BLBS102-c05 BLBS102-Simpson March 21, 2012 12:2 Trim: 276mm X 219mm Printer Name: Yet to Come
5 Water Chemistry and Biochemistry 103food services and technologies. Wastewater from the food in-
dustry needs treatment, and the technology is usually dealt with
in industrial chemistry (Lacy 1992).
When food is plentiful, beneficial and pathogenic organisms
thrive. Pathogenic organisms present in drinking water cause
intestinal infections, dysentery, hepatitis, typhoid fever, cholera,
and other diseases. Pathogens are usually present in waters that
contain human and animal wastes that enter the water system
via discharge, runoffs, flood, and accidents at sewage treatment
facilities. Insects, rodents, and animals can also bring bacteria to
the water system (Coler 1989, Percival et al. 2000). Testing for
all pathogenic organisms is impossible, but some organisms have
common living conditions. These are calledindicator bacteria,
because their absence signifies safety.Water and State of FoodWhen a substance and water are mixed, they mutually dissolve,
forming a homogeneous solution, or they partially dissolve in
each other, forming solutions and other phases. At ambient pres-
sure, various phases are in equilibrium with each other in iso-
lated and closed systems. The equilibria depend on temperature.
A plot of temperature versus composition showing the equilibria
among various phases is aphase diagramfor a two-component
system. Phase diagrams for three-component systems are very
complicated, and foods consist of many substances, including
water. Thus, a strict phase diagram for food is almost impossi-
ble. Furthermore, food and biological systems are open, with a
steady input and output of energy and substances. Due to time
limits and slow kinetics, phases are not in equilibrium with each
other. However, the changes follow a definite rate, and these
aresteady states. For these cases, plots of temperature against
the composition, showing the existences of states (phases), are
calledstate diagrams. They indicate the existence of various
phases in multicomponent systems.
Sucrose (sugar, C 12 H 22 O 11 ) is a food additive and a sweetener.
Solutions in equilibrium with excess solid sucrose are saturated,
and their concentrations vary with temperature. The saturated
solutions contain 64.4 and 65.4% at 0 and 10◦C, respectively.
The plot of saturated concentrations against temperature isthe
equilibrium solubility curve,ES, in Figure 5.17. The freezing
curve, FE, shows the variation of freezing point as a function of
temperature. Aqueous solution is in equilibrium with ice Ih along
FE. At theeutectic point,E, the intersection of the solubility and
freezing curves, solids Ih and sucrose coexist with a saturated
solution. The eutectic point is the lowest mp of water–sugar
solutions. However, viscous aqueous sugar solutions or syrups
may exist beyond the eutectic point. These conditions may be
present in freezing and tempering (thawing) of food.
Dry sugar is stable, but it spoils easily if it contains more
than 5% water. The changes that occur as a sugar solution is
chilled exemplify the changes in some food components when
foods freeze. Ice Ih forms when a 10% sugar solution is cooled
below the freezing point. As water forms Ih, leaving sugar in the
solution, the solution becomes more concentrated, decreasing
the freezing point further along the FE toward the E. However,
when cooled, this solution may not reach equilibrium and yieldFigure 5.17.A sketch showing the phase and state diagram of
water-sucrose binary system.sugar crystals at the eutectic point. Part of the reason for not
having sucrose crystals is the high viscosity of the solution,
which prevents molecules from moving and orienting properly to
crystallize. The viscous solution reaches a glassy or amorphous
state at theglass transition temperature (Tg), point G. The glass
state is a frozen liquid with extremely high viscosity. In this
state, the molecules are immobile. The temperature, Tg, for
glass transition depends on the rate of cooling (Angell 2002). The
freezing of sugar solution 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 Oand
C 12 H 22 O 11 ·3.5H 2 O hydrated crystals formed at temperatures
higher than the Te, which is for anhydrous sucrose. The water-
sucrose binary system illustrates that the states of food com-
ponents 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 interesting.INTERACTION OF WATER AND
MICROWAVEWavelengths of microwave range from meters down to a mil-
limeter, their frequencies ranging from 0.3 to 300 GHz. A typi-
cal domestic oven generates 2.45 GHz microwaves, wavelength
0.123 m, and energy of photon 1.62× 10 −^24 J(10μeV). 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 test-
ing at the Raytheon Corporation in 1945. As a further test, he