Optimal Frying Conditions

The fats in a fryer go through stages that influence the quality of the fried product. At the “new” and “break-in” stages, foods absorb too little oil; just the right amount is absorbed at the “fresh” and “optimum” levels that follow; and then too much soaks in at the “degraded” and “runaway” phases. Many professional chefs claim that foods fry best in oil that has been used at least once. Oils pick up proteins and carbohydrates from the foods that are fried, so slightly used oil tends to produce more desirably browned crusts. Eventually, however, the browning becomes too dark and the fat must be replaced. Also, as the fat deteriorates, the surface tension of the frying oil decreases, making foods more likely to soak up the fat. Repeated use of a frying fat will also lower its smoke point, because each heating hydrolyzes some of the triglycerides into smaller molecules.

Foods should be as free of surface moisture as possible before being submerged in the heated fat. Water causes spattering of hot oil, which can cause burns; it requires more energy to maintain temperatures; it may result in longer frying times; and it causes the fat to break down chemically, reducing its frying life.

Inevitably, particles of food or breading break off or fall through the basket and build up in a deep-fat fryer over time. These food particles should be filtered out daily (or every eight hours of use) or they will darken the oil’s color, lower its smoke point, and reduce its keeping time. On the other hand, excessive filtering introduces oxygen into the oil, resulting in rancidity, gum development, and foaming, the latter observed as a persistent layer of bubbles on the surface.

A frying fat should theoretically stay fresh for several months if it is cooled immediately after use and stored in an air-tight container in a dark, cool place. Refrigeration will also increase its shelf life. Large commercial fryers contain too much fat to be cooled completely and then efficiently reheated, so they are turned down to approximately 225^oF (107^oC). Decreasing the temperature during down time prevents the fat’s breakdown and extends its usefulness.

There is no easy method for determining when oil that has been used repeatedly should be discarded. The first indication that an oil needs to be replaced is that its color and the color of the fried food starts to darken. This transformation takes place soon before the flavor and odor of the oil start to deteriorate. An experienced person can tell by looking at it if the oil needs changing, but food service establishments may purchase a commercial kit that allows anyone to determine an oil’s freshness by checking its color against a standard provided in the kit. A further indication that the oil is too old is that the food fried in it is greasier than normal because of increased oil absorption. Other factors to consider in the decision to discard oil include the type of oil used, the type of foods being fried, the number of times the oil is used, the presence of many particles, excessive foaming or smoking, and the quality of the foods cooked in the oil.