Frying is a great way to cook food quickly, but it also leads to some unexpected side effects. One of these is the foam that rises from the oil when you first drop in your food. You might think this is an indication that your oil is hot enough, but in fact it happens because of a chemical reaction between the food and the oil.
The reason for this reaction can be explained if you know how fats and oils are made. Fats and oils are made up of triglycerides — three fatty acids attached to glycerol (a type of alcohol). When you heat fats and oils, they break down into free fatty acids and glycerol — this process is known as saponification. The free fatty acids are what creates foam when they come into contact with water in the food being fried.
Frying oil is usually heated above its smoke point, which is when it starts to break down into free fatty acids and glycerol. When the temperature drops below this point, it begins to recombine into triglycerides again. This process creates foam as small bubbles form on the surface of the oil and rise up through it because they are lighter than other parts of the liquid mixture.
Last modified: October 26, 2022