Food Biochemistry and Food Processing (2 edition)

BLBS102-c05 BLBS102-Simpson March 21, 2012 12:2 Trim: 276mm X 219mm Printer Name: Yet to Come

92 Part 1: Principles/Food Analysis

Figure 5.8.Density of water mg m−^3 as a function of temperature

(◦C).

pressures of ice Ih and water are the same, 0.611 kPa, and the

boiling point (373.15 K, 100◦C) is the temperature at which

the vapor pressure is 101.325 kPa (1 atm). At slightly below

394 K (121◦C), the vapor pressure is 202.65 kPa (2.00 atm). At

473 and 574 K, the vapor pressures are 1553.6 and 8583.8 kPa,

respectively. The vapor pressure rises rapidly as temperature

increases. The lowest pressure to liquefy vapor just below the

critical temperature, 373.98◦C, is 22,055 kPa (217.67 atm), and

this is known as the critical pressure. Above 373.98◦C, water

cannot be liquefied, and the fluid is called supercritical water.

The partial pressure of H 2 O in the air at any temperature

is theabsolute humidity. When the partial pressure of water

vapor in the air is the equilibrium vapor pressure of water at

Figure 5.9.Equilibrium vapor pressure of water as a function of

temperature.

the same temperature, therelative humidityis 100%, and the

air is saturated with water vapor. The partial vapor pressure in

the air divided by the equilibrium vapor pressure of water at

the temperature of the air is therelative humidity, expressed

as a percentage. The temperature at which the vapor pressure

in the air becomes saturated is thedew point, at which dew

begins to form. Of course, when the dew point is below 273 K

or 0◦C, ice crystals (frost) begin to form. Thus, the relative

humidity can be measured by finding the dew point and then

dividing the equilibrium vapor pressure at the dew point by

the equilibrium vapor pressure of water at the temperature of

the air. The transformations between solid, liquid, and gaseous

water play important roles in hydrology and in transforming the

earth’s surface. Solar energy causes phase transitions of water

that make the weather.

Transformation of Solid, Liquid, and Vapor

Food processing and biochemistry involve transformations

among solid, liquid, and vapor of water. Therefore, it is impor-

tant to understand ice–water, ice–vapor, and water–vapor trans-

formations and their equilibria. These transformations affect our

daily lives as well. A map or diagram is helpful in order to com-

prehend these natural phenomena. Such a map, representing

or explaining these transformations, is called aphase diagram

(see Fig. 5.10). A sketch must be used because the range of

pressure involved is too large for the drawing to be on a linear

scale.

The curves representing the equilibrium vapor pressures of

ice and water as functions of temperature meet at thetriple point

(see Fig. 5.10). The other end of the vapor pressure curve is the

critical point. The mps of ice Ih are 271.44, 273.15, and 273.16 K

at 22,055 kPa (the critical pressure), 101.325 kPa, and 0.611 kPa

(the triple point), respectively. At a pressure of 200,000 kPa, Ih

melts at 253 K. Thus, the line linking all these points represents

the mp of Ih at different pressures. This line divides the condi-

tions (pressure and temperature) for the formation of solid and

liquid. Thus, the phase diagram is roughly divided into regions

of solid, liquid, and vapor.

Ice Ih transforms into the ordered ice XI at low temperature. In

this region and under some circumstance, Ic is also formed. The

transformation conditions are not represented in Figure 5.10, and

neither are the transformation lines for other ices. These occur at

much higher pressures in the order of gigapascals. A box at the

top of the diagram indicates the existence of these phases, but

the conditions for their transformation are not given. Ices II–X,

formed under gigapascals pressure, were mentioned earlier. Ice

VII forms at greater than 10 GPa and at a temperature higher

than the boiling point of water.

Formation and existence of these phases illustrate the various

hydrogen bonding patterns. They also show the many possibili-

ties of H 2 O-biomolecule interactions.

Subcritical and Supercritical Waters

Water at temperatures between the boiling and critical points

(100–373.98◦C) is called subcritical water, whereas the phase