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 93

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

above the critical point issupercritical water. In the seventeenth

century, Denis Papin (a physicist) generated high-pressure steam

using a closed boiler, and thereafter pressure canners have been

used to preserve food. Pressure cookers were popular during the

twentieth century. Pressure cooking and canning use subcriti-

cal water. Plastic bags are gradually replacing cans, and food

processing faces new challenges.

Properties of subcritical and supercritical water such as di-

electric constant, polarity, surface tension, density, viscosity,

and others, differ from those of normal water. These properties

can be tuned by adjusting water temperature. At high tempera-

ture, water is an excellent solvent for nonpolar substances such

as those for flavor and fragrance. Supercritical water has been

used for wastewater treatment to remove organic matter, and this

application will be interesting to the food industry if companies

are required to treat their wastes before discharging them into the

environment. The conditions for supercritical water cause poly-

mers to depolymerize and nutrients to degrade. More research

will tell whether supercritical water will convert polysaccharides

and proteins into useful products.

Supercritical water is an oxidant, which is desirable for the

destruction of substances. It destroys toxic material without the

need of a smokestack. Water is a “green” solvent and reagent, be-

cause it causes minimum damage to the environment. Therefore,

the potential for supercritical water is great.

AQUEOUS SOLUTIONS

Water dissolves a wide range of natural substances. Life began

in natural waters,aqueous solutions, and continuation of life

depends on them.

Water, a polar solvent, dissolves polar substances. Its po-

larity and its ability to hydrogen bond make it a nearly uni-

versal solvent. Water-soluble polar substances arehydrophilic,

whereas nonpolar substances insoluble in water arehydropho-

bicorlipophilic.Substances whose molecules have both polar

and nonpolar parts areamphiphilic. These substances include

detergents, proteins, aliphatic acids, alkaloids, and some amino

acids.

The high dielectric constant of water makes it an ideal solvent

for ionic substances, because it reduces the attraction between

positive and negative ions inelectrolytes: acids, bases, and salts.

Electrolyte solutions are intimately related to food and biological

sciences.

Colligative Properties of Aqueous Solutions

Vapor pressure of an aqueous solution depends only on the

concentration of the solute, not on the type and charge of the

solute. Vapor pressure affects the mp, the bp, and the osmotic

pressure, and these arecolligative properties.