Principles of Deep-Fat Frying
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Principles of Deep-Fat Frying book
Deep-fat frying is used widely around the world as a major food processing operation. In recent years, frying has become one of the fastest growing processes used in the fast food industry generating billions of dollars annually worldwide. Despite its popularity, frying is still very much an art and the theoretical aspects of the process are highly complex and continue to present difficulties to scientists. There are a series of different phenomena occurring simultaneously during a deep-fat frying process. In particular, there is simultaneous heat, moisture, and fat transfer taking place between the product and the heating medium (frying oil). The formation of a crust layer on the outer surface of the product is another characteristic event that takes place. To complicate issues even further, the composition of the oil and the properties of the products are steadily changing throughout the process. It is important to have a good understanding of these activities during the frying process in order to optimize and control the process. Optimization allows for simplification of the control documentation aspects of the process. This should translate to several benefits including increased frying oil life, decreased oil absorption by the product, decreased product rejection rates through tightened process control specifications, energy conservation, and reduced operating costs (Blumenthal, 1991). Deep-fat frying operation involves immersing a food product into a hot-oil bath for a given period of time. Oil temperatures are typically in the range of 175 to 205°C (350 to 400°F), resulting in moisture loss due to cooking of the food product and evaporation. Fat uptake by the food product occurs simultaneously with moisture loss. For a breaded product, there is the bread coating at the surface and an inner substrate product core which changes the nature of heat, moisture, and fat transfers. Rapid heat transfer from the frying oil quickly sets the structure of the coating, allowing only limited moisture loss and fat uptake. A crust forms at the surface during frying, especially within the breading region of the food where there is rapid evaporation of water. The hardening of the surface locks moisture inside the product, resulting in a relatively moist interior and dry exterior. Thus, the coating is a barrier to mass transfer during frying. The bread crust is one of the most important characteristics that give fried foods their unique crunchy texture.