How to test Fat for Frying
Heat the fat. When a blue smoke begins to appear, drop in a cube of bread; if the bread browns a golden brown in forty seconds, the fat is hot enough for any food which has previously been cooked. Uncooked food requires a longer, slower frying; the cube of bread should brown in sixty seconds for such food.
Food cooked in fat tested in this way should never soak fat if one is careful to put a small enough quantity of food in at a time—not enough to cool the fat.