Introduction to Food Physics Chapter 4

Introduction to Food Physics - Chapter 4: Thermal Properties

• Chapter Outline: Covers temperature, heat capacity, enthalpy, heat transfer in food, thermal conductivity, thermal diffusivity, caloric value of food, and thermal analysis. Refer to e-learn handouts for further details.

Thermodynamic Laws

• First Law: Energy cannot be created or destroyed, only transformed. Energy transfer occurs through mass transfer, external work, or heat exchange across a boundary. This results in a change in stored energy within a system.

• Second Law: In all energy exchanges, if no energy enters or leaves a system, the potential energy of the state will always be less than that of the initial state. The measure of this disorder is entropy. Energy transfer often results in a loss of usable energy, dissipating as heat. Entropy is the amount of energy unavailable to do work .

Heat

• Heat Transfer Equation: dU = dQ + dW (Eq. 4.11 and 4.12)

  • This equation expresses the change in internal energy (dU) related to heat transfer (dQ) and work (dW).
  • In thermodynamic systems, work is most often associated with displacement.

• Work Displacement: dW = -pdV (Eq. 4.13)

  • This equation relates work done to the change in volume.

• Internal Energy Equation: dU = dQ - pdV (Eq. 4.14)

  • This equation shows the internal energy change due to heat transfer and work.

• Alternative internal energy equation: dQ = dU + P⋅dV (Eq. 4.15)

  • It represents another way to express the heat transfer.

Enthalpy

• Definition: Enthalpy (H) is the sum of internal energy (U) and the product of pressure (p) and volume (V)

• H = U + pV (Eq. 4.16)

• Change in enthalpy: dH = dU + pdV + Vdp (Eq. 4.17)

  • If pressure is constant (dp = 0), then dH = dU + pdV (Eq. 4.18)
  • For constant pressure conditions the change in enthalpy equals to the heat transfer to the system.

• Explanation: A thermodynamic measurement of energy; encompassing internal energy and the product of volume and pressure. This energy value is useful when studying processes under constant pressure.

Heat and Enthalpy

• Heat Definition: Heat transfer occurs due to a temperature difference across a system boundary.
• Energy Conservation: Governed by the First Law of Thermodynamics, where energy is neither created nor destroyed, but transformed.
• Heat and Efficiency: In theory, energy transformations can be 100% efficient. However, in reality, the Second and Third Laws of Thermodynamics limit efficiency to less than 100%.

Heat Transfer in Food

• Method types: Conduction, convection, and radiation.
• Conduction: Direct heat transfer through a material by atomic or molecular interaction. Directly relates to temperature change and the material properties.
  • Governing Equation: q = − k (ΔT/Δx)
• Convection: Heat transfer through fluid (liquid or gas) movement. Important in heat transfer applications, especially in food processes.
  • Governing Equation: q = hA ΔT
• Radiation: Heat transfer through electromagnetic waves. Critical factor in many food processes.
  • Governing Equation: q = εσA(T₄ - T₄)