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 101

soup and gravy, and starch is added to food for desirable tex-

ture and appearance. Water increases the molecular mobility of

starch, and starch slows the movement of water molecules. Food

processors are interested in a quantitative relationship between

water and starch and the viscosity of the suspension.

Glycogen, animal starch, is easily hydrolyzed to yield glu-

cose, which provides energy when required. In the hydrolysis of

polysaccharides, water molecules react at the glycosidic links.

Certain enzymes catalyze this reaction, releasing glucose units

one by one from the end of a chain or branch.

Polysaccharide chains in cellulose are very long, 7000–15,000

monosaccharides, and interchain hydrogen bonds bind them into

fibers, which further stack up through interfiber hydrogen bonds.

Many interchain hydrogen bonds make the penetration of water

molecules between chains a time-consuming process. Heating

speeds up the process.

Water, Minerals, and Vitamins

Most minerals are salts or electrolytes. These are usually in-

gested as aqueous solutions of electrolytes, discussed earlier.

Ions (Ca^2 +,Mg^2 +,Na+,K+,Fe^2 +,Zn^2 +,Cu^2 +,Mn^2 +,Cl−,I−,

S^2 −,Se^2 −,H 2 PO 4 −, etc.) present in natural water are leached

from the ground. Some of them are also present in food, be-

cause they are essential nutrients for plants and animals that

we use as food. A balance of electrolytes in body fluid must

be maintained. Otherwise, shock or fainting may develop. For

example, the drinking water used by sweating athletes contains

the proper amount of minerals. In food, mineral absorption by

the body may be affected by the presence of other molecules.

For example, vitamin D helps the absorption of calcium ions.

Small amounts of a group of organic compounds not syn-

thesized by humans, but essential to life, are called vitamins;

their biochemistry is very complicated and interesting; many

interact with enzymes, and others perform vital functions by

themselves. Regardless of their biological function and chem-

ical composition, vitamins are divided into water-soluble and

fat-soluble groups. This division, based on polarity, 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-gulofuranolactone, 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 isolated together

in an aqueous solution. It includes thiamine (B1), riboflavin (B2),

niacin (or nicotinic acid, B3), pantothenic acid (B5), cyanocobal-

amin (B12), and vitamin B6 (any form of pyridoxal, pyridoxine,

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, composition 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 probably fair to

suggest that food chemistry is the chemistry 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 funda-

mental 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 transfer,

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 correlate water with special properties such as perishability,

shelf life, mobility, smell, appearance, color, texture, and taste.

Water as a Common Component of Food

Water is a food as well as the most common component 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, C O, NH, and polar

groups. These sites strongly interact with water molecules by

hydrogen bonding and dipole-dipole interactions. Furthermore,

dipole-ion, hydrophilic, 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 molecules next to them. There is no clear

boundary for affected and unaffected water molecules. Yet it

is convenient to divide them intobound waterandfree water.

This is a vague division, and a consensus definition is hard to

reach. Fennema and Tannenbaum (1996) give a summary of var-

ious 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.

Water Activity

Interactions of water and food molecules mutually change their

properties. Water in food is not pure water. Water molecules in