Great article about deep-frying in this month's Food Arts by Chris Young (co-author of Modernist Cuisine:The Art and Science of Cooking and head of The Fat Duck Experimental Kitchen for five years). Assuming you already know the basics of frying (the oil should be clean and hot, the food very dry), here are ten things that you may not have thought about:
10. Those bubbles you see in the oil after placing food in the fryer represents the evaporation of moisture -- the heat, in other words, is being used to vaporize water rather than to increase the temperature of the food. In fact, these bubbles actually serve to cool the surface of the food, so while the temperature of the fryer may be 400F, the effective cooking temperature is only 200F. Once the water is evaporated, the heat then goes towards browning the surface of the food.
9. If your frying oil is in peak condition, the food will directly contact the oil for no more than half the frying time -- the rest of the time those streaming bubbles serve to push the oil away from the food. This means that very little oil is absorbed into food while it is in the fryer. Surprisingly, most oil is trapped and absorbed by the surface after the food is removed from the fryer -- which is why it is important to blot excess oil after removal from the fryer.
8. Don't blot all the oil away -- the oil is what provides the flavor, texture, and mouthfeel that we love about fried foods.
7. Still, some oil is absorbed during frying. Temperature of the oil has a lot to do with how much: Deep-frying a crusty food in 340F oil, rather than 360F or hotter, can increase the oil absorption by 40 percent.
6. Deep-frying large pieces of food doesn't work very well, which is why we cut foods into smaller pieces before putting them in the fryer. You knew that already I'm sure, but did you realize that cutting the size of the food in half can cut the cooking time by a factor of four?
5. Saturated oils are more stable than unsaturated, and therefore no less healthful when frying. The key to keeping either oil stable is to frequently strain out any food particles.
4. When frying delicate foods such as fish, you should use a batter -- the water in the batter will steadily boil and will keep the temperature beneath the surface cooler -- so while the crust is browning, the fish inside won't overcook. Alcohol boils at a lower temperature than water, so using a neutral spirit such as vodka in your batter will keep the fish even cooler while the batter itself becomes dry and crisp sooner.
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3. Leaveners such as baking powder provide even more insulation and even crisper crusts, as the acids and alkaline ingredients react and fill the batter with bubbles of carbon dioxide.
2. Old frying oil doesn't just ruin the flavor of the foods being fried, and doesn't just make them greasier, but also increases the risk of a dangerous grease fire -- the breakdown of oil decreases the smoking and flash points.
1. On the other hand, some of the chemical changes that occur in used oil actually improve it ("used", not "old"). For instance, oil that is "broken in" has split fat molecules into compounds such as emulsifiers and gums. These allow the oil and water to mix so that the food being fried spends more time in contact with the oil, which in turn delivers the heat more rapidly and evenly -- thus a faster cooking time and a more golden-brown color.