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Last night I tried deep frying stuff for the first time.

As the oil heated in the pot, I thought it would be a good idea to stir the oil (I was really scared of it catching on fire). It was probably around 250F when I stirred it. Big mistake, should have just left it along.

For a few minutes after I stopped stirring, tiny little bubbles appeared in the pot, like the ones you would see in water just as it started approaching a boil. There weren't a lot of them, but they made these sharp, loud explosive popping sounds.

Why were they so loud and what is the science behind this happening?

Thanks! Matthew

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Was there any water on your stirring implement? How vigorously did you stir? What material was your stirring implement made out of? –  baka Jun 12 '12 at 19:05
    
I stirred with a set of stainless steel bbq tongs. I'm nearly certain there was no water on the tongs when I stirred the oil, and just gave the oil a brief (~10 seconds) swirl, nothing vigorous. The tiny bubbles appeared from the bottom of the pot, then subsided after the 2-4 minute firecracker-sounding show. –  Matthew Jun 12 '12 at 23:19
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2 Answers

up vote 14 down vote accepted

It definitely sounds like you had some water on whatever you stirred the oil with. When water droplets get in the oil, they sink since oil is lighter than water. Then the water droplets turn to steam because the boiling point of water is much below the boiling point of oil. At this point, the steam rapidly rises out of the oil and escapes with a noise and a splashing of oil.

It's good to be aware of the temperature of your oil when deep frying, but the oil will start to smoke long before it spontaneously combusts. Use a thermometer, and know the smoke point of the oil you are using, and you will be OK. The real danger is oil touching the burner, or the oil spilling on you, rather than the oil in the pot exploding. Stirring is unnecessary because the density of oil reduces as temperature increases, allowing the hotter oil to make its way to the top (but in a nice controlled manner, unlike water).

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Excellent sciencey answer, exactly what I was looking for. If there was any water on the tongs I used to stir, it would have been a very, very small amount (~1ml, washed and I did dry before stirring). The tiny bubbles that appeared came from the bottom of the pot, but were relatively evenly spaced out over the entire bottom of the pot and lasted around 2-4 minutes before subsiding. Would this happen if there was only about 1ml of water on the tongs when I stirred? –  Matthew Jun 12 '12 at 23:27
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His explanation matches my intuition of what happened as well. If you're absolutely certain that the implement was dry, the water could've been at the bottom of the pot before you poured in the oil. In that case, the pot was just too cool to boil the water yet. The stirring was a red herring in this case. –  Eric Hu Jun 13 '12 at 0:12
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The stirring may have been the catalyst to send super-heated water into a boil. It's all speculation though. –  michael Jun 13 '12 at 0:43
    
Any ideas why it was so loud? –  Matthew Jun 13 '12 at 16:11
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@matthew it was so loud because these were real explosions, only small ones. It was gas (most probably steam at this temperature) suddenly increasing in volume a lot. –  rumtscho Jun 14 '12 at 13:11
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A vegetable oil is not a single fat, it is a mixture of many different fats with different boiling or pyrolysis temperatures (fat molecules are so big that they fall apart before they can reach their boiling point, this is called pyrolysis). The temperatures for deep frying are very high, much above 250 F. 200 C are normal, but inexperienced cooks can easily reach 250 C and above if they can't recognize the signs of overheating. I suspect that at this temperature, you had superheated fats in the oil, just the way you get superheated water. If you plunge a stirrer into the pot, you provide the oil with nucleation sites, the same way it does with water. The fats pyrolise to gaseous components at these nucleation sites, and create the bubbles.

Of course, if the tongs were wet, you would have gotten steam bubbles too, just as @michael explained. And even small amounts of water can create a vigorous foaming. But you will also get some bubbles with dry tongs, and I think that my explanation covers this. Even if they were wet, you will get both effects (nucleation sites for the oil and water turning into steam) at once, rather than just the steam. Especially with you insisting that the tongs were dry, I think that this effect created a large proportion of your bubbles, if not all of them.

From the practical side, splatter is expected and unavoidable with deep frying. The bubbles you get with the tongs are nothing compared to what happens when the food (which contains a fair amount of water) hits the pan. Do it somewhere where you can clean well afterwards. Be on your guard for signs of overheating - if you notice a slight vapor above the pot, this means it already has reached its smoke point. It is best to measure with an infrared thermometer (or a candy thermometer, if yours can withstand the high temperatures). Be aware that it is very possible to reach the self-ignition temperature of oil and cause an oil fire! Always leave at least three inches of the pot empty and keep a pot cover at hand - this way you will have the time to throw the cover on the pot at the first sign of a spark. As a nice side effect, the walls of a tall pot also contain lots of the splatter (you'll still have to clean the stove afterwards, but maybe not the kitchen walls).

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Thanks, great science and practical advise. I'd upvote if I had enough rep (new to this community). I used a fry thermometer though and the popping happened at around 250F while I was heating it so I don't think it was from the oil being superheated. There must have been trace amounts of water on the tongs when I heated. I know the bubbles are expected putting stuff in, but it was surprising to get such loud, crisp popping sounds - with a frequency of pops mimicking that of popcorn. –  Matthew Jun 13 '12 at 16:11
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