OK, I’ve always wondered how much time do you save when you preheat an over for a pizza? Do you even save time at all? Secondly does it matter which side of the aluminum foil is facing the food? Some kid told me that to keep it warm you want it the shiny side down and vice versa for cooler food. Is this just BS? Thanks
IANA physicist, but I don’t see how you’d save any time by preheating an oven. I always assumed preheating was intended to ensure that cooking times could be given accurately (for example, if a recipe said a pizza needed fifteen minutes in a 350 degree oven, the time would be longer if you put the pizza in BEFORE the oven was fully heated).
I seem to remember watching a program about aluminum foil on TV (I must have been really bored) in which representatives for one of the big foil companies claimed that there was no practical difference between the shiny and dull sides of the foil - that the difference in appearance was only a result of the manufacturing process.
One of the reasons to preheat an over is for more even cooking. If you put an uncooked pizza into an oven that isn’t preheated, then the bottom of the pizza will be getting much more heat than the top as it will be above the heating elements (or burners) as they go full bore to warm up the oven. If you like your pizza with the bottom burned and the top still raw, by all means do not preheat the oven.
The temperature of the oven is of paramount importance to insure that a pizza is properly cooked. Time in the oven is second, and third involves whether of not you use a pizza stone, wire oven rack (open cooking both sides), or the hearth of a commercial pizza oven. No need for aluminum foil with any of the above.
BTW: The last rolling of aluminum foil is two layers and the facing sides acquire the frosty appearance during rolling. The two layers are separated and packaged separately.
It’s probably not the big of a deal with pizza but it can be critical for baking.
Haj
I have to say that’s incorrect as I just cooked a pizza w/o preheating the oven last night and it came out just fine. I didn’t actaully do the cooking so I don’t know how much the time was altered by, and while the cheese looked slightly unmelted it was more then hot enough.
Oh, and the old Frescetta pizzas required you to put the frozen pizza in a cold oven (and then turn it on of course).
In my old oven, if I put a pizza in without preheating, it still wouldn’t be up to temperature after 17 (19 for supreme) minutes were up. I suppose the pizza is cooking some in that time, so the total time might be less if I started cold, but how long it would take me would not necessarily bear any relation to how long it might take with a better oven. So the oven has to start at a uniform temperature for them to be able to give one set of directions.
And back in my Boy Scout days, I figured that it wouldn’t matter which side of the foil was out on a foil meal, because any heat penetrating the foil in either direction still has to get through all the layers. None of the guys in my troop bought that reasoning, until I did the experiment. There was in fact no detectable difference between the two meal packets I wrapped different ways.
May not matter too much for pizza (although it may depend on how long your oven takes to preheat), but for some things, failing to preheat can alter the end result; because the food item will be exposed to a gradual buildup of heat over a slightly longer period, it might dry out more and things like cake mixture may not behave in the same way (i.e. rise) as when put straight into a hot oven.
Here’s a page on it, but the experts aren’t very unanimous or convincing: http://www.newton.dep.anl.gov/askasci/gen01/gen01407.htm
If there’s a significant difference in reflectivity between the shiny side and dull side at infrared wavelengths, you would want the shiny side on the inside if you’re trying to keep something warm or heat it up, and shiny out if you’re trying to keep something cold or cool it off. I don’t have any data on the relative reflectivity of dull vs. shiny in the IR spectrum, so I can’t say.
If the food is touching the foil it wouldn’t mater, since the foil will conduct directly to or from the food, with no chance for inner IR transmission.
The cooking method would make the most difference. If you’re cooking with one of those new halogen ovens, where IR is a significant part of the heating, then the reflection could make a difference. In a convection oven, it would make very little difference.
Do what you think sounds better, and just assume it’s the right way. 
I would expect that a gradual rise in temperature, with an unheated oven, would dry out the toppings and dough before it was able to cook and make the dough more compact, rather than ‘fluffy’.
At higher temperatures moisture would be trapped inside the food matrix by the dextrin and oils from the cheese. Gas in the dough will also be trapped if cooked quickly.
In a slowly heating over water and gas would be driven off before being sealed in.