The mechanisms controlling heat and mass transfer on frying of beefburgers

Sammanfattning: The aim of the present work was to gain better understanding of the mechanisms controlling heat and mass transfer during thermal processing of beefburgers. The effects of different factors such as the chemical composition and comminution of the meat raw material, the pan temperature and the meat patty diameter on the characteristics of heat and mass transfer were studied. The evolution of the characteristics of heat and mass transfer was investigated as a function of frying time. Ultimately, a technique was developed to measure the permeability of the porous meat patty. This property and the related mass transfer were followed at different cooking temperatures up to 80°C. A higher initial water content in the beefburgers resulted in a faster thawing time. The greater patty diameter shrinkage caused more fat and total losses, which in turn was related to the amount of connective tissue. The fat influences mainly the heat transfer in the central core of the thawed beefburgers, whereas in the neighbourhood of crust formation at the surface of the burger, water evaporation losses mainly prolong the frying time. It was shown that the degree of comminution had no significant effect on the heat and mass transfer. Patty diameter had a significant affect on the thawing and total frying time, with faster temperature increase in burgers having a smaller diameter. The most determinant factor for the water flux is the temperature gradient, and for the fat flux, the fat content. The pressure-driven water loss is the main mechanism governing the water loss in the frying of beefburgers. The higher the heat penetration by using beefburgers of the smaller diameter and the higher the cooking temperature that induces higher water losses, the faster is the crust formation, which in turn results in less shrinkage and higher porosity of the heat processed meat. The water loss based on the initial water content increased linearly (0.4 %/°C) with the increase of the average temperature of the beefburger during the heating process. The fat content governs both the amount and the initial time of the fat release during frying. The diameter shrinkage increased linearly from 0 to 10-12 % for all types of meats, except for the fattiest burger, during the frying. High porosity was achieved in the heat treated burgers and was especially evident after crust formation and with high water and fat losses. The main mechanism for mass transfer during frying was suggested to be a flow of fat and water created due to the denaturation and contraction of the meat proteins, and moving in the radial direction. The water and fat drip that was released from the meat migrated out through the circumference of the burger. The intrinsic permeability of water in the heat-treated beefburgers made of different meat raw materials was measured between the temperature of 50°C and 80°C. The highest permeability value was found in beefburgers prepared from the lean meat cooked at 60°C. The meat network porosity calculated by image analysis from the micrographs visualising the structural changes in the meat patties caused by the heat treatment was well-correlated to the permeability. As a complementary method, permeability was also determined by centrifugation.