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It seems that if I have fully loaded our oven, everything takes longer to cook.
A pan of veggies will roast in an hour at 400. But if we are also baking chicken and fries in there at the same time, nothing is ready in time.
Here's what I don't understand. The veggies are still exposed to 400 degrees for an hour. What physical effect could cause the food to take longer to cook? Or am I imagining it?
For the record, I'm a regular person cooking in a regular electric oven at home.
Regular domestic oven - so, not convection. Fill it up with pans, poor air circulation. The area near the thermostat sensor may well be at 400 °F (or depending how much your oven lies, it may not - I've seen multiple cases of domestic ovens always reporting they are at temperature once they get there, even if the door is opened or they otherwise are not actually at temperature - but they don't want to tell you that, so they don't) but temperatures in different parts of the oven will vary considerably.
Any food containing water will cause an area around it of "pretty much 212 °F/100°C" due to water boiling off from the food. Get enough of that going on and much of it won't see 400 °F, really.
A convection oven (at least a half-decent one) will generally be more even due to forced air circulation.
It is not true that your oven is at 400 degrees, that's a convenient fiction which is easy to put on a dial.
In fact, an oven has two heating elements, which are pumping energy into the inside of the oven. The oven dial just determines the energy that will be emitted, averaged over a longish period of time. The energy is transferred to the oven walls, the air in the oven, the food, and the pan material.
Out of those, the food is the worst heat sink you have. The air has almost no mass, and is heated quickly. The metal conducts heat well, heats up quickly, and starts emitting itself once heated. The organic matter in your food is slow to heat up, especially by conduction from the warm air. Direct radiation from the heat element contributes a lot, especially to surface browning. And of course, for the conduction to happen, you need the air layer close to the food to be warm, not a wad of cooled down air which just exchanged its energy with the food.
As soon as you fill your oven up, these effects are disturbed. For conduction, you get the same amount of energy, but it has to be exchanged with a much higher mass, so you get less energy per unit of food. For convection, Ecnerwal already explained that the foods block the flow of air, so the air surrounding the food is colder than it would have been. And for radiation, the upper crust of the uppermost food absorbs everything coming from the upper element, and the lower crust of the lowermost food absorbs from below.
The oven is not a completely dumb device, it does have a sensor, and so it should keep the heating elements turned on for longer in such a situation than in an emptier oven. But first, as Ecnerwal pointed out, the sensor has an arbitrary position that is not so close to the food, and you have created a situation in which the heat distribution within the oven is very unequal. And second, I would guess that, even if it goes on for longer, it is just not sufficient to cook your food quicker. This is especially true with modern baking techniques, where we use light ovens and barely preheat them, so that they are nowhere close to thermal equilibrium when we add our food. This is a really good thing for both our wallets and the environment, but it does require you to get used to cooking times that vary with oven loading.
Decreased air circulation and inaccurate oven temperature regulation are factors, but so is the amount of energy your oven can deliver per hour.
If you put two glasses on the counter, one with 1 ice cube and the other with 50 ice cubes, you probably wouldn't expect them to both convert fully to water at the same time. This is because the amount of energy to melt 50 ice cubes is much more than the energy to melt a single ice cube. More energy transfer means more time required.
just read your post. The problem is everything cooks at a different temperature. If you're cooking a nice size chicken with veggies and fries, they will overcook and slow the cooking process of your chicken down. For example, if you are frying fish, and you take out pieces that are done and add raw fish to the hot fish, the temperature will drop your grease and the best way to fix this is to place a lid on your pan to increase the temperature back up. Hope this helps.
Felicia
