Frying Oil Temperature
You don’t want to rely on guesswork for determining when frying oil is hot. Here's the test kitchen’s method.
When deep-frying, heating the oil to just the right temperature is essential. Properly heated oil ensures that food will become crisp and browned. Oil that's too cool (the most common mistake made by home cooks) will yield pale, greasy food. A good candy thermometer or instant-read thermometer that registers high temperatures is a great tool, but with a little observation you can tell if the oil is ready for frying by making a test run of sorts.
Heating oil to the proper frying temperature (usually between 325 and 375 degrees—recipes will vary) takes a few minutes, depending on the amount of oil in the pot. When you think the oil might be ready, drop a small piece of bread (with the crust removed) or a small spoonful of batter into the hot oil.
If the bread or batter sizzles and fine bubbles appear, the oil is just right and you can add your first batch of food. If there's no action in the pan and your bread or batter just sits there, the oil isn't hot enough. Remove the bread or batter using a slotted spoon, wait a minute or two, and try again with more bread or more batter.
If the oil bubbles furiously or the bread or batter colors almost instantly, the oil is much too hot. Take the pan off the heat and wait a few minutes before adding the first batch of food.