BLBS102-c05 BLBS102-Simpson March 21, 2012 12:2 Trim: 276mm X 219mm Printer Name: Yet to Come
5 Water Chemistry and Biochemistry 91Figure 5.6.Equilibrium vapor pressure (Pa) of ice as a function of
temperature (K).This simplified equation gives an estimate of the vapor pressure
(in Pa) of ice at temperatures in a narrow range around the triple
point.P= 611 .15 exp(
− 6148(
1
T−1
273. 16))The value 6148 is the enthalpy of sublimation (subH=51.1 J/
mol, varies slightly with temperature) 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 temperature) of water at
273 K.Liquid H 2 O—WaterWe started the chapter by calling the compound H 2 O water,
but most of us considerwaterthe liquid H 2 O. In terms of food
processing, the liquid is the most important state. Water is con-
tained 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 commodi-
ties. Various forms of water ingested help digest, absorb, and
transport nutrients to various part of the body. Water further fa-
cilitates 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 constant of liq-
uid 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 rising temperature, these properties decrease. To show theFigure 5.7.Variation of viscosity (1.793 mPa s), dielectric constant
(87.90), surface tension (75.64 mN/m), heat capacity Cp (4.2176 J
g−^1 K−^1 ), and thermal conductivity (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.variation, the properties at other temperatures are divided by the
same property at 273 K. The ratios are then plotted as a func-
tion of temperature. At 273.15 K (0◦C), all the ratios are unity
(1). The thermal 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 important
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 temperature is well known, and ac-
curate values are carefully measured and evaluated especially
between 273 K 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 density within this temperature range, and theCRC Hand-
book of Chemistry and Physics(Lide 2003) has a table listing
these values. The variation of water density between the freez-
ing point and the bp is shown in Figure 5.8. The densities are
0.9998426, 0.9999750, and 0.9998509 Mg m−^3 at 0, 4, and
8 ◦C, respectively. The decrease in density is not linear, and at
100 ◦C, the density is 0.95840 Mg m−^3 , a decrease of 4% from
its maximum.Vapor Pressure of Liquid H 2 OEquilibrium vapor pressure of water (Fig. 5.9) increases with
temperature, similar to that of ice. At the triple point, the vapor