Goal: When you have finished this laboratory exercise you will understand
- the importance of thermophysical properties (thermal conductivity, specific heat, density and thermal diffusivity) of food products
and you will learn
- to use predictive equations to determine thermophysical properties of a food product of a known composition, and
- to learn about the effects of temperature on the changes in the properties of foods.
The most common processes found in a food processing plant are heating and cooling of food products.
Unit operations such as refrigeration, freezing, thermal sterilization, drying and evaporation, involve heat transfer between a product and some heating or cooling medium.
For design and optimization purposes, the thermal properties (thermal conductivity, specific heat and density) of the foods, heating and cooling media, and materials used in the equipment are extensively used.
The composition of the nectarines used in the experiments is given as follows:
- Protein: 0.6%
- Carbohydrate: 16.7%
- Water: 82.2%, and
- Ash: 0.5%
The initial freezing point of the nectarines based on the given composition is -1.11oC.
Changes in thermal properties will be determined using the predictive equations for thermal properties of protein, carbohydrate, water and ash.
Thermophysical properties -- thermal conductivity, heat capacity, density and thermal diffusivity -- of foods are important during heating, cooling, refrigeration and freezing processes. The composition of the material affects these properties.
Thermal conductivity (k) of a material can be defined as a measure of its ability to conduct heat. In foods, it mostly depends on the composition and any factor that affects heat flow through the material such as percent void space, homogeneity and orientation of fibers.
Heat capacity (Cp) is the amount of heat required to change the temperature of 1 kg of product by 1 K. The heat required to heat a material of mass M from an initial temperature of T1 to a final temperature of T2 is equal to the product of mass, heat capacity and temperature difference (T1 - T2), or MCp (T1 - T2).
Density of biological materials varies with species. It also depends on the temperature and composition.
Thermal conductivity is the relative ability of a material to conduct heat and its ability to store heat. It can be calculated as the thermal conductivity of the product divided by heat capacity and density. Another definition for thermal diffusivity is the rate at which heat is diffused into or out of a material.
Analytical and numerical methods of heat transfer require accounting for changes in the thermal properties of the product which is being heated or cooled.
In this experiment, we will determine the changes in the thermal conductivity, heat capacity, density and thermal conductivity of nectarines due to temperature.
In the data analysis, the effect of temperature on the given thermal and physical properties will be determined.
In this laboratory exercise, you obtained the changes in thermophysical properties of the nectarines in the temperature range of -20 to 20 oC.
To analyze the data:
1. Copy the experimentally obtained thermophysical property values in a new Excel file. The experimental data is shown as a function of temperature.
2. Prepare separate plots for each thermophysical property's change with respect to temperature for above and below the initial freezing point.
3. Discuss the effect of temperature on the thermophysical properties.
- What is the effect of temperature on the change of thermal conductivity, heat capacity and density?
- Why does the specific heat of a food show a dramatic increase near the initial freezing point?
- Review published literature to determine different experimental techniques used for measuring thermophysical properties of foods. Discuss advantages and disadvantages of each selected method.
- Choi, Y. and Okos, M.R. (1986). Effects of temperature and composition on the thermal properties of food. In "Food Engineering and Process Applications," Vol 1. "Transport Phenomena."(M. LeMagure and P. Jelen, eds.), pp.93-101. Elsevier Applied Science Publishers, London, UK.
- Heldman, D.R. and Singh, R.P. (1981). Food Process Engineering. 2nd Edition. AVI Publ. Co., Westport, CT.
- Singh, R.P. and Heldman, D.R. (2009). "Introduction to Food Engineering," 4th ed., Academic Press, London.