Heated Fats - Frying

B. Heat and mass transfer during deep-fat frying 214... 

FIGURE 5.1 Schematic diagram of simultaneous heat transfer (left-hand side of the figure) and mass transfer (right-hand side of the figure) during deep-fat frying (with the courtesy of M. C. Moreno). 

From an engineering perspective, deep-fat frying can be defined as a unit operation where heat and mass transport phenomena occur simultaneously. Convective heat is transferred from the frying media to the surface of the product, which is thereafter conducted within the food. Mass transfer is characterized by the loss of water from the food as water vapor and the movement of oil into the food (Singh, 1995). 

Baumann, B. and Escher, E. (1995). Mass and heat transfer during deep fat frying of potato slices I. Rate of drying and oil uptake. Lebensmittel-Wissenschaft Technologie. 28, 395-403. Besaratinia, A. and Pfeifer, G. (2004). Genotoxicity of acrylamide and glycidamide. J. Natl. Cancer Inst. 96,1023-1029. 

Moreira, R. G., Palau, J., and Sun, X. (1995b). Simultaneous heat and mass transfer during the deep fat frying of tortilla chips. J. Food Process Eng. 18,307-320. 

Singh, R. P. (1995). Heat and mass transfer in foods during deep-fat frying. Food Technol. 49, 134 137. 

Deep-fat frying is one of the oldest and most common unit operations used for cooking foods by immersing them in an edible oil or fat heated above the boiling point of water (Farkas et al.. 

The isolation and structure elucidation of dimers formed as the result of thermal oxidation of fats during deep frying has been of great interest. However, the compositional determination of this fraction from used or heated fats and oils has been neglected because of the difficulties encountered during the isolation of dimers from heated fats and the complexity of the structures present within this fraction. 

Oils containing highly unsaturated acids like linolenic acid undergo oxidation in air. This reaction, called oxidative rancidity, is accelerated by heat, explaining why saturated fats are preferred for deep fat frying. 

Oil acts as a heat transfer medium during the frying process which rapidly heats, cooks, and sometimes puffs, dehydrates, or forms a crust around the product. The mechanics of deep fat frying are shown in Fig. 34.32.173 The entering material carries oxygen that temporarily aerates the hot oil, and water that forms steam, with both rapidly swept out... 

Vitrac, O., Trystram, G., and Raoult-Wack, A. (2000). Deep-fat frying of food Heat and mass transfer, transformations and reactions inside the frying material. European Journal of Lipid Science and Technology, 102(8-9), 529-538. 

Ghavami, M. I.D. Morton. Effect of heating at deep-fat frying temperature on the sterol content of soya bean oil./. Sci. Food Agric. 1984,35, 569-572. 

There are two main methods of commercial frying (1) the deep-fat frying and (2) the swallow frying. In deep-fat frying or immersion frying, the food is covered by the oil and all surfaces receive a similar heat treatment to produce a uniform color and appearance (Figure 59.1). Heat is transferred... 

---