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OBJECTIVES

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The learner will be able to --

draw and describe the structure of water and the manner it influences
crystallization, solvation, and activity and other colligative properties.
describe the structure of water and the importance of phase transition.
differentiate hard and soft water.
differentiate free and bound water.
describe ways that hardness, activity, and bonding of water influences food quality.
list the roles of water in influencing food quality.
discuss the manner in which water affects appearance, texture and flavor of foods.

The chemical nature and physical structure of water determine the properties of raw foods and the nature of the changes that take place during food processing and preparation. In milk, Tang, and tea, water serves as a primarily as a solvent. It is also a dispersion medium for the emulsified fat in the milk. In bread, it assists in the formation of gluten and in the gelatinization of starch. Many food processes and treatments are designed to add or remove water, or change the physical state of the water. For example, in ice cream, we remove energy to change the dominant liquid into a dominant solid.

Thus, water can take on a variety of roles, each of which depends on water's unique characteristics and the food it is in. The following are examples of selected roles:

At least two types of chemical reactions are of importance to food scientists. In the most simple one, water acts as a solvent, or dispersing agent, bringing two reactants together. For example, baking powder in the can does not react. Once water is added, the sodium bicarbonate and the acid salt dissolve and react to form carbon dioxide and by-products. A second type of chemical reaction is such as where wheat gliadin and glutenin are brought together and these proteins and water change the compound to gluten.

	Water's ability to serve as a universal solvent is discussed in the solutions section. This ability has tremendous impact on its other characteristics. For example, the fact that water serves as a solvent sugar and tea impacts the osmotic pressure that solution.
	Water's ability to freeze in tiny crystalline particles contributes to the structure of emulsions such as ice cream.
	serves as a heat transfer medium:When cooking peas, water in the pan absorbs heat energy from the heat source and releases it to the peas.
	Due to its polarity, water is a cleansing agent. Polar matter attaches to water molecules and is dispersed and carried away from the surface it originated on.
	Water facilitates reactions between acids and bases.
	Water is the dispersing medium in numerous food mixtures. Examples are cake batter, gravy, and soups.

What is the probable role(s) of water in a can of beer?

What is the probable role(s) of water in the preparation of a sherbet?

What is the probable role(s) of water in heating a cup of instant coffee?

The diagram below gives an indication of the relative affect on the water molecule of the degree of bonding. As noted, if steam the molecules show no evidence of intermolecular hydrogen bonding to like water molecules. As you note, as energy is removed there is an increasing amount of hydrogen bonding until, ice, shows evidence of rigid structured bonding.

Water is an important constituent of most foods with each food having its own characteristic content. The range of values summarized by the table to the left indicates water may play differing roles as to its importance in a foods appearance, texture, or flavor of the food. Reviewing this differing percentage of water and correlating it to its quality characteristics is particularly interesting.

The chemical nature and physical structure of water give this component of food materials great significance in determining the properties of raw foods and the nature of the changes that take place during food processing and preparation. Water is an unusual compound with unusual properties. The water molecules, which is made up of two hydrogens and one oxygen, or H2O. However, there are some 3 different components -- because of isotopes, of possible oxygens(6) and hydrogens(3). Water is visualized as occupying the center of a tetrahedron, a solid with four faces.

THREE STATES OF MATTER

On this planet, water is the only substance that occurs abundantly in all three physical states. It is our only common liquid and is our most widely distributed pure solid, being ever-present somewhere in the atmosphere as suspended ice particles or on the earth's surface as various types of snow and ice. Of course, it is well known that water is found in liquid state in our oceans, lakes and rivers, and streams. We even find water in the third state in nature -- as steam.

In the crystalline state we know that water shows a definite pattern of hydrogen bonding, the volume of water expands as the temperature drops from 4C to 0C. At OC, crystals are formed due to the heat removal. This requires 80 calories of heat to be removed per gram of water to form this crystalline network. The water molecules align and form this lattice partly due to its dipole moment. If the crystalline water molecules have energy supplied back and water becomes more "fluid" and becomes a liquid. The calories required to change the state from solid to liquid or from liquid to gas are the latent heats.

Because of water's ability to engage in three-dimensional hydrogen bonding, it does have unusually large values for heat capacity, melting point, boiling point, surface tension, heats of fusion, vaporization, and sublimation--all related to the extra energy needed to break intermolecular hydrogen bonds. This is important from processing and food preparation standpoint.

Physical Constants Molecular weight	18.0153
Melting point at 1 atm	0.00 C
Boiling point at 101.3 kPa atm	100.00C
Heat of fusion at 0 C	6.012 kJ (1.436 kcal)/mol
Surface Tension at 0C

Phase transition properties Heat of vaporization at 100 C	40.657 kJ (9.711 kcal)/mol
Heat of sublimation at 0 C	50.91 kJ (12.16 kcal)/mol

The affect of dispersed particles upon the colligative properties in different dispersion systems. It can most accurately be summarized as follows:

Colligative Property	Affect of Dispersed Particles
	Solutions	Colloids	Suspensions
Boiling Point	significantly affected by solute	slightly affected by dispersed particle	no affect of dispersed particle
Freezing Point	significantly affected by solute	slightly affected by dispersed particle	no affect of dispersed particle
Water Activity	significantly affected by solute	slightly affected by dispersed particle	no affect of dispersed particle
Osmosis	significantly affected by solute	slightly affected by dispersed particle	no affect of dispersed particle

Solutions, unsaturated, saturated, supersaturated solutions. In each case, these are defined by the amount of solute in the dispersing substance, usually water in foods, at a specific temperature. The latter phrase, ...at a specific temperature is critical for understanding. That is, solutions can be defined as saturated by adjusting the temperature. For example, in the making of fondant, at 115C a saturated solution of sugar and water is prepared. This identical solution becomes supersaturated when carefully cooled without stirring to 40C. However, when a solution is supersaturated it is very unstable and if any nuclei are formed (whether by 'seeding or agitation) crystallization will occur.

This diagram emphasizes the role of solute on the ability for bacteria, yeast and molds to grow. Generally, the lower the water activity the less likely these microorganisms are to grow.

The affect is consistent with added solute. Boiling point is influenced by added electrolytes with a nonionizing electrolyte decreasing the freezing point of water -1.86C for every gram molecular weight per liter and increasing the boiling point 0.52C above a hundred for every gram molecular weight per liter.

Another concept is to recognize that dissolved substances make water impure in a chemical sense. This happens in nature as can be seen in the above colored bank from Yellowstone; the coloring is caused by the dissolved substances. In discussing the dissolved substances, we talk about the hardness of water.

The hardness of water is due to calcium or magnesium salts which may make the water temporarily or permanently hard depending on whether they are present in the form of soluble carbonates or sulfates. Although food scientists are minimizing these concepts of degree of hardness in scientific world, it is still used for a practical application format. Temporary hard water contains soluble carbonates such as calcium bicarbonate or magnesium bicarbonate as they are fairly easily eliminated by boiling the water. This can be easily seen by looking at the bottom of a double boiler where often a grayish-tan layer of sediment is accumulated.

Permanently hard water contains either calcium sulfate or magnesium sulfate and is expressed in parts per million or in units called "grains," with one grain equivalent to 0.064 gram of calcium carbonate. Probably the big difference as far as food preparation is concerned is that, whereas the temporary hard water can have the ions easily removed, the permanent hard water requires an ion exchange. Washing soda is one way to remove the ions from the water. Foods where hardness is important are items such as pears, cranberry juice, canned apples, pies, fondant, and other foods, the hard water may serve to firm or soften, or color or discolor the food.