Food Biochemistry and Food Processing

114 Part II: Water, Enzymology, Biotechnology, and Protein Cross-linking

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 tempera-
ture meet at the triple point(see Fig. 5.10). The oth-
er end of the vapor pressure curve is the critical
point. The melting points 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 melting point of Ih at different pres-
sures. This line divides the conditions (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 circum-
stance, Ic is also formed. The transformation condi-

tions 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 giga-

pascals. 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 tem-

perature higher than the boiling point of water.

Formation and existence of these phases illustrate

the various hydrogen bonding patterns. They also

show the many possibilities of H 2 O-biomolecule in-

teractions.

SUBCRITICAL ANDSUPERCRITICALWATERS

Water at temperatures between the boiling and criti-

cal points (100–373.98°C) is called subcritical

water, whereas the phase above the critical point is

Figure 5.10.A sketch outlining the phase diagram of ice, water, and vapor.