Modelling of heat and mass transfer during deep frying process

CAMPAÑONE L; GARCIA M.A.; ZARITZKY N

Mathematical Analysis of Food Processing

Año: 2007;

Frying process is considered one of the oldest cooking methods. It is used to cook food in oil and to confer unique textures and tastes; the organoleptic characteristics depend on frying conditions [1, 2]. Frying is usually done by the immersion of the product in oil at a temperature of 150-200 ºC, where a simultaneous heat and mass transfer take place [2, 3]. Heat conduction occurs in the core of the solid food and it is strongly influenced by the physical properties of the food that are continuously changing during the frying process [2]. Simultaneously, heat transfer by convection takes place between the oil and the surface of the food. When the formation of bubbles begins, the heat transfer is accelerated because bubbles contribute to the turbulence in the frying medium; however, the formation of foam in the oil produces a significant decrease in the heat transfer rate [2]. Convection heat transfer decreases as the amount of vapor decreases due to the depletion of water in the core of the food. The rate of heat transfer towards the food core is influenced by the thermal properties and viscosity of the frying medium and the agitation conditions. During the production of bubbles on the surface, forced convection becomes the main  regimen. Water transfer is produced by surface boiling and when evaporation decreases, diffusion of water from the food core towards the surface is the dominant physical phenomenon. Water leaves the product as bubbles of vapor while migrates internally by means of different mechanisms [2-4]. After an initial time, when surface moisture is evaporated, a dehydrated zone begins to form on the surface of the food. Under these conditions, surface temperature becomes closer to that of the frying medium while moisture is strongly reduced reaching values close to the bound water value. Thus, an important gradient of moisture between the core and the surface is established. In starchy products a water competition is found between the moisture necessary to gelatinize the starch and the moisture that is leaving the product. The mechanism of oil penetration is a subject of controversy [5-7]. Absorption of oil on the surface of the fried product occurs when samples are removed from the frying medium; the oil that remains on the piece surface enters the product [3, 4, 7-13]. According to these authors, oil does not invade the product itself and oil uptake during frying is negligible. Conditions at which products are removed from the frying oil seem decisive for the uptake of oil; this would be related to the adhesion of oil to the surface and how it is drained. Many factors affect oil uptake in deep fat frying, such as oil quality, frying temperature, residence time, product shape and size, product composition (initial moisture, and protein content), pore structure (porosity and pore size distribution), and pre-frying treatments (drying, blanching, surface coating). Interfacial tension was also reported by Pinthus and Saguy 5 to have a significant influence on oil uptake after deep-fat frying. Many researchers have suggested that fat absorption is primarily a post-frying phenomena [10, 11, 14-16], thus fat transfer occurs in food product during the cooling stage. Moreira and Barrufet 9 reported that 80% of the total oil content was absorbed in tortilla chips during the cooling period. They explained that as the product temperature decreases, the interfacial tension between gas and oil increases, raising capillary pressure. This sucks the surface oil into the porous medium, thus increasing the final oil content. Ufheil and Escher 8 also suggested that the absorption theory was based on surface phenomena, which involve the equilibrium between adhesion and the drainage of oil during cooling. Fat absorption in chips due to surface adherence was the result of steam condensation, as was also suggested by Rice and Gamble17. Oil uptake depends on structural changes during the process; differences in the starting food microstructure can be expected to be important in the evolution of the characteristics of the end product. Meat products are mainly an arrangement of protein fibers, which may relax and denature upon heating [18] while flour based products, such as doughnuts and tortilla, have a different microstructure. Several models have been developed for heat, moisture and fat transfer during frying of foods [10, 19, 20]. Ateba and Mittal 21 developed a model for heat, moisture and fat transfer in deep-fat frying of beef meatballs. Rice and Gamble 17 used a simplification of the Fick’s equation to predict moisture loss and the oil absorption during potato frying. Farkas, Singh and Rumsey 22, 23 developed a model for frying considering heat and moisture transfer and Singh 24 treated the crust as a moving boundary. Farid and coworkers 25, 26 have made an important contribution to the moving boundary analysis that can be applied to describe large number of processes such as frying, melting, solidification, microwave thawing, spray-drying, and freeze-drying. The large volume of production of fried foods and its influence on the consumption of lipids enhances the study of frying process because of its strong economical and nutritional impacts. From economical point of view, higher oil content in fried food products increases production costs [4]. However, the reduction of lipid content in fried foods is required mainly owing to its relation with obesity and coronary diseases. An alternative to reduce oil uptake in fried foods is the use of edible films or coatings. The application of hydrocolloid coatings allows to reduce oil content of deep-fat fried products due to its lipid barrier properties; the most widely studied are gellan gum and cellulose derivatives [7, 14, 15, 27]. Cellulose derivatives, including methylcellulose (MC) and hydroxypropyl-methylcellulose (HPMC) exhibit thermogelation. When suspensions are heated they form a gel that reverts below the gelation temperature, and the original suspension viscosity is recovered [28]. These cellulose derivatives reduce oil absorption through film formation at temperatures above their gelation point, or they reinforce the natural barrier properties of starch and proteins, especially when they are added in dry form [29]. With regard to the modeling of deep-fat frying of coated foods, Mallikarjunan, et al. 30 and Huse et al. 31 demonstrated the effectiveness of various edible coatings in reducing oil absorption in starchy products. Williams and Mittal 14, 15 working on frying of foods coated with gellan gum, determined the effectiveness of edible films to reduce fat absorption in cereal products, and developed a mathematical model for the process. In the present chapter the following objectives were proposed: 1. To model heat and moisture transfer during deep-fat frying of food by solving numerically the partial differential equations of heat conduction and mass transfer within the food, which allows the prediction of temperature profiles and moisture concentrations, as a function of operating conditions. 2. To validate experimentally the mathematical model with regards the temperature profiles and the water losses from the food product.  3. To relate oil uptake measurements after the frying process with the effects of frying time on sample microstructural changes as well as on the growth of the dehydrated zone check the English 4. To analyze the performance of applying an edible coating based on methylcellulose (MC) on a food model dough system by measuring moisture content and oil uptake in deep-frying process. 5. To analyze the quality attributes such as texture and color and to observe using Scanning Electron Microscopy the surface microstructure of coated and uncoated fried products at different processing times