Thermal Food Processing

434 Thermal Food Processing: New Technologies and Quality Issues
14.2.2 MAIN PARAMETER: ELECTRICAL CONDUCTIVITY
Probably the most important parameter in OH modeling is σ. 20 Some of the
characteristics of this property are summarized below:
• σ can be anisotropic (varies in different directions).
• Changes in the value of σ reflect changes in the matrix structure, e.g.,
during starch gelatinization or cell lysis.
• The value of σ of a material that is not suitable for ohmic processing,
can be suitably modified, e.g., by blanching.
However, perhaps the most striking feature of σ is its dependence on temperature,
as it has been shown to increase with increasing T. This observation is
due to a variable opposition (drag force) to the movement of the ions responsible
for conducting the electricity in food materials: for higher temperatures, that
opposition is less important than for lower temperatures. 21
For most solid foods, σ increases sharply with temperature at around 60°C,
and this has been attributed to the breakdown of cell wall materials, 22 releasing
ionic compounds to the bulk medium. Figure 14.3a represents this dependence
for several electric field (F) strength values, demonstrating that the relationship
between σ and T becomes linear for increasing values of F (the lower limit of
F — i.e., F = 0 V· m –1 — represents conventional heating). This effect has been
demonstrated by, among others, Palaniappan and Sastry, 5 who suggested that it
Electrical conductivity (σ)
Temperature (T)
Increasing
values of F
(a) (b)
FIGURE 14.3 (a) Variation of σ with T for solids and for several electric field strength
values (F), showing that the relationship between σ and T becomes linear for increasing
values of F (the lower limit of F, i.e., F = 0 V· m –1 , represents conventional heating).
(b) Variation of σ with T for liquids, showing that σ always has a linear relationship
with T, with the value of σ decreasing for increasing concentration of nonpolar (thus
nonconductive) constituents (e.g., soluble solids). (Adapted from Sastry, S.K. and
Palaniappan, S., Food Technol., 45, 64–67, 1992.)
Electrical conductivity (σ)
Increasing solids
content
Temperature (T)