Engineering Principles

It is possible to calculate the many aspects of heat and mass transfer most important to the frying process based on the expected loss of water and the mass of food to be dehydrated. Unfortunately, these calculations do not take into account the constant changes occurring in a degrading heat transfer medium (the oil) and the accumulation of surfactant species in the oil due to both food and process influences. Numerical modeling has not yet advanced beyond examining fresh oil model systems.

There are five stages in the life of frying oil that produce, in sequence, raw, cooked, and overcooked food. The following is a description of how frying proceeds and why surfactant chemicals control the kinetics and dynamics of the frying process. From the perspective of physical chemists and process specialists, the cooking of food in an oil can be reduced to simple engineering principles with parallel simple measurement and control procedures. This is a new paradigm of frying and is different from that of the paradigm of organic chemists and food scientists, who initially studied the complexities of frying oil and food chemistry to develop databases of results and observations.

The model of understanding frying in terms of physical chemistry and engineering leads to the belief that the foremost way to judge frying and frying oils is by evaluating the physical properties of fried foods. Only the process variables affecting the physical properties of fried foods can be controlled in the engineering sense. Temperature profiles, water loss from food, and oil absorption into food are amenable to process control. On rare occasions, the pressure over the frying oil is also controlled.

Factors such as flavor development and typical finished food color are not primarily controlled by the process. Rather, they are dependent on the source of the oil, the content and type of surfactants, the type and composition of food fried, and a range of organic reactions, only some of which depend directly on process variables.

Two traditional means of controlling the transfer of thermal energy to frying food are heater temperature control (at designed energy flux) and residence time of the food in the heated oil bath. Overheated oil at the surface of the heaters is reduced to carbon deposits (coke) on the heater surfaces. As the carbon layer builds and becomes an insulating jacket, the heater cycle on times and temperatures are increased to keep the oil at frying temperature. A new frying oil somewhat resists this process, and heat transfer rates and ratios are essentially a static system. As the oil degrades, however, and products such as thermally formed and food-formed surfactants increase, the dynamics of the heat transfer system accelerates. Oil makes successively better contact with the food exterior and thus excessively dehydrates that layer. Thermal energy is expended to convert more and more surface water to steam. This ever-deepening dehydration of the crust phenomenon robs energy that otherwise would go to heat and cook the interior of the food.

The time food spends in heated oil can be varied to achieve a particular degree of cooking. Times vary from blanching for 30 s to cooking a chicken for 20 min. Manual placement and removal of basketloads of food control residence time in smaller fryers. Larger, or continuous, fryers often use conveyor systems to control frying food residence time. Unfortunately, heater designs and residence time variations leave out considerations of the chemical changes that take place in the oil with time. Such changes have been described in depth by analytical and organic chemists, but physicochemical changes that affect heat and mass transfer of the oil with use have been largely, although not completely, ignored. Although electromechanical devices for controlling fryer heaters and residence time are well developed (based on the incomplete engineering concept that the thermal properties of oils are constant), further analysis of frying systems has ignored available information related to chemical changes taking place in the frying oil.

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