The oil or fat you use for deep-frying should have a high smoke point — the temperature to which it can be heated without smoking. Butter and margarine have low smoke points, so they aren’t good for frying but work for light sautéing. The best oils for deep-frying and high temperatures are refined safflower and sunflower oils, peanut, safflower and soy oils. Refined almond, avocado and cottonseed oil are also great if you can find and afford them, and canola oil is usually not a problem either.
Remove food particles from used deep-frying oil by straining it through a coffee filter, or a sieve or funnel lined with a double layer of cheesecloth. Cover, tightly seal and refrigerate strained oil; it can then be used one more time.
The temperature of the fat is all-important if the fat isn’t hot enough, food will absorb fat and be greasy, oils that can't take the heat will get too hot, and burn. The normal temperature range for frying is 325°F to 375°F, however, it'd quite likely that higher temperatures of 375°F to 400°F also are used. Most foods cook rapidly in the 325°F to 375°F range and develop a golden color, crisp texture and good flavor. High-temperature frying leads to thinner crusts and less oil absorption. Foods fried in this normal temperature range absorb 8 to 25 percent oil. Frying time is longer at lower temperatures. Frying at lower temperatures results in lighter color, less flavor development and increased oil absorption.
The most accurate method of testing the temperature of oil for deep-frying is a deep-fat thermometer. Make sure the bulb of your thermometer is completely immersed in the oil, but not touching the bottom of the pan. Otherwise, the reading could be affected. If you don’t have one, use the age-old method of dropping a square of bread into the hot oil; if it rises to the surface crackling and frying, the oil's hot enough. If it browns uniformly in:
60 seconds, the temperature is 350 to 365°F
40 seconds, the temperature is about 365 to 382°F
20 seconds, the temperature is about 382 to 390°F
Flash & Fire Points
Other heat points for fats include "flash" and "fire" points at 600 and 700 degrees, respectively. Do not put out an oil-fire with water, the water will splatter the burning oil and spread it more quickly. Smother the fire with a tight-fitting lid. If the fire has spread outside the pan, suffocate it with baking soda or a fire extinguisher formulated for oil fires.
Cooking oil that has reached boiling point (bubbling) is very dangerous. If the oil starts to boil, remove it from the heat source immediately. Simply turning off the heat source may not be enough to reduce the heat immediately for electric appliances, or cook tops because they retain heat even after they are turned off. An oil reaches its flash point at about 600°F. when tiny wisps of fire begin to leap from its surface. If the oil is heated to its fire point 700° F. for most oils, its surface will start vaporizing and spontaneously ignite, surging up and out almost instantly.
Healthy Oil Tip
While all oils are pure fat, not all fats are created equal. Saturated and trans-fatty acids seem to be the culprits if you have health concerns. Margarine or Trans Fat Partially hydrogenated vegetable oils contain Trans fatty acids. Read your food labels, if it says, “partially hydrogenated”, try to avoid them. They are present in many commercially made products and have unsaturated fats which can be damaged at high temperatures and converted to a trans fat. Look for fats labeled super-unsaturated (like flax seed) or monounsaturated (like olive and canola oil). Polyunsaturated oils are also healthy choices.