
within solids, and makes it possible to process solids
and liquids at similar rates. This article reviews the
principles that govern electrical heating, the factors
which control the heating rate, and the design of
processes.
Electrical Heating
0002 Thermal processing aims to reduce contamination so
that the food will not cause a health hazard during its
shelf-life. Each part of the material must be processed
to a level prescribed to insure product safety. Heat
transfer to solids is slow, however; by the time enough
heat is conducted to the centre of a solid to insure
sterility, much of the rest is overcooked.
0003 Several sets of reactions occur when a food is
heated: those which reduce the level of bacterial con-
tamination, those which result in losses in product
quality, in terms of nutrition, taste, and texture, and
some that improve taste and texture. The reactions
which reduce the number of bacterial spores have a
higher activation energy than those which lower
product quality: to maximize quality for a given level
of sterility it is best to process at as high a temperature
as possible. Continuous ultrahigh-temperature (UHT)
or high-temperature, short-time (HTST) processes
exploit this to produce food of a higher quality than
canning. In these processes, food is rapidly heated to
c. 140
C, held there for a short period and then
rapidly cooled. At 140
C, food can be sterilized in a
few seconds, rather than the several minutes needed
at canning temperatures. This requires rapid heating
rates to minimize time spent at high temperatures,
and thus quality loss, and is best done in a continuous
process.
0004 Processing single-phase liquid foods this way is
straightforward, because they can be heated and
cooled rapidly. High heating and cooling rates
(> 1
Cs
1
) are possible. This type of process can
efficiently cook liquids such as milks, fruit juices,
soups, and sauces, but cannot readily be applied to
foods which contain particles, because of the slow-
ness of thermal conduction into the solid. Conduction
heating requires a temperature driving force between
particles and liquid, and for particles larger than
about 2 mm, rapid heating is not feasible. It is pos-
sible to use conduction/convection methods to pro-
cess foods with a high solids fraction (for example, in
scraped-surface exchangers), but low heating rates
and long hold times are needed, giving poor product
quality.
0005 The need to conduct heat is the limiting factor in
the thermal processing of particles, and thus for
solid–liquid mixtures. Volumetric heat generation
techniques can solve this problem. Various techniques
are available which use electric fields. In microwave
or RF (radio frequency) heating, a high-frequency
electric field excites the water molecules within
the material, whilst in electrical resistance heating
(ohmic heating), however, the passage of electrical
current results in heating throughout the material.
The process is more energy-efficient than microwave
heating, because nearly all of the electrical energy
goes to heat the food. It requires the passage of
electric current through the material; electrodes
that make good contact with the food are required,
unlike microwave heating which needs no physical
contact. When an electric current flows through a
material, heat is generated according to the familiar
Ohm’s law:
W ¼ I
2
R ð1Þ
Here W is power, I is current, and R is the resistance.
Heat is generated throughout the material as a result
of its inherent electrical resistance.
0006If the electrical conductivities and thermal capaci-
ties of different components of a food are similar, they
can heat at similar rates. In practice, as a result of
their thermal capacities and electrical conductivities,
solids can heat faster than liquids, a result which is
impossible by conventional means. Figure 1 shows
that rapid heating rates are possible for real foods.
Short process times thus make it possible to apply
UHT techniques to particulate foods.
100
80
60
40
20
0
0 50 100 150 200 250 300
Time (s)
Temperature (⬚C)
fig0001Figure 1 Electrical heating of a piece of lamb meat 20 20
15 mm, electrical conductivity 0.45 S m
1
at 15
C, in a saline
solution, conductivity 0.454 S m
1
at 15
C. Applied voltage
gradient 11 V cm
1
. Dashed line, solid; continuous line, liquid.
Reproduced from Heat Treatment, Electrical Process Heating,
Encyclopaedia of Food Science, Food Technology and Nutrition,
Macrae R, Robinson RK and Sadler MJ (eds), 1993, Academic
Press.
HEAT TREATMENT/Electrical Process Heating 3045