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.

3. D. Kimball, "Grapefruits, Lemons and Limes," in L. P. Somogyi, D. M. Barrett, and Y. H. Hui, eds., Processing Fruits: Science and Technology. Major Processed Products, Technomic, Lancaster, Pa., 1996, pp. 305-336.

4. J. S. Wu and M. J. Scheu, "Tropical Fruits," in L. P. Somogyi, D. M. Barrett, and Y. H. Hui, eds., Processing Fruits: Science and Technology. Major Processed Products, Technomic, Lancaster, Pa., 1996, pp. 387-418.

5. C. E. Mumaw, "Pineapples," in L. P. Somogyi, D. M. Barrett, and Y. H. Hui, eds., Processing Fruits: Science and Technology, Major Processed Products, Technomic, Lancaster, Pa., 1996, pp. 337-360.

6. S. Lakshminarayana, "Sapodilla and Prickly Pear," in S. Nagy and P. E. Shaw, Tropical and Subtropical Fruits, AVI, Westport, Conn., 1980, pp. 415-441.

F. J. Francis Editor-in-Chief University of Massachusetts Amherst, Massachusetts

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