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

88 Part 1: Principles/Food Analysis

absorption intensities and regions of water vapor are different

from those of ozone, but both are responsible for UV absorption

in the atmosphere. Incidentally, both triatomic water and ozone

molecules are bent.

Hydrogen Bonding and Polymeric Water

in Vapor

Attraction between the lone pairs and hydrogen among water

molecules is much stronger than any dipole–dipole interac-

tions. This type of attraction is known as the hydrogen bond

(O H O), a very prominent feature of water. Hydrogen bonds

are directional and are more like covalent bonds than strong

dipole–dipole interactions. Each water molecule has the capac-

ity to form four hydrogen bonds, two by donating its own H

atoms and two by accepting H atoms from other molecules. In

the structure of ice, to be described later, all water molecules,

except those on the surface, have four hydrogen bonds.

Attractions and strong hydrogen bonds among molecules form

water dimersand polymericwater clustersin water vapor. Mi-

crowave spectroscopy has revealed their existence in the atmo-

sphere (Goldman et al. 2001, Huisken et al. 1996).

As water dimers collide with other water molecules, trimer

and higher polymers form. The directional nature of the hydro-

gen bond led to the belief that water clusters are linear, ring,

or cage-like rather than aggregates of molecules in clusters

(see Fig. 5.4). Water dimers, chains, and rings have one and

two hydrogen-bonded neighbors. There are three neighbors per

molecule in cage-like polymers. Because molecules are free to

move in the gas and liquid state, the number of nearest neigh-

bors is between four and six. Thus, water dimers and clusters are

entities between water vapor and condensed water (Bjorneholm

et al. 1999).

By analogy, when a few water molecules are intimately as-

sociated with biomolecules and food molecules, their properties

would be similar to those of clusters.

CONDENSED WATER PHASES

Below the critical temperature of 647 K (374◦C) and under

the proper pressure, water molecules condense to form a liq-

uid or solid—condensed water. Properties of water, ice, and

vapor must be considered in freezing, pressure-cooking, and

microwave heating. In food processing, these phases transform

among one another. The transitions and the properties of con-

densed phases are manifestations of microscopic properties of

water molecules. However, condensation modifies microscopic

properties such as bond lengths, bond angles, vibration, rota-

tion, and electronic energy levels. The same is true when water

molecules interact with biomolecules and food molecules. All

phases of water play important parts in biochemistry and food

science.

Water has many anomalous properties, which are related to

polarity and hydrogen bonding. The melting point (mp), boil-

ing point (bp), and critical temperature are abnormally high for

water. As a rule, the melting and boiling points of a substance

are related to its molecular mass; the higher the molar mass, the

higher the mp and bp. Mp and bp of water (molar mass 18, mp

273 K, bp 373 K) are higher than those of hydrogen compounds

of adjacent elements of the same period, NH 3 (molar mass 17,

Figure 5.4.Hydrogen bonding in water dimers and cyclic forms of trimer and tetramer. Linear and transitional forms are also possible for

trimers, tetramers, and polymers.