Thermophysical Properties of Processed Meat and Poultry

Data on thermophysical properties are essential for modeling and evaluation of food processing operations involving heat transfer when safety, quality and energy cost are considered. Thermophysical properties of various meat and poultry emulsions were evaluated at 4 temperatures (20, 40, 60 and 80C). Thermal conductivities (0.26 W/mK to 0.48 W/mK) increased linearly with the temperature from 20 until 60C. From 60-80C, it remained stable for most product except the bologna. Curves of thermal conductivities as a function temperatures could roughly be grouped in two categories; products containing meat particles and emulsions. Densities decreased slightly as a function of temperature from 20 to 40C. A transition phase was observed from 40 to 60C. It was followed by a decrease from 60 to 80C. There was a decrease of about 50 kg/m3 between the density of a raw product at room temperature (at maximum 1070 kg/m) and the product heated to 80°C (at minimum 970 kg/m) due to the gelation or setting of the structure. After a transition period from 10-30C, the heat capacity increased linearly from 30 until 80C. Values ranged from 2850-3380 J/kgC. Densities and heat capacities were strongly influenced by the carbohydrate content (i.e. as the carbohydrate content increased the density decreased). The fat proportion negatively affected the thermal conductivity and diffusivity. As the water content increased the thermal conductivity and diffusivity increased. The density, heat capacity, thermal conductivity, and diffusivity were not affected by the salt proportion due to the limited amount incorporated in products. INTRODUCTION: Thermo-physical properties of food are needed to describe various thermal processes as well as to optimize the design and the operation of heating, cooking, freezing and cooling systems (Karunakar et al., 1998). Thermal properties are also essential for the modeling and evaluation of food processing operations involving heat transfer, especially when energy costs, food quality and safety are considered. Many examples of safety considerations are given in available publications (Unklesbay et al., 1999). For example, the temperature at the centre of a typical sausage must be above a certain level (72°C) by the end of heating and below certain temperature (15C) at the end of cooling in order to achieve microbiological stability (Akterian, 1997). There are many methods to measure thermophysical properties (Baik et al., 2001). Thermal conductivity is highly temperature dependent especially in temperature region where a phase change occurs. According to Karunakar et al. (1998), in the low temperature range (0 to 40°C), the thermal conductivity did not show very significant difference for different temperatures. At high temperatures (>50°C), it increases gradually as the temperature increases (Pan and Singh, 2001). Both thermal conductivity and specific heat are known to increase with moisture content increase (Shmalko et al., 1996). Water content will specific heat more than other components, the lower specific heat values generally occurred with the lower moisture content values (Unklesbay et al., 1999). Thermal conductivity and density of foods vary with temperature during thermal processing due to the changes in texture and/or composition (Karunakar et al., 1998). A decrease in density values will become important for its effect on other thermal properties (Mohsenin, 1980). Most changes in meat products occur during heating, shrinkage, tissue hardening, moisture loss, fat loss and discoloration, and are caused by the changes in muscle protein denaturation (Pan and Singh, 2001). All these changes in the meat will affect the thermophysical properties. The objective of the study was to measure thermophysical properties of various meat and poultry emulsions as a function of temperature. MATERIALS AND METHODS Products Five types of meat products were used: fine emulsion of bologna and wieners, coarse emulsion of pepperoni, turkey emulsion and flaky ham. Turkey emulsions and the flaky ham contained muscle particles. Raw products were taken at a typical industrial plant and measurements were made the day after. Products were kept at 4°C in a cold room until they were analyzed. All experiments were performed three times as well as with three different batches. Thermo-physical properties of meat and poultry emulsions were gathered at different temperatures from raw product to cooked product temperature. The composition of meat and poultry emulsions is listed in Table 1. Table 1. Composition of various meat and poultry emulsions Moisture (%) Fat (%) Salt (%) Protein (%) Carbohydrate (%) Bologna 61.59 20.31 2.39 11.49 2.39 Pepperoni 57.26 21.72 2.53 12.32 5.22 Wieners 60.51 20.90 2.43 12.40 2.00 Turkey 74.88 1.67 1.54 15.46 1.30 Ham 72.70 6.35 2.78 11.62 2.25 Thermal conductivity Thermal conductivity measurements of various meat samples were performed using the probe method based on the line-heat source method developed by Sweat (1974). In the probe, there was a heater wire insulated over its length and a thermocouple in the center of this length. The probe was 38 mm long with an outside diameter of 0.66 mm, it consisted of a constantan heater wire and a chromel-constantan thermocouple (type E) (Sweat, 1995). The probe was connected to a power supply (Hewlett-Packard, 6236B) and to a multimeter (TES 2610 multimeter) in order to read the current more precisely; the multimeter was set to the scale mA DC. The thermocouple wires were connected to a data acquisition system (Data Shuttle by Strawberry Tree) that was connected to a computer (Baik et al., 1999). The software “Workbench for Windows version 3” was used to convert the analog signal of the thermocouple in a digital signal, to set the acquisition rate at 1 reading every 2 seconds. The probe had a theoretical internal resistance of 226.67 Ω/m. It was calibrated with glycerol. Values were within 10% of the literature value of 0.284 W/m.K at 20°C. Four constant temperature water baths were used for the analyses with an increment of 20C: room temperature bath at 20C, a 40°C water bath, a 60°C water bath and an 80°C oil bath to prevent evaporation. A copper cylinder (12.7 cm height and 2.54 cm inside diameter with a maximum wall thickness of 0.159 cm) with a high thermal conductivity was used. Samples were inserted using a syringe for the emulsions and by hand for the turkey and the flaky ham because of the muscle parts. A rubber cover was placed at both ends of the cylinder to insulate them in order to keep the heat flow coming only from the side of the cylinder or radial direction. An infinite cylinder was assumed for thermal conductivity calculations. Three cylinders were placed in each of the four temperature controlled water baths. As the core temperature of samples reached the equilibrium with the water bath, the top rubber cover was replaced by a thinner one and the probe was inserted in a small hole