Food Biochemistry and Food Processing

5 Water Chemistry and Biochemistry 129

sucrose crystals is the high viscosity of the solution,
which prevents molecules from moving and ori-
enting properly to crystallize. The viscous solution
reaches a glassy or amorphous state at the glass
transition temperature (Tg), point G. The glass
state is a frozen liquid with extremely high viscosity.
In this state, the molecules are immobile. The tem-
perature, Tg, for glass transition depends on the rate
of cooling (Angell 2002). The freezing of sugar so-
lution may follow different paths, depending on the
experimental conditions.
In lengthy experiments, Young and Jones (1949)
warmed glassy states of water-sucrose and observed
the warming curve over hours and days for every
sample. They observed the eutectic mixture of 54%
sucrose (Te 13.95°C). They also observed the
formation of phases C 12 H 22 O 11 2.5H 2 O and C 12 H 22
O 11 3.5H 2 O hydrated crystals formed at tempera-
tures higher than the Te, which is for anhydrous
sucrose. The water-sucrose binary system illustrates
that the states of food components during freezing
and thawing can be very complicated. Freshly made
ice creams have wonderful texture, and the physics
and chemistry of the process are even more inter-
esting.

INTERACTION OF WATER AND

MICROWAVE

Wavelengths of microwave range from meters down

to a millimeter, their frequencies ranging from 0.3 to

300 GHz. A typical domestic oven generates 2.45

GHz microwaves, wavelength 0.123 m, and energy

of photon 1.62  10 -24J (10 eV). For industrial

applications, the frequency may be optimized for the

specific processes.

Percy L. Spencer (1894–1970), the story goes,

noticed that his candy bar melted while he was

inspecting magnetron testing at the Raytheon Corp-

oration in 1945. As a further test, he microwaved

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 explanations on how microwaves heat food.

Water’s high dipole moment and high dielectric

Figure 5.17.A sketch showing the phase and state diagram of water-sucrose binary system.