Thermal properties of food materials
Thermal properties are the properties that control the transfer of heat in a specified food. They must be known before any heat transfer calculation can be trusted — whether that calculation sits behind the design of storage and refrigeration equipment, or behind the estimation of process times for refrigerating, freezing, heating or drying foods and beverages. Thermophysical properties of foods encompass a wide range of parameters associated with heat transfer operations in food processing.
Four properties do most of the work. Specific heat capacity (Cp) is the amount of heat needed to raise the temperature of one gram of a substance by one degree Celsius, expressed in J/kg K in the SI system; Heldman (1975) proposed an equation to estimate the specific heat of foodstuffs from their mass fractions of water, protein, fat, carbohydrate and ash. Thermal conductivity (k) is the ability of a material to conduct heat, measured in Watts per meter-kelvin, and depends mainly on chemical composition, molecular structure and the temperature at which the product is processed — it determines how fast heat can be evenly transferred to the entire food mass, which in turn affects the quality of the final product. Thermal diffusivity (α) is the rate at which heat is diffused out of the material, the ratio between thermal conductivity and volumetric heat capacity, in m²/s.
The fourth pair of properties concerns phase change. Boiling point elevation is the raising of a solvent's boiling point due to the addition of a solute, and freezing point depression is the lowering of a solvent's freezing point due to the addition of a solute; as the boiling point of a solvent increases, its freezing point decreases. The increase in the boiling point, ΔTb = Tb – Tb0, is known as the elevation of boiling point, and it is directly proportional to the lowering of vapour pressure (Δp). Adding salt to ice illustrates the effect: the solute reduces the freezing point of the ice, allowing it to melt at a lower temperature.
One caution governs how these values are applied. In general, the thermophysical properties of a food or beverage are constant when its temperature is above its initial freezing point. Below the initial freezing point they vary greatly because of the complex processes involved during freezing, and because the thermophysical properties of ice and water are quite different, the thermophysical properties of frozen foods vary dramatically with temperature.