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.
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.
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.
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.
"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?"