130 Part II: Water, Enzymology, Biotechnology, and Protein Cross-linking
constant caused it to absorb microwave energy, lead-
ing to an increase in its temperature. Driven by the
oscillating electric field of microwaves, water mol-
ecules rotate, oscillate, and move about faster,
increasing water temperature to sometimes even
above its boiling point. In regions where water has
difficulty forming bubbles, the water is overheated.
When bubbles suddenly do form in superheated
water, an explosion takes place. Substances without
dipole moment cannot be heated by microwaves.
Therefore, plastics, paper, and ceramics won’t get
warm. Metallic conductors rapidly polarize, causing
sparks due to arcing. The oscillating current and
resistance of some metals cause a rapid heating. In
contrast, water is a poor conductor, and the heating
mechanism is very complicated. Nelson and Datta
(2001) reviewed microwave heating in the Hand-
book of Microwave Technology for Food Applica-
tions.
Molecules absorb photons of certain frequencies.
However, microwave heating is due not only to ab-
sorption of photons by the water molecules, but also
to a combination of polarization and dielectric
induction. As the electric field oscillates, the water
molecules try to align their dipoles with the elec-
tric field. Crowded molecules restrict one another’s
movements. The resistance causes the orientation of
water molecules to lag behind that of the electric
field. Since the environment of the water molecules
is related to their resistance, the heating rate of the
water differs from food to food and region to region
within the same container. Water molecules in ice,
for example, are much less affected by the oscillat-
ing electric field in domestic microwave ovens,
which are not ideal for thawing frozen food. The
outer thawed layer heats up quickly, and it is cooked
before the frozen part is thawed. Domestic micro-
wave ovens turn on the microwave intermittently or
at very low power to allow thermal conduction for
thawing. However, microwaves of certain frequen-
cies may heat ice more effectively for tempering
frozen food. Some companies have developed sys-
tems for specific purposes, including blanching,
tempering, drying, and freeze-drying.
The electromagnetic wave form in an oven or in
an industrial chamber depends on the geometry of
the oven. If the wave forms a standing wave in the
oven, the electric field varies according to the wave
pattern. Zones where the electric field varies with
the largest amplitude cause water to heat up most
rapidly, and the nodal zones where there are no
oscillations of electric field will not heat up at all.
Thus, uniform heating has been a problem with
microwave heating, and various methods have been
developed to partly overcome this problem. Also,
foodstuffs attenuate microwaves, limiting their pen-
etration depth into foodstuff. Uneven heating re-
mains a challenge for food processors and micro-
wave chefs, mostly due to the short duration of
microwaving. On the other hand, food is also sel-
dom evenly heated when conventionally cooked.
Challenges are opportunities for food industries
and individuals. For example, new technologies in
food preparation, packaging, and sensors for moni-
toring food temperature during microwaving are re-
quired. There is a demand for expertise in microwav-
ing food. Industries microwave-blanche vegetables
for drying or freezing to take advantage of its energy
efficiency, time saving, decreased waste, and reten-
tion of water-soluble nutrients. The ability to quick-
ly temper frozen food in retail stores reduces
spoilage and permits selling fresh meat to cus-
tomers.
Since water is the heating medium, the tempera-
ture of the food will not be much higher than the
boiling point of the aqueous solutions in the food.
Microwave heating does not burn food; thus, the
food lacks the usual color, aroma, flavor, and texture
found in conventional cooking. The outer layer of
food is dry due to water evaporation. Retaining or
controlling water content in microwaved food is a
challenge.
When microwaved, water vapor is continually
removed. Under reduced pressure, food dries or
freeze-dries at low temperature due to its tendency
to restore the water activity. Therefore, microwaving
is an excellent means for drying food because of its
savings in energy and time. Microwaves are useful
for industrial applications such as drying, curing,
and baking or parts thereof.
Microwave ovens have come a long way, and
their popularity and improvement continue. Food
industry and consumer attitudes about microwav-
able food have gone up and down, often due to mis-
conceptions. Microwave cooking is still a challenge.
The properties of water affect cooking in every way.
Water converts microwave energy directly into heat,
attenuates microwave radiation, transfers heat to
various parts of the foodstuff, affects food texture,
and interacts with various nutrients. All properties of