Heated up oil for frying, stirred it, tiny bubbles appeared and rose slowly but with very loud popping sound - why?

Question

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

Answer 1

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).

Answer 2

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).