RHEOLOGICAL PROPERTIES OF
FRUITS AND FRUIT PRODUCTS
Most processed and many freshly consumed fruits receive some type of heating or cooling during handling or manufacturing. Design and operation of processes involving heat transfer needs special attention due to heat sensitivity of fruits. Both theoretical and empirical relationships used when designing, or operating, heat processes need knowledge of the thermal properties of the foods under consideration. Food thermal properties can be defined as those properties controlling the transfer of heat in a specified food. These properties are usually classified (Perry and Green, 1973) into thermodynamical properties, viz, specific volume, specific heat, and enthalpy; and heat transport properties, namely, thermal conductivity and thermal diffusivity. When considering the heating or cooling of foods, some other physical properties must be considered because of their intrinsic relationship with the ‘‘pure’’ thermal properties mentioned, such as density and viscosity. Therefore, a group of thermal and related properties, known as thermophysical properties, provide a powerful tool for design and prediction of heat transfer operation during handling, processing, canning, and distribution of foods (Fig. 4.1). Abundant information on thermophysical properties of food (Polley et al., 1980; Wallapapan et al., 1983; Choi and Okos, 1986; Rahman, 1995) is available to the design engineer. However, finding relevant data is usually the controlling step in the design of a given food operation, and the best solution may be the experimental determination. This chapter provides data and information for thermal process calculation for fruits and fruit products, including a brief description of more commonly used methods for measurement and determination of thermophysical properties.
4.2. THERMOPHYSICAL PROPERTIES’ IDENTIFICATION
Thermophysical properties include different types of parameters associated to the heat transfer operations present during fruit processing. It is well known that heat can be transferred by three ways: radiation, conduction, and convection. Radiation is the transfer of heat by electromagnetic waves. The range of wavelength 0.8–400 mm is known as thermal radiation, since this infrared radiation is most readily 73
k (W/m−1/ K−1)
a (m2/ s−1)
r (kg / m−3)
cp (kJ/kg−1/ C−1)
m (Pa s)
Heat transport properties
Figure 4.1. Thermophysical properties associated to fruit processing.
absorbed and converted to heat energy. A body emitting or absorbing the maximum possible amount of radiant energy is known as a ‘‘black body.’’ Energy emitted by a black body is given by the Stefan–Boltzmann law:
Q ¼ sAT 4
where s is the Stefan–Boltzmann constant; A the area of transfer, and T the absolute temperature. For no ‘‘perfect’’ black bodies, as real bodies are, Eq. (4.1) is corrected by as factor known a emissivity («):
Q ¼ s«AT 4
Emissivity values of foods are in the range 0.5–0.97 (Karel et al., 1975). Conduction is the movement of heat by direct transfer of molecular energy within solids (for example, heating of a fruit pulp by direct fire through metal containers). Convection is the transfer of heat by groups of molecules that move as a result of a ´
gradient of density or agitation (for example, the stirring of tomato puree). Heat transfer may take place: (i) in steady-state way by keeping constant the temperature difference between two materials or (ii) under unsteady-state way when the temperature is constantly changing. Calculation of heat transfer under these conditions is extremely...