When using oil for frying, you need to have your oil temperature dialed in pretty well.
Without a candy or fat thermometer, this can be challenging. There is, however, a pretty good way to test the temperature of your oil to within around ten degrees Fahrenheit or five degree Celsius, using nothing more than an ordinary piece of white bread.
Now, you will need to start your oil out near the appropriate burner setting for your target temperature. The most common deep-frying temperatures are from 350°F (178°C) to 375°F (190°C). For that, you generally want to set your burner to medium-high heat or too high.
Recommendation: Polder Stainless Steel Deep Fry/Jelly/Candy Thermometer
For 345° to 355°F, start with a little less than medium and adjust the heat to medium as needed. Obviously, for lower heats, start with low and adjust upwards. For very high frying temperatures just start with a high setting and wait.
The temperature of your oil will not remain constant as you are cooking. When you add cold ingredients, it will drop, perhaps by as much as 20 degrees. Do not add too much at once to the pan, to avoid dropping the temperature too much. Monitor the heat as you cook. Usually, once you begin cooking, you will be able to go by how fast your food is frying, but a good thermometer is always the easiest way.
How to Test Cooking Oil Temperature With White Bread
You will need a timer and a piece of white bread. A slice of sandwich bread will do, especially older bread. Cut out several pieces of bread from the center of the slice, avoiding the crust. Have them ready while you are heating up your oil, along with a kitchen timer or stopwatch, etc.
Chances are you will be cooking at a moderate frying temperature, so start your oil over medium heat and allow it to heat for a few minutes.
Lower a piece of the bread into the oil and time how long it takes to turn a crisp golden-brown. The length of time it takes will correspond to a certain temperature range. Warning: This is not a highly accurate test. Much depends on the bread and its moisture content. Use this test just to give yourself a general cue, and adjust as needed. The longer it takes, the lower the heat.
The following are the approximate times it should take for the bread to turn golden-brown and what temperature this time corresponds to:
If the oil starts out too hot, turn the heat down, wait a few minutes, and test again. If the bread turns brown in 20 seconds or less, indicating the oil is very hot and probably much hotter than you need it (perhaps dangerously so), you might want to turn off the heat completely to let it cool, and then when your oil has begun to cool enough, turn the heat back to a lower setting than before.
Try not to heat your oil to the point that it smokes. You can check here for the smoke point of various cooking oils, and find much more information about cooking oil safety.
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