella a real public health problem, mass hysteria, or an urban myth?

Perhaps I'm an obsessive nerd, but I spent a good part of my career solving heat diffusion equations, and my eldest son is a chef. We often discuss del squared theta and the appropriate wine.

PS for BC alancalverd, Sat, 30th Nov 2013

"If cooking didn't kill bugs, nobody would suffer from salmonella from undercooked chicken. So is salmonella a real public health problem, mass hysteria, or an urban myth?"
It's a real risk of course, and cooking does kill bugs why ask?
Re the Tandoor, Yes, I know. So what?

I think it would be better to talk of preheating ensuring an even temperature throughout the oven, rather than the food. The food never reaches a temperature much above 100C or it would be dry and probably inedible. Bored chemist, Sat, 30th Nov 2013

Science has yet to discover why some of my relatives do this. cheryl j, Sat, 30th Nov 2013

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