BLBS102-c05 BLBS102-Simpson March 21, 2012 12:2 Trim: 276mm X 219mm Printer Name: Yet to Come
104 Part 1: Principles/Food Analysismicrowaved popping corns, which popped. A team at Raytheon
developed microwave ovens, but it took more than 25 years and
much more effort to improve them and make them practical and
popular. Years ago, boiling water in a paper cup in a microwave
oven without harming the cup amazed those who were used to
see water being heated in a fire-resistant container over a stove
or fire. Microwaves simultaneously heat all the water in the bulk
food.
After the invention of the microwave oven, many offered ex-
planations on how microwaves heat food. Water’s high dipole
moment and high dielectric constant caused it to absorb mi-
crowave energy, leading to an increase in its temperature. Driven
by the oscillating electric field of microwaves, water molecules
rotate, oscillate, and move about faster, increasing water temper-
ature to sometimes even above its bp. In regions where water has
difficulty forming bubbles, the water is overheated. When bub-
bles 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 theHandbook of
Microwave Technology for Food Applications.
Molecules absorb photons of certain frequencies. However,
microwave heating is due not only to absorption of photons by
the water molecules, but also to a combination of polarization
and dielectric induction. As the electric field oscillates, the wa-
ter molecules try to align their dipoles with the electric field.
Crowded molecules restrict one another’s movements. The re-
sistance 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 oscillating electric field in domes-
tic 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 microwave ovens
turn on the microwave intermittently or at very low power to
allow thermal conduction for thawing. However, microwaves of
certain frequencies may heat ice more effectively for temper-
ing frozen food. Some companies have developed systems 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 mi-
crowaves, limiting their penetration depth into foodstuff. Uneven
heating remains a challenge for food processors and microwavechefs, mostly due to the short duration of microwaving. On the
other hand, food is also seldom evenly heated when convention-
ally cooked.
Challenges are opportunities for food industries and indi-
viduals. For example, new technologies in food preparation,
packaging, and sensors for monitoring food temperature dur-
ing microwaving are required. There is a demand for expertise
in microwaving food. Industries microwave-blanche vegetables
for drying or freezing to take advantage of its energy efficiency,
time saving, decreased waste, and retention of water-soluble nu-
trients. The ability to quickly temper frozen food in retail stores
reduces spoilage and permits selling fresh meat to customers.
Since water is the heating medium, the temperature 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. Un-
der reduced pressure, food dries or freeze-dries at low tempera-
ture 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 atti-
tudes about microwavable food have gone up and down, often
due to misconceptions. Microwave cooking is still a challenge.
The properties of water affect cooking in every way. Water con-
verts 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 proper-
ties of water must be considered in order to take advantage of
microwave cooking.WATER RESOURCES AND THE
HYDROLOGICAL CYCLEFresh waters are required to sustain life and maintain living stan-
dards. Therefore, fresh waters are calledwater resources.En-
vironmentalists, scientists, and politicians have sounded alarms
about limited water resources. Such alarms appear unwarranted
because the earth has so much water that it can be called a water
planet. Various estimates of global water distribution show that
about 94% of earth’s water lies in the oceans and seas. These
salt waters sustain marine life and are ecosystems in their own
right, but they are not fresh waters that satisfy human needs. Of
the remaining 6%, most water is in solid form (at the poles and
in high mountains before the greenhouse effect melts them) or
underground. Less than 1% of earth’s water is in lakes, rivers,
and streams, and waters from these sources flow into the seas
or oceans. A fraction of 1% remains in the atmosphere, mixed
with air (Franks 2000).
A human may drink only a few liters of water in various forms
each day, but ten times more water is required for domestic
usages such as washing and food preparation. A further equal