Thermal Food Processing

Ohmic Heating for Food Processing 437
compared to the fluid, and it may be underprocessed. In the case of Figure 14.4a,
underheating may result from the major part of the current bypassing the particle,
thus heating more the surrounding fluid. Alternatively (Figure 14.4d), underheating
may result from the particle transmitting most of the current, thus creating a
low field gradient within the particle and low current densities in its vicinity. A
similar situation will be that of a cluster of particles (more conductive than the
fluid) blocking the cross section of the heater. However, the authors conclude that
both the latter cases are avoidable in an industrial application (e.g., controlling
particle size or online sensing the σ of the product to divert possible particle
clumps from the product stream prior to heating), the former one causing the
greater concern. In practice, if a single low-conductivity particle enters the system,
there is potential for underprocessing that particle, because its cold spot would
have a significantly lower temperature than that of the fluid. 15 Such a particle
might be a fat globule that, if carrying a microbial load, could present a serious
risk for the safety of the product being processed.
Most of the conclusions presented above have been drawn for a static ohmic
heater. What happens then in a continuous-flow ohmic heater? In this case, the
fluid will be agitated, and therefore the fluid–particle heat transfer will be higher
than in the static situation. Sastry and Salengke 32 have shown that the static
situation is not always related to the worst-case scenario. In fact, when the solid
is less conductive than the fluid. the worst situation will be that of a mixed fluid
(continuous situation). Nevertheless, if the solid is more conductive than the fluid,
then the worst case will be that of a static fluid (static situation). Evidently, the
rheological properties of the fluid have great influence, and viscous fluids tend
to make a continuous system behave as a static system. 33
The concentration and size of the particles are also responsible for alterations
of the heating rate of particle/fluid mixtures (Figure 14.5a and b). In fact, the
values of σ of orange and tomato juices decreased with solids content. 6 The same
observation has been made by other authors using other food systems (e.g.,
Zareifard et al. 23 used carrot puree or cubes immersed in a starch solution and
Castro et al. 8 used strawberry cubes immersed in strawberry pulp (Figure 14.9a)).
A possible explanation for this finding is given by Palaniappan and Sastry, 6 who
establish a comparison with the case of a fluid without particles. In this case (as
mentioned in the beginning of this section), the increase of σ with T is due to
the reduced opposition (drag force) to the movement of ions. At constant T, this
opposition is increased when solid particles are present (e.g., due to the increased
tortuosity of the path that the ions have to follow, among other effects), and
this may be the reason for the observed decreasing trend of σ with increasing
particles content (Figure 14.5a), which is in line with what happened with
dissolved solids concentration (see Figure 14.3b). This same explanation may be
applied to justify the decreasing of σ when particle size increases (Figure 14.5b).
This effect has been observed by several authors in various systems. 6,8,23
If a solid particle with a lower σ than the fluid, in which the particle is immersed,
is subjected to OH, it will heat slower than the fluid (assuming a particle with an
aspect ratio close to unity or aligned as in Figure 14.4a). However, if a number of