Info

Source: From Ref. 7. Reprinted with permission.

Source: From Ref. 7. Reprinted with permission.

heat content of the system or the amount of heat transferred is, therefore:

APE is the change in potential energy, x2 — xlt AE is the change in internal energy, E2 — E 1( Apv is the change in pv work, p2v2 — pivly and AKE is the change in kinetic energy, V\/2g - V\!2g.

In considering food processes, there are many simplifications to the rigorous thermodynamic relationships. This is possible since most of the fluids are being heated or cooled and there are negligible changes in specific volume, and kinetic energy and no shaft work is being done by the flowing product. Therefore, since AE + Apv = AH = enthalpy, the work required for compressing a gas or the heat change in cooling or heating is equal to the enthalpy and equation 7 becomes:

Heating or Cooling Foods

The amount of heat required to raise or lower the temperature of a food is dependent on the specific heat. As previously discussed, over the temperature ranges encountered in food processing, the specific heat (cp) can be considered constant and the heat added or lost (called sensible heat) is

When a change in phase takes place, there is an additional heat input required called the "latent" heat. Hence, the latent heat of vaporization or the latent heat of fusion must be considered when a product is being evaporated or frozen. Although certain foods (eg, pure fats) have independent latent heat values for phase change, the latent heat of most foods is associated with water. When foods are dehydrated or frozen, water is normally the major component that determines the latent heat required for the phase change. In fact, water is the sole consideration in dehydration processes since that is the only component that is removed by a change of state.

Figure 1 is a generalized diagram showing the stages of heat change taking place as a product is heated and goes through phases changes as follows:

1. Heating in frozen stage to freezing point, Qi = cpAT (cp is that for the frozen product).

2. Changing from frozen to solid or liquid food, Q2 = Latent heat of fusion.

3. Heating liquid or solid food, Qa = cpAT. (cp is that for the nonfrozen product).

4. Changing to vapor phase, Q = Latent heat or vaporization qt = qi + q2 + qa + q4

Values for thermodynamic properties of foods and other substances are available in many scientific handbooks. These give the thermal properties as related to temperature, pressure, and volume. Most frequently used are Tables giving the basic properties of water (Tables 1, Table 2) and saturated steam (Table 3). The base or reference point for published thermodynamic properties have been established by international agreements as E = 0 and entropy at 0.01°C.

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