In an oven, when following cooking times from a recipe, do I have to adjust times if I change quantity? I don't mean thicker or thinner food, I mean more or less of the same size, like e.g. more or less potato wedges.

Or does the thermostat in the oven just make the burner stay on for different times naturally and thus deliver the same amount of energy per mass of food over time (assuming I don't put too much food in at once).

  • This is a duplicate, I'm virtually certain. Here we go: cooking.stackexchange.com/questions/55678/… – Ecnerwal Jan 1 '17 at 20:29
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    @Ecnerwal : not quite a dupe. For potato wedges, you're also trying to get them crispy, and the increased evaporation will result in more moisture in the oven, throwing things off. I'd say make sure you're not over-crowding the trays, rotate them half way through (top to bottom) if two trays, and maybe crack the oven open once near the first quarter of the cooking. Check them at the normal recommended time, and possibly let them go a couple minutes more if it's not quite done. – Joe Jan 1 '17 at 23:03
  • @Ecnerwal It's fundamentally probably the same question. But the answer here is better. Mine might be more general too. Maybe mark as dupe the other way? Or whatever, y'all are more than welcome to close this, or just do nothing. – Jason C Jan 2 '17 at 1:38

Many ovens do not have a thermostat. They have a timer, which turns the heating elements on and off in a predetermined pattern. As a result, the oven is a very imprecise heating instrument. It is frequently miscalibrated, and even for a calibrated oven, the temperature you set is the average temperature, while the actual temperature of the air inside varies in a roughly sinusoidal pattern, and the internal temperature of the baked good is even more irregular depending on its composition, location, choice of baking tin, oven type, etc. Even if the oven has a thermostat, it only (roughly) regulates the air temperature, not the internal temperature of the food.

As a result, the change in heating speed caused by adding a little bit more mass is really insignificant when compared to the huge variability of heating speed which is inherently present between ovens.

Luckily, many recipes are not very sensitive, and taste good no matter how much heat transfer actually happened during the time prescribed in the recipe. For these, you don't have to sweat it, keeping the original time is mostly OK. If you create major occlusion (e.g. two separate pans of gratin above each other), add some time by gut feeling, and check frequently for doneness toward the end.

For sensitive recipes (roasts, custards, some cakes), baking by time doesn't work anyway. The best you can expect from a time given in the recipe is a rough guide to help with planning when to start preparation for a meal. You have to always bake these until they are done, regardless of what the recipe says. In this case, you again don't have to do any time adjustments, because you are not baking by time.

To summarize, no, you don't really have to do any adjustments to time. Just bake everything until it is done, and if your time-based expectations were wrong, adjust based on doneness.

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    Your first paragraph is self-contradictory, probably because your first sentence is wrong. The thermostat might not be very good but it's definitely a thermostat (unlike on an electric hotplate). It's easy to test that this is true; open the door for a few minutes and the element will come on continuously until it's back up to temperature. After that you're right - the calibration isn't up to much and is the average air temperature (though it's not much like sinusoidal as the element can deliver heat much faster than the oven cools with it off. – Chris H Jan 1 '17 at 22:51
  • Can confirm above comment. My oven has a thermostat, behavior when opening door confirms, and also I can actually buy a replacement sensor. But it is really poorly calibrated, in the past I've observed this oven holds at about 85% of its set point. PS off topic I've found leaving a large cast iron pan in the oven stabilizes the temperature swings slightly. – Jason C Jan 2 '17 at 1:26
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    OK, changed the claim that ovens don't use thermostats. – rumtscho Jan 3 '17 at 16:39
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    @ChrisH yes, any form of temperature control, including a primitive one such as a timer, can be calibrated. And those of ovens I have worked with have usually been poorly calibrated, settling at a temperature far away from that shown on the knob. I have read books also discussing that this is a common problem. I can see why you were focused on thinking of calibrating a thermostat, but calibrating something else is also possible. Anyway, glad that we could clear the whole matter up. – rumtscho Jan 3 '17 at 17:28
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    I don't have my sources here, but they should be from the 2010s, roughly. Certainly not 1970s. Of course, who knows when the authors updated their knowledge... I am quite certain I have seen this discussed in Cookwise, and I know Cooking for geeks has something on the topic, although not 100% sure it also mentions thermostatlessness. You have to also take into account geographic differences, I was shocked to hear that US ovens don't allow people to turn the bottom and top heater elements separately, and that's before we consider gas or solid-fuel ovens, which are still the norm in some areas. – rumtscho Jan 3 '17 at 19:36

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