Even if the pan appears to be at the same temperature
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The food contains water, which is released as the food cooks. Water boils at a lower temperature than oils used in cooking will burn at. While there’s water to boil, the oil can’t get too much hotter than the boiling point of water that it’s in close contact with, because any additional heat goes into boiling more water.
The pan isn’t the same temperature. The mass of the food is acting as a heat sink and keeps the oil/butter from getting as hot. The oil is heated by the pan (which is heated by the stove). But this heat energy then moves into the food rather than staying in the oil, because the food is at a lower temp. So before the oil/butter can get hot enough to burn it has to heat the food up too. Eventually the oil/butter will get hot and burn….. along with the food.
Generally, the more stuff in a pan, the slower everything will burn – this is because heat always wants to go from a higher heat area into a lower heat area. If you have only butter in a pan, then the path for heat to travel is from the flame, into the pan, and into the butter, heating the butter hotter and hotter until it burns.
But add some diced onions into that butter, and now the path becomes flame>pan>butter>onion, and until those onions get to a very hot temperature, heat will continue to transfer into them, taking away from the heat added into the butter. Though, if you leave them alone long enough, their temperature will increase until they start burning.
You can keep the process going, as well. When I make minestrone, I start with onions, carrots, and celery, all sweating down in oil for a few minutes until they’re soft. Then I add more veggies to saute before I add my liquids – potatoes, zuchinni, etc. – and the new veggies absorb enough heat to slow down the cooking of those first veggies, so that nothing burns or overcooks (as long as I’m paying attention).
The point of water (food typically has water in it) is to limit the temperature of whatever you’re cooking, to 100 Celsius 212 Fahrenheit. When a liquid starts to boil, the heat energy from the flame no longer raises the temperature of the liquid, but rather all of it goes into breaking away the liquid molecules so that they’re a (widely spaced) gas rather than a (closely spaced) liquid.
So oil and butter boil at a much higher temperature than the water in the food. And as soon as the food runs out of water (all of it boiled away), the temperature will start going up again and the food will be charred. The flames will start happening in the pan when the temperature of the oil is high enough to ignite.
And that’s why firefighters typically use water hoses at and around a fire: all of the heat from the fire goes into trying to boil all that water, and the wood materials get *a lot colder* and go under the “ignite” temperature.
Same thing happens with ice, by the way. No matter how hot it is outside, in a cup with ice water (or ice and soda), all of that heat energy goes into melting the water (ice) molecules from solid to liquid. So the entire cup stays at 0 Celsius 32 Fahrenheit until the ice has fully melted.
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