Food Biochemistry and Food Processing

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

and others perform vital functions by themselves.
Regardless of their biological function and chemical
composition, vitamins are divided into water-soluble
and fat-soluble groups. This division, based on po-
larity, serves as a guide for food processing. For
example, food will lose water-soluble vitamins
when washed or boiled in water, particularly after
cutting (Hawthorne and Kubatova 2002).
The water-soluble vitamins consist of a complex
group of vitamin Bs, vitamin C, biotin, lipoic acid,
and folic acid. These molecules are either polar or
have the ability to form hydrogen bonds. Vitamins A
(retinal), D2, D3, E, and K are fat soluble, because
major portions of their molecules are nonpolar
organic groups.
Vitamin C, L-ascorbic acid or 3-oxo-L-gulofura-
nolactone, has the simplest chemical formula
(C 6 H 8 O 6 ) among vitamins. This diprotic acid is
widely distributed in plants and animals, and only a
few vertebrates, including humans, lack the ability
to synthesize it.
Vitamin B complexis a group of compounds iso-
lated together in an aqueous solution. It includes thi-
amine (B1), riboflavin (B2), niacin (or nicotinic
acid, B3), pantothenic acid (B5), cyanocobalamin
(B12), and vitamin B6 (any form of pyridoxal, pyri-
doxine, or pyridoxamine). Biotin, lipoic acid, and
folic acid are also part of the water-soluble vitamins.
These vitamins are part of enzymes or coenzymes
that perform vital functions.

FOOD CHEMISTRY OF WATER

Water ingestion depends on the individual, composi-
tion of the diet, climate, humidity, and physical
activity. A nonexercising adult loses the equivalent
of 4% of his or her body weight in water per day
(Brody 1999). Aside from ingested water, water is
produced during the utilization of food. It is proba-
bly fair to suggest that food chemistry is the chem-
istry of water, since we need a constant supply of
water as long as we live.
Technical terms have special meanings among
fellow food scientists. Furthermore, food scientists
deal with dynamic and nonequilibrium systems,
unlike most natural scientists who deal with static
and equilibrium systems. There are special concepts
and parameters useful only to food scientists. Yet,
the fundamental properties of water discussed above
lay a foundation for the food chemistry of water.

With respect to food, water is a component, solvent,

acid, base, and dispersing agent. It is also a medium

for biochemical reactions, for heat and mass trans-

fer, and for heat storage.

Food chemists are very concerned with water

content and its effects on food. They need reliable

parameters for references, criteria, and working

objectives. They require various indicators to corre-

late water with special properties such as perishabil-

ity, shelf life, mobility, smell, appearance, color, tex-

ture, and taste.

WATER AS ACOMMONCOMPONENT OFFOOD

Water is a food as well as the most common compo-

nent of food. Even dry foods contain some water,

and the degree of water content affects almost every

aspect of food: stability, taste, texture, and spoilage.

Most food molecules contain OH, CO, NH, and

polar groups. These sites strongly interact with

water molecules by hydrogen bonding and dipole-

dipole interactions. Furthermore, dipole-ion, hydro-

philic, and hydrophobic interactions also occur

between water and food molecules. The properties

of hydrogen-bonded water molecules differ from

those in bulk water, and they affect the water mole-

cules next to them. There is no clear boundary for

affected and unaffected water molecules. Yet it is

convenient to divide them into bound waterand

free water. This is a vague division, and a consen-

sus definition is hard to reach. Fennema and Tan-

nenbaum (1996) give a summary of various criteria

for them, indicating a diverse opinion. However, the

concept is useful, because it helps us understand the

changes that occur in food when it is heated, dried,

cooled, or refrigerated. Moreover, when water is the

major ingredient, interactions with other ingredients

modify the properties of the water molecules. These

aspects were discussed earlier in connection with

aqueous solutions.

WATERACTIVITY

Interactions of water and food molecules mutually

change their properties. Water in food is not pure

water. Water molecules in vapor, liquid, and solid

phases or in solutions and food react and inter-

change in any equilibrium system. The tendency to

react and interchange with each other is called the

chemical potential, . At equilibrium, the potential