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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.

nuggethead
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    If you're cooking three things at once, my guess is that you're opening the oven at least once during cooking to add/remove items. If you are, the more times you open the oven, the longer it will take. – Dan C Aug 13 '22 at 13:33
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    There are two aspects to heating something: How hot can it get (regardless of how long it takes to get there, and how well it can hold there), and how much energy can be put into making that thing hotter. A very powerful heater might take something to X temperature with ice-cold water running over it no problem, but a weak heater (that could get to X normally) may not be able to overcome the cooling effect of the water. – Steve Aug 15 '22 at 22:33
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    More cold food = more cooling effect. – Steve Aug 15 '22 at 22:34

4 Answers4

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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.

Ecnerwal
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  • Interesting! You've made me curious now. In a standard non convection oven, does air circulate at all? There isn't a fan or anything – nuggethead Aug 12 '22 at 23:11
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    Sure it does - convection currents driven by thermal gradients (also, there's usually a small vent, too.) Without fan forcing they are comparatively weak, though. – Ecnerwal Aug 13 '22 at 00:58
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    @nuggethead -- in a conventional oven, air circulates by convection; in a convection oven it circulates by fan. – Pete Becker Aug 13 '22 at 13:10
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    Besides the poor convection, you also block radiant heat, so the food is being heated from above or below but not both. There are some cookie recipes that specifically mention not to cook more than one tray at a time because it causes issues. (How it spreads before setting, browning, etc) – Joe Aug 14 '22 at 15:43
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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.

rumtscho
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    Any experience with cranking up the oven temperature setting, to try to correct for the temp difference between the air near the food vs. the air near the sensor? Presumably that shifts the balance more towards radiant heat, as the element is on more. Depending on the pan (glass vs. metal vs. ceramic), that might lead to more browning or even charring of thin parts sticking up; where hot air could get around and heat "shaded" parts, radiation is more of a line from the elements, especially if you use the top element (broil instead of bake on my oven). I haven't experimented myself. – Peter Cordes Aug 13 '22 at 08:38
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    "The oven dial just determines the energy that will be emitted, averaged over a longish period of time." Are you sure that's true? Having a look at oven thermostats suggests that they work by having that heating element on full until the requested temperate is reached and then turning off (until the temperature falls). So an oven that claims to have reached 400 should be (more or less) at 400, at least at the place where the thermostat is. – Richard Tingle Aug 13 '22 at 17:00
  • @RichardTingle The controlling in most ovens is very loose, with the thermostat allowing several tens of degrees deviation before turning on or off, so the supposed 400 is only the average temperature over time, it fluctuates considerably. And ovens tend to turn their preheating lamps off quite early, long before their walls have warmed enough to ensure a quick return to the "reached" temperature despite opening the door (which loses the hot air) and shoving a room-cold pan with food inside. – rumtscho Aug 13 '22 at 17:49
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    @PeterCordes most oven recipes profit from a low-and-slow cooking process. Trying to artificially speed it up by giving it more speed is rarely good, from a taste point of view. And you cannot well shift towards radiant heat when most parts of most food pans are shielded from it, in an oven filled with several pans stacked over each other. So while it might be possible, it is usually not desirable. – rumtscho Aug 13 '22 at 17:52
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    @rumtscho of course, oven thermostats aren't great. But your answer implies that the dial actually directly controls energy output. And could be labelled in watts. But that's not the case and an oven will have a far higher enegy usage while warming up than after it has reached temperature – Richard Tingle Aug 13 '22 at 17:54
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    I think this answer is mostly right, but not terribly close to *entirely* right, and definitely disagree with your wording on the thermostat - sure, ovens aren't super accurate, but it's not just "heat level", it's (attempting to) adjust by actual temperature. – Joe M Aug 14 '22 at 03:12
  • I wasn't trying to speed things up relative to a recipe, just achieve similar oven temperature *around the food*, to get the desired cooking speed. But yes, food on both racks will obstruct radiative heating, good point. Heat distribution would be very uneven, and thus very hard to manage. Maybe possible in theory (with enough sensors and/or computer modeling of oven heat distribution), but utterly impractical except maybe with a few foods that can cook at a range of temps, and probe thermometers so you don't need to know the temp or time them. – Peter Cordes Aug 14 '22 at 05:15
  • @RichardTingle this wasn't what I was trying to say. If it is how it comes across, do you have ideas how to reword it? I wanted to emphasize that the oven is very far from holding a constant heat level, and that the PID controlling contributes much less to the final heat output than most people appear to think, but not that it doesn't exist. – rumtscho Aug 14 '22 at 08:48
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    @rumtscho Maybe "It is not true that the whole of your oven is at exactly 400 degrees, that's a convenient fiction which is easy to put on a dial." and then later "The oven dial just determines what one particular part of your oven with roughly be at. There may be significant parts of the oven not at that temperature" – Richard Tingle Aug 14 '22 at 10:26
  • The thermostat doesn’t affect how much energy is put out (unless you have a fancy oven with ‘fuzzy logic’), it’s just a threshold at which the elements stop heating. Opening the door, food absorbing heat, evaporative cooling, poor convection from crowding, blocked radiation, etc are all factors here, and the oven element likely can’t maintain the temperature requested – Joe Aug 14 '22 at 15:40
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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.

JS.
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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

Felicia McGee
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  • Congratulations on making your first SA answer! I'm afraid it got downvoted, though. The reason for that is that you're not actually answering the OP's question. When answering, it's worth reading the question carefully so that your answer is on target. – FuzzyChef Aug 21 '22 at 18:51
  • Thank you for explaining where I went wrong. I will work on this. – Felicia McGee Sep 09 '22 at 22:19