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Water is abundant in all living things and, consequently, is in almost all foods, unless steps have been taken to remove it. It is essential for life, even though it contributes no calories to the diet. Water also greatly affects the texture of foods, as
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Water
Introduction Water is abundant in all living things and, consequently, is in almost all foods, unless steps have been taken to remove it. It is essential for life, even though it contributes no calories to the diet. Water also greatly affects the texture of foods, as can be seen when comparing grapes and raisins (dried grapes), or fresh and wilted lettuce. It gives crisp texture or turgor to fruits and vegetables, and it also affects perception of the tenderness of meat. For some food products, such as potato chips, salt, or sugar, lack of water is an important aspect of their quality, and keeping water out of such foods is important to maintain quality. Almost all food processing techniques involve the use of water or modification of water in some form: freezing, drying, emulsification (trapping water in droplets or trapping oil in a water phase to give salad dressings their characteristic mouthfeel), breadmaking, thickening of starch, and making pectin gels are a few examples. Further, because bacteria cannot grow without water, the water content has a significant effect on maintaining quality of the food. This explains why freezing, dehydration, or concentration of foods increases shelf life and inhibits bacterial growth. Water is important as a solvent or dispersing medium, dissolving small molecules to form true solutions, and dispersing larger molecules to
form colloidal solutions. Acids and bases ionize in water; water is also necessary for many enzyme catalyzed and chemical reactions to occur, including hydrolysis of compounds such as sugars. It is also important as a heating and cooling medium and as a cleansing agent. Since water has so many functions that are important to a food scientist, it is important to be familiar with some of its unique properties. When modifying the water content of a food, it is necessary to understand these functions in order to predict the changes that are likely to occur during processing of such foods. Drinking water is available to the consumer in convenient bottled and aseptic containers in addition to the tap.
Chemistry of Water The chemical formula for water is H2O. Water contains strong covalent bonds that hold the two hydrogen atoms and one oxygen atom together. The oxygen can be regarded to be at the center of a tetrahedron, with a bond angle of 105 between the two hydrogen atoms in liquid water and a larger angle of 109 60 between the hydrogens in ice (Fig. 2.1). The bonds between oxygen and each hydrogen atom are polar bonds, having a 40 % partial ionic character. This means that the outer-shell electrons are unequally shared between the
V.A. Vaclavik and E.W. Christian, Essentials of Food Science, 4th Edition, Food Science Text Series, DOI 10.1007/978-1-4614-9138-5_2, # Springer Science+Business Media New York 2014
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H
H 150° Water
Oxygen
Water
Hydrogen Bond
Oxygen
190° 6’ Ice
Covalent Bond
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Fig. 2.1 Bond angle of water and ice
oxygen and hydrogen atoms, the oxygen atom attracting them more strongly than each hydrogen atom. As a re
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