Thermal Food Processing

Ohmic Heating for Food Processing 445
Other in-depth problems have been addressed that arose during the modeling
effort during the previous two decades. Lacey 42,43 and Lacey et al. 44 studied the
problem of thermal runaway while modeling OH, first by considering only heat
transfer by conduction and second by including heat convection, which was found
to dominate heat conduction. 45 In the same line, a diffusion–convection problem
was addressed by Kavallaris and Tzanetis, 46 with the Heaviside function representing
the food resistivity (which is the inverse of the conductivity).
14.3 THERMAL AND NONTHERMAL EFFECTS OF OH
This section reveals the research efforts to determine the effects of OH, both
thermal and nonthermal, on microbes and on food constituents such as enzymes
and vitamins. The information on proteins is limited to the studies made with
surimi and its gel-forming ability due to the presence of the myosin heavy chain
(see, e.g., Yongsawatdigul et al. 47,48 ).
14.3.1 MICROBIAL KINETICS
Microbial inactivation in foodstuffs is predominantly carried out by thermal
processes, and the thermal inactivation kinetics of most of the target microorganisms
is well studied. The need to reduce processing time and the increasing interest in
using OH as an alternative heating technology to conventional heat transfer during
commercial processes of sterilization or pasteurization were the impetus for the
study of nonthermal mechanisms of microbial inactivation.
The destruction of microorganisms by nonthermal effects such as electricity
is still not well understood and generates some controversy. Little work
has been done in this field. Moreover, most of the published results do not
refer to the sample temperature, or cannot eliminate temperature as a variable
parameter.
The application of OH in the fermentation by Lactobacillus acidophilus, a
lactic acid bacterium used in the dairy industry and with human health implications,
12 was studied. The lid of the fermentation vessel was equipped with ports
for the thermocouple, pH probe, inoculation, water circulation coil, medium
circulation, and two stainless steel plate electrodes. Metal surfaces were coated
with epoxylite for electrical insulation and inertness. Temperature control was
carried out under either conventional heating (by continuous water circulation)
or OH (a constant voltage of 15 V, low voltage, or 40 V, high voltage, was applied),
at different temperatures (30, 35, and 40°C). The results indicated that the lag
phase for fermentations at 30°C was significantly lower (18-fold) under lowvoltage
ohmic conditions, which was also the lowest lag period of all the conditions
tested. Although additional investigation is needed to explain the shortening
of lag phase, this may be due to the improvement of absorption of nutrients and
minimization of the inhibitory action of fresh medium. The application of an electrical
field may induce pore formation in membranes (similar to the electroporation
mechanism used to transform cells in molecular biology studies), allowing a faster