Food borne diseases cause 76 million illnesses in the United States each year [1]. Since many of these diseases result from insufficient destruction of pathogens, safety of cooked meat is one of the primary concerns among food scientists and technologists. Safety of a food material includes protection from microbial, chemical and physical hazards or contamination that may occur at any stage of food production and handling. As far as meat cooking is concerned, time-temperature history determines microbial destruction and thus, is the primary indicator of safety. The cooking recommendations for safe for consumption meat are prescribed in terms of temperature and holding times of the slowest heating points [2, 3] to guard against lethal microbial activity. In addition to safety, the eating quality of the meat product, characterized by nutrient composition, flavor, color and texture/tenderness, is of importance to the consumer. It is reported that as meat is cooked, its toughness lasts till 50 °C, after which the meat product starts becoming tender and thus tasteful [4]. Elasticity of meat is adversely affected above 65 °C leading to a tougher, more brittle meat as one goes to higher temperatures. Thus, quality and safety put opposing demands on the temperature range and the most tasty and microbially safe meat requires cooking within a narrow temperature range. Apart from temperature, water and fat content are other two important components determining meat quality. Moisture content and distribution inside the meat protein fibers and extracellular spaces determine its juiciness, while flavor is mainly a property of fats and lipids. With more than 32% of its population as obese (Body Mass Index, BMI > 30) and another 34% overweight (25 < BMI < 30) [5], US needs improvements in the quality of its food products esp. in controlling fat content. Computer modeling of a meat cooking process help a long way in establishing a better understanding of the transport processes that take place inside which is vital for quality prediction and improvement. However, underlying physics is complex due to highly heterogeneous nature of the material.
In the current study, a macroscopic multiphase model based on unsaturated fluid flow in a hygroscopic porous media is developed, which accounts for all the important physical phenomena that take place while cooking of meat. Frozen meat is considered as a porous solid comprising of water, fat and protein with gas trapped in its pores. As it is heated, water and fat melt and are gradually released from the solid to the pore space according to their holding capacities in the solid matrix, which are assumed to be functions of temperature. As temperature further increases, free transportable water evaporates. Since four fluid components (liquid water, liquid fat, water vapor and air) are present in the pores, a mass balance equation is written for each component. Thermal equilibrium assumption leads to a single energy balance equation for the whole system. Melting of solid water and fat is handled by effective specific heat method. The other two phase changes are accounted for by adding source terms to the respective mass balance equations. The model is numerically solved in COMSOL Multiphysics and is validated for double sided contact heating of hamburger patties by comparing temperature and moisture profiles with experimental studies and numerical results available in literature. Dominant transport modes for water as well as fat under different conditions are identified by performing a sensitivity analysis on various parameters such as surface temperature, patty diameter, meat composition etc.
References
1. [cited May 8, 2007] http://www.cdc.gov/foodsafety/
2. Food Code. U.S. Public Health Service, Food and Drug Administration -- 1999 Washington, D.C.
3. USDA urges consumers to use food thermometer when cooking ground beef patties. FSIS News Release. Food Safety and Inspection Service. U.S. Department of Agriculture -- 1998 Washington, D.C.
4. Tornberg, E., Effects of heat on meat proteins - Implications on structure and quality of meat products. Meat Science, 2005. 70(3): p. 493-508.
5. [cited May 8 2007] http://win.niddk.nih.gov/statistics/index.htm.