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

5 Water Chemistry and Biochemistry 85

chemical principles, and water chemistry is a key for the begin-

ning of primitive life forms billions of years ago. The properties

of water molecules give us clues regarding their interactions

with other atoms, ions, and molecules. Furthermore, water va-

por in the atmosphere increases the average temperature of the

atmosphere by 30 K (Wayne 2000), making the earth habitable.

Water remains important for human existence, for food produc-

tion, preservation, processing, and digestion.

Water is usually treated before it is used by food industries.

After usage, wastewaters must be treated before it is discharged

into the ecological system. After we ingest foods, water helps us

to digest, dissolve, carry, absorb, and transport nutrients to their

proper sites. It further helps hydrolyze, oxidize, and utilize the

nutrients to provide energy for various cells, and eventually, it

carries the biological waste and heat out of our bodies. Oxida-

tions of various foods also produce water. How and why water

performs these functions depend very much on its molecular

properties.

THE COMPOUND WATER

Acompoundis a substance that is made up of two or more basic

components calledchemical elements(e.g., hydrogen, carbon,

nitrogen, oxygen, iron) commonly found in food.Wa t e ris one

of the tens of millions of compounds in and on earth.

The chemical equation and thermal dynamic data for the for-

mation of water from hydrogen and oxygen gas is

2H 2 (g)+O 2 (g)+2H 2 O(l),
H^0 =− 571 .78 kJ

The equation indicates that 2 mol of gaseous hydrogen, H 2 (g),

react with 1 mol of gaseous oxygen, O 2 (g), to form 2 mol of

liquid water. If all reactants and products are at theirstandard

statesof 298.15 K and 101.325 kPa (1.0 atm), formation of

2 mol of water releases 571.78 kJ of energy, as indicated by the

negative sign for
H^0. Put in another way, theheat of formation

of water,
Hf^0 ,is−285.89 (=−571.78/2) kJ/mol. Due to the

large amount of energy released, the water vapor formed in the

reaction is usually at a very high temperature compared with its

standard state, liquid at 298.15 K. The heat of formation includes

the heat that has to be removed when the vapor is condensed to

liquid and then cooled to 298.15 K.

The reverse reaction, that is, the decomposition of water, is

endothermic, and energy, a minimum of 285.89 kJ per mole of

water, must be supplied. More energy is required by electrolysis

to decompose water because some energy will be wasted as heat.

A hydrogen-containing compound, when fully oxidized, also

produces water and energy. For example, the oxidation of solid

(s) sucrose, C 12 H 22 O 11 , can be written as

C 12 H 22 O 11 (s)+12O 2 (g)=12CO 2 (g)+11H 2 O(l),


H^0 =−5640 kJ

The amount of energy released,−5640 kJ, is called thestandard

enthalpy of combustionof sucrose. The chemical energy derived

this way can also be used to produce high-energy biomolecules.

When oxidation is carried out in human or animal bodies, the

oxidation takes place at almost constant and low temperatures.

Of course, the oxidation of sucrose takes place in many steps to

convert each carbon to CO 2.

THE POLAR WATER MOLECULES

During the twentieth century, the study of live organisms evolved

from physiology and anatomy to biochemistry and then down

to the molecular level of intermolecular relations and functions.

Atoms and molecules are the natural building blocks of matter,

including that of living organisms. Molecular shapes, structures,

and properties are valuable in genetics, biochemistry, food sci-

ence, and molecular biology, all involving water. Thus, we have

a strong desire to know the shape, size, construction, dimension,

symmetry, and properties of water molecules, because they are

the basis for the science of food and life.

The structure of water molecules has been indirectly studied

using X-ray diffraction, spectroscopy, theoretical calculations,

and other methods. Specific molecular dimensions from these

methods differ slightly, because they measure different proper-

ties of water under different circumstances. However, the O H

bond length of 95.72 pm (1 pm= 10 −^12 m) and the H O H

bond angle of 104.52◦have been given after careful review of

many recent studies (Petrenko and Whitworth 1999). The atomic

radii of H and O are 120 pm and 150 pm, respectively. The bond

length is considerably shorter than the sum of the atomic radii.

Sketches of the water molecule are shown in Figure 5.1 (spher-

ical atoms assumed).

Bonding among the elements H, C, N, and O is the key for bio-

chemistry and life. Each carbon atom has the ability to form four

chemical bonds. Carbon atoms can bond to other carbon atoms

as well as to N and O atoms, forming chains, branched chains,

rings, and complicated molecules. These carbon-containing

compounds are calledorganic compounds, and they include

foodstuff. The elements N and O have one and two more elec-

trons, respectively, than carbon, and they have the capacity to

form only three and two chemical bonds with other atoms.Quan-

tum mechanicsis a theory that explains the structure, energy,

and properties of small systems such as atoms and molecules.

Figure 5.1.Some imaginative models of the water molecule, H 2 O.