112 Part II: Water, Enzymology, Biotechnology, and Protein Cross-linking
Plot of ln Pversus 1/Tshows a straight line, and
this agrees well with the Clausius-Clapeyron equa-
tion. The negative slope gives the enthalpy of the
phase transition, H, divided by the gas constant R
(8.3145 J/K/mol).
This simplified equation gives an estimate of the
vapor pressure (in Pa) of ice at temperatures in a
narrow range around the triple point.
The value 6148 is the enthalpy of sublimation
(subH51.1 J/mol, varies slightly with tempera-
ture) divided by the gas constant R. Incidentally, the
enthalpy of sublimation is approximately the sum of
the enthalpy of fusion (6.0 J/mol) for ice and the
heat of vaporization (45.1 J/mol, varies with temper-
ature) of water at 273 K.
LIQUIDH 2 O—WATER
We started the chapter by calling the compound H 2 O
water, but most of us consider waterthe liquid H 2 O.
In terms of food processing, the liquid is the most
important state. Water is contained in food, and it is
used for washing, cooking, transporting, dispersing,
dissolving, combining, and separating components
of foods. Food drying involves water removal, and
fermentation uses water as a medium to convert raw
materials into commodities. Various forms of water
ingested help digest, absorb, and transport nutrients
to various part of the body. Water further facilitates
biochemical reactions to sustain life. The properties
of water are the basis for its many applications.
Among the physical properties of water, the heat
capacity (4.2176 J g^1 K^1 at 273.15 K) varies little
between 273.15 and 373.15 K. However, this value
decreases, reaches a minimum at about 308 K, and
then rises to 4.2159–4.2176 J g^1 K^1 at 373.15 K
(see Fig. 5.7).
The viscosity, surface tension, and dielectric con-
stant of liquid H 2 O decrease as temperature increases
(see Fig. 5.7). These three properties are related to
the extent of hydrogen bonding and the ordering of
the dipoles. As thermal disorder increases with ris-
ing temperature, these properties decrease. To show
P
T=−−⎛
⎝⎜⎞
⎠⎟⎛
⎝⎜⎞
⎠⎟611 15 614811
273 16.exp
.((ln ))
((/ ))dP
dTH
1 R=−∆the variation, the properties at other temperatures
are divided by the same property at 273 K. The
ratios are then plotted as a function of temperture. At
273.15 K (0°C), all the ratios are unity (1). The ther-
mal conductivity, on the other hand, increases with
temperature. Thus, the thermal conductivity at 373
K (679.1 W K^1 m^1 ) is 1.21 times that at 273 K
(561.0 W K^1 m^1 ). Warm water better conducts
heat. Faster moving molecules transport energy
faster. The variations of these properties play impor-
tant roles in food processing or preparation. For
example, as we shall see later, the dielectric constant
is a major factor for the microwave heating of food,
and heat conductivity plays a role cooking food.
Densities of other substances are often determined
relative to that of water. Therefore, density of water
is a primary reference. Variation of density with tem-
perature is well known, and accurate values are care-
fully measured and evaluated especially between
273 and 313 K (0–40°C). Two factors affect water
density. Thermal expansion reduces its density, but
the reduced number of hydrogen bonds increases its
density. The combined effects resulted in the highest
density at approximately 277 K (4°C). Tanaka et al.
(2001) has developed a formula to calculate the den-
sity within this temperature range, and the CRC
Handbook of Chemistry and Physics(Lide 2003)
has a table listing these values. The variation ofFigure 5.7.Variation of viscosity (1.793 mPa s), dielec-
tric constant (87.90), surface tension (75.64 mN/m),
heat capacity Cp (4.2176 J g^1 K^1 ), and thermal con-
ductivity (561.0 W K^1 m^1 ) of water from their values at
273.16 K to 373.16 K (0 and 100°C). Values at
273.15°K are given.