Doughnuts are unique among baked goods, as they are deposited into a fluid frying fat in order to “bake” the doughnut. The frying fat is not only the heat transfer medium, but also becomes a major part of the finished product as fat is absorbed during the frying process. On average, the total fat content of a finished cake doughnut is 20-25%, and most of that comes from fat that is absorbed during frying. This makes frying fat a major ingredient in the finished product, and one that contributes to tenderness, shelf life, flavor, color, and texture. Therefore, the quality of the finished product is dependent not only on the formulation and process, but it is also directly tied to the quality of the frying fat and the frying conditions. It is important to understand how our operating procedures impact the quality of our frying fat.
A fat is comprised of one molecule of glycerol combined with three molecules of fatty acids to form a triglyceride. Breakdown of the fat occurs primarily at the point where the fatty acid bonds to the glycerol backbone or at points of unsaturation in the fatty acid chain. There are four factors that will adversely affect fat quality and accelerate degradation:
- Foreign materials
During regular production, frying fat is exposed to all of these factors for an extended period of time. The degree to which the fat is negatively affected depends on its fatty acid makeup, treatments to which the fat has been subjected during processing, and the severity of conditions to which the fat is exposed during frying.
Let's focus on water and heat. Hydrolysis is one of the main reactions of concern during the frying process. This is a reaction of water with fat, resulting in the breaking of a bond between the fatty acids and the glycerol, forming free fatty acids, monoglycerides, diglycerides, and free glycerol. A small amount of free fatty acid in the fat is necessary for proper flavor and appearance, and fresh fat should be “broken in” before use by heating, thereby causing some breakdown of the fat to begin. Fresh frying fat will typically have a free fatty acid content of less than 0.10%, while a good range for frying doughnuts is 0.3-0.75%. Levels in excess of 1.0% will cause defaults in the product.
As the dough is fried, some of the moisture released from the product is retained in the fat and the combination of this moisture and high temperatures can cause fat breakdown to occur. Excessive hydrolysis will cause the doughnuts to have a darker crust, off flavors, and can become rough and non-uniform in shape. Fat absorption will become excessive and can cause problems with adhesion of coatings and a stained appearance in sugar-coated doughnuts. We can see this transition occur by noticing foaming of the fat and excessive smoke on the surface. Another type of hydrolysis is saponification, and occurs in the presence of residual alkaline cleaning materials. When the fat is heated, it produces “soap” with its associated flavor and foaming problems.
We can preserve the quality of our frying fat within acceptable limits through closely controlled operating procedures. The rate of breakdown is approximately doubled for each 18°F (10°C) rise in fat temperature. Needlessly high temperatures of the frying fat not only wastes energy, but also contributes to fat deterioration. The heating element should be turned off as soon as the frying operation is completed, or held at a low temperature during shorter downtimes. Under no conditions should the fat be held higher than 400°F (204°C).
As fat is absorbed by the doughnuts during frying, fresh fat must be added to maintain the proper level in the fryer. If the fat chamber is properly sized, the continual addition of fresh fat will result in the correct levels of free fatty acids. If the fryer is oversized, or only used for a short period of time each day, the fat will be held in the fryer for too long and breakdown will become excessive. In this case, periodic partial replacement of the fat will be necessary in order to maintain a satisfactory product. Ideally, at least 1/3 of the fryer fat capacity should be turned over each day.
There are many other aspects of the process that will contribute to preserving the quality of your frying fat, and therefore to a satisfactory product. Filtering foreign materials and limiting oxidation, as well as selecting an appropriate fat are just a few. The quality of your frying fat is critical to the quality and success of your product, and can be maintained with careful attention to your process and operating conditions.