I wanted to make some burned onion rings, so I chopped up some onions and let it cook in oil on the stove. Heating the oil was taking a long time so I decided to leave the oven alone for a bit.

I came back after hearing the oil making crackling sounds and I immediately turned off the heat. When I lifted the lid slightly to assess what had happened, the oil suddenly started crackling again violently.

Did I just almost start a fire? As I understand, there can be no fire without oxygen, and by lifting the metal lid I introduced oxygen. Also my entire house smells like smoke.

  • Mostly off topic, but what's a burned onion ring? Are they battered? – Preston Oct 2 '14 at 3:45
  • Just chopped onions in oil and later I added some brown sugar (which probably led to the smoke). – Dissenter Oct 2 '14 at 4:25
  • What kind of oil was it? – ElendilTheTall Oct 2 '14 at 8:07
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    You said "on the oven" and mentioned lifting a lid - did you mean on the stove? – Cascabel Oct 2 '14 at 12:40
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    Is it possible that the heat of the oil caused water to condense inside the lid? If so, then small droplets would form and drop back into the oil, causing small "explosions" as they immediately evaporate in the hot oil. Lifting the lid would also disturb the droplets, causing even more to drop in. – valverij Oct 2 '14 at 17:18

When you lift the lid on something that's been fryed in hot oil while the oil's still hot, any water that's cooked out of the oil and condensed on the lid has a chance to fall back into the oil. That can cause violent spattering. Usually being gentle in lifting is enough to prevent the problem.

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    and water entering hot oil will literally explode and burn off your face. It happened to someone in my high school culinary class; my friend described it as "his face turned white and melted off". The chef was fired from his job shortly thereafter. – seasonedaddict Oct 2 '14 at 23:23

Although oil can spontaneously combust if heated hot enough, the typical reason for grease fires is the following combination of events:

  • the oil level is too high in the vessel
  • wet ingredients cause the oil to violently bubble
  • the bubbles go over the side of the vessel
  • the spilled oil ignites from the burner below

... and when it happens, it's really not good. (trust me, I speak from experience). And the 'cover with a lid to keep out the air' trick doesn't work if your stove is designed in such a way as that there's venting from underneath the burner. See How do you put out a grease fire? .

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Note I answered this question assuming that the onions are in the oven, not on the stove. See the other answers for a frying pan on top of the stove.

It is highly unlikely that you started an oil fire in the oven. The flash point of oil is close to 400 Celsius, domestic ovens don't go that high. And even if you had managed to heat the oven to 300 Celsius, this wouldn't be the temperature of the oil in the pan. Without having been there, I can't tell for sure, but I don't see how you can have gotten it hot enough. Besides, oil burns with visible flames, if it doesn't give you an outright flameball.

A better explanation for the cracking would be steam explosions. They are unavoidable and harmless. The sugar would explain the smoke, sugar chars at 190 Celsius.

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  • They are harmless in that they won't start a fire, but they can still spatter hot grease on you and cause burns. – Nate Eldredge Oct 2 '14 at 18:49
  • Is it possible that hot grease spatter was hitting an electric heating element? While the oven won't hit 400 C, the individual coils definitely will. – Tenway Norsing Oct 3 '14 at 8:53
  • With the lid on, obviously not. Without a lid, you'd need an oven with open heating elements, which is not so common. And even then, you're unlikely to start a fire. I have roasted lots of meat in a toaster oven/rotisserie with open elements. The inside is all splashed with fat coming from the skin crisping, but I've never seen fire or smelled smoke. I suspect that the drops getting to the element are too small and burn out almost instantly, without anything else catching on fire. – rumtscho Oct 3 '14 at 10:09

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