For fried food that’s light, crisp, and not greasy, the proper oil temperature is critical.
Most deep frying starts with oil between 325 and 375 degrees, but the temperature drops when food is added. Once the oil recovers some heat, it should remain somewhere between 250 and 325 degrees (depending on your recipe) for the duration of cooking. To maintain the proper oil temperature, use a clip-on deep-fry thermometer and keep close watch.
If the oil starts lightly smoking, that’s a sign that it’s overheated and starting to break down; remove the pot from the heat until the oil cools to the correct temperature. If the oil has given off a significant amount of smoke, it will impart an off-flavor to foods and should be discarded. (Make sure to thoroughly pat food dry before frying because water can cause oil to decompose, lowering its smoke point by as much as 30 degrees.)
On the other hand, food fried in oil that’s too cool will retain too much moisture and emerge soggy. If the temperature drops too low, bring the oil back up to your target range before frying the next batch.