Mathematical Modelling of Heat And Mass Transfer During Frying of Coated and Uncoated Products

BERTOLINI SUAREZ R.; CAMPAÑONE L.; GARCIA M.A.; ZARITZKY N.

Buenos Aies, Argentina

Congreso; XXII Interamerican Congress of Chemical Engineering; 2006

The great volume of production of fried food and its influence on lipids consumption  enhances the study of the frying process because of its economic and nutritional impact. Lipid content is related to obesity and coronary diseases; an alternative to reduce oil uptake in fried foods is the use of hydrocolloid edible coatings with oil barrier properties. The objectives of the present work were: -To apply an edible coating based on methylcellulose (MC) on a food model system to analyze the reduction of  oil uptake during deep-fat frying process. -To model the heat and mass transfer during the deep-fat frying process of food systems (with and without MC coating), numerically solving the differential equations that allow to predict temperature and water concentration, as a function of the operating conditions -To validate experimentally the model evaluating the temperature profiles and the loss of water. An edible MC coating, showing thermal gelation properties, was applied to reduce oil uptake during  frying of a dough system formulated with 64.5% wheat flour. Coating formulation included 1% MC and 0.75% sorbitol as plasticizer. Dough samples were dipped in the coating suspensions for 10 seconds and immediately fried at 160oC in sunflower oil. Temperature profiles, loss of water and oil uptake were measured as a function of frying time. Lipid content of fried products was determined on dried samples using a combined technique of successive batch and semicontinuous Soxhlet extractions. Quality attributes of fried dough such as color and texture were measured along the experiments. Thermal histories of coated and uncoated samples were obtained using thermocuples located at the border and center of the samples. A mathematical model of the deep-fat frying process based on the simultaneous numerical solution of the heat and mass transfer differential equations under unsteady state conditions was solved. During frying water diffuses from the food core towards the surface and leaves the product as bubbles of vapor. Heat transfer is produced by conduction in the core of the solid food, and convection in the oil. The formation of bubbles accelerates the heat transfer because they contribute to the turbulence of the frying medium. 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. After an initial time, when surface moisture is evaporated, the crust begins to form at the surface of the food. Under these conditions, crust temperature is closer to that of the frying medium while moisture is strongly reduced to the value of bound water. Thus, an important gradient of moisture between the core and the crust is established and a water competence is produced between that necessary to gelatinize the starch and the water that is leaving the product. Absorption of oil on the surface of the fried product occurs when samples are removed from the frying medium; oil that remains on the piece surface enters into the product. Scanning electron microscopy (SEM) was used to observe the structure of the fried dough. The mathematical model considered the evaporation of water at the interface and a formation of a crust that grows towards the core, with different thermal properties                (moving boundary problem). Heat transfer and water diffusion equations were simultaneously solved using finite differences method.  The model was validated with experimental data. The oil uptake reduction was 30% for coated dough discs compared to the uncoated ones; MC coating did not modify water content of the samples. The final lipid content values were 0.0894 g oil /g dry solid for control samples and 0.0626 g oil /g dry solid for the coated ones. The equilibrium water content values, that correspond to bounded water, were 0.157 g water / g dry solid and 0.1695 g water / g dry solid for control and coated samples, respectively. With reference to quality attributes similar texture and color parameters  were obtained for coated and uncoated fried samples at processing times lower than 720sec. The application of MC coatings allowed to reduce oil uptake without affecting sensorial properties and quality attributes of the products. The mathematical model satisfactory predicted heat and mass transfer processes during frying of coated and uncoated products.