Water: TheUniversal Solvent

Water is a vital resource. It plays a tremendousrole in everything from chemical reactions to foodproduction and processing to health and nutrition.

Chapter 7

Few things necess~ry for life ~re asunderr~ted ~nd overlooked ~s w~ tel'. It is theonly substance in nature found in abund~nce

in ~ solid, liquid, and gas state. People oftenmarvel at the beauty of a snowflake ~nd thedi~mond-like glitter of ice. However, few~ppreci~te accumulations of snow th~t downpower lines, close roads, and damage treesand property. People are seldom amazed bycrop dalllage caused by freezing \Ivater inplant tissue, either.

..

Objectives

After studying this ch~pter,

you will be able to

describe how the structure of a water mole­CLue affects water's physical ch~r~cteristics.

demonstrate a function of water in foodpreparation.

explain how the water content of ~ foodaffects how the food will react during prepa­ration ~nd storage processes.

identify four functions of w~ter in the body.

list COlnn1on contanLinants in \·vater.

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Key Terms

nutrient

nonpolar cov~lent

bond

pol~r covalent bond

hydrogen bond

intermolecul~r

surface tension

~tmospheric pressure

impurity

free w~ter

bound water

hydrate

~J1hydrous

\<\Iater activity

hydr~ted

contamin~nt

pollut~nt

Chapter 7 Water: The Universal Solvent

Water is a nutrient. Nutrients are foodcomponents necessary to sustain life. Thereare six groups of nutrients. Besides water, thenutrient groups are carbohydrates, fats, pro­teins, vitamins, and minerals. You will studythe chemistry of each of these nutrient groupsin later chapters in this book.

Water is the main component of manyfoods. Water influences the texture, appear­ance, and taste of food. Nutritionally, it helpscontrol your body's temperature. It also trans­ports nutrients and wastes and provides thesolution for hundreds of chemical reactions. Itis an important heat medium in cooking andthe main ingredient in beverages. Withoutwater, cleanup and sanitation would beextremely difficult. It is critical to all forms offood preservation and its presence or absencedetermines a food's likelihood of spoilage.Where there is water, there is life.

The Structure of WaterWater is chemically composed of one oxy­

gen atom and two hydrogen atoms. It is a rel­atively small compound held together bycovalent bonds. Compounds such as methane

145

(CH4) and ammonia (NH3) are similar to waterin size and"atomic construction. However, thephysical properties of water are quite differentfrom these compounds. Water has highermelting and boiling points, more surface ten­sion, a lower density, and a greater ability toconduct energy. These characteristics arecaused by the nature of the molecule.

Types of Covalent BondsWater molecules are unique because of

how the electrons are shared between the oxy­gen and hydrogen atoms. Covalent bondsshare electrons in two ways: equally andunequally. Methane (CH4), hydrogen (H2), andoxygen (02) are examples of molecules thatshare electrons equally. The protons and elec­trons within the atoms have opposite electri­cal charges or poles. This is similar to theearth's opposite ends or poles. When the elec­trons are shared equally, these oppositecharges balance or neutralize each other. As awhole, the molecule does not have an electri­cal charge. A nonpolar covalent bond is elec­trically neutral because the electrons areshared equally. That is, they spend the sameamount of time orbiting the nucleus of eachatom. See 7-1.

Nonpolar Covalent Bonds of a Hydrogen Molecule

1. The electrons of two hydrogen atoms repel orpush away from each other.

3. The proton of one atom attracts the electron ofanother atom. The outer orbital prefers to havetwo electrons.

2. The protons of two hydrogen atoms also repeleach other.

-1

4. In covalent bonds, each electron travels in theorbitals around both nuclei.

7-1 A hydrogen molecule (H2) is electrically neutral because the electrons are equally shared between thetwo hydrogen atoms.

Polar Covalent Bond of a Water Molecule

hydrogen bonds

Hydrogen Bonds Among Polar Molecules

Unit II Basic Chemistry

7-3 The positively charged hydrogen end of onewater molecule is attracted to the negativelycharged oxygen end of another water molecule.

Surface TensionHydrogen bonds give water a greater sur­

face tension than most compounds. 511ljacetension refers to the force between moleculesat the outside edge of a substance. If you over­fill a glass slowly, the water will form a convexsurface that rises above the rim. The force ofthe hydrogen bonds keeps water fromspilling. The water will spill over only whenthe force of gravity exceeds the force of thehydrogen bonds. It is impossible to observethis phenomenon with alcohol. Alcohol lacksthe hydrogen bonds that would allow it to riseabove the level of the container. This demon­stration is also an example of the cohesivenature of watet~ or its ability to cling to itself.

The polar nature of water causes water toappear to climb the sides of a container. Thewater forms an upward curve or "smile"shape on its top surface. This curve of the topsurface, as you learned in Chapter 2, is called

each person's axms to his or her torso. It iseasy for the two people to separate by drop­ping their hands (breaking the hydrogenbond). Jt is much harder (and more danger­ous!) to separate a person's arm from his orher body (breaking a covalent bond).

hydrogen atom

+

104.5°

hydrogen atom

+

oxygenatom

146

7-2 Because the oxygen and hydrogen atoms in awater molecule share the electrons unevenly, adivision of charge is created. The oxygen endbecomes negatively charged and the hydrogenend becomes positively charged.

Hydrogen BondsLike d1arges repel aJ1d opposite charges

attract. Therefore, the positive end of onewater molecule is pushed away from the pos­itive ends of other water molecules. However,the positive hydrogen end of one water mole­cule is drawn toward the negative oxygenends of other water molecules. This attractionis called a hydrogen bond. See 7-3.

Hydrogen bonds are intermoleCillar, orbetween molecules. Intermolecular bonds aremuch weaker than the covalent bonds withina molecule. You can think of two molecules ofwater as two people. Think of a hydrogenbond as the two people holding hands. Thinkof covalent bonds as the structure that holds

The water molecule has a V shape with theangle between the two hydrogen atoms being104.5°. This placement causes the electrons tobe shared unequally. The larger oxygen nucle­us tends to pull the shared electron toward theoxygen atom with greater frequency than thehydrogen atom. This polarizes the molecule, orcauses it to develop regions of opposite elec­trical chaxge. The hydrogen end tends to beslightly positive and the oxygen end slightlynegative. When a molecule with covalentbonds has a polar nature, its bonds aTe calledpolar covalellt bonds. A polar covalent bondmeans there is an unequal sharing of electronswithin the molecule. See 7-2.

Chapter 7 Water: The Universal Solvent

a meniscus. The meniscus is caused by water'sadhesive property. An adhesive clings to some­thing else as well as to itself. The curve formsbecause the adhesion between water and thecontainer is stronger than the cohesion amongwater molecules. This characteristic of wateris more obvious in narrow tubes like buretsthan it is in larger containers.

The increased surface tension of wateraccolli1ts for its higher melting and boilingpoints as compared to other liquids. It takesmore energy for polar molecules than for non­polar molecules to break free from a rigidsolid like ice. It also takes more energy for apolar molecule to break out of a liquid.Water's great surface tension also explainswhy water freezes faster than it thaws.Hydrogen bonds can easily form and holdwater molecules into a rigid structme. It ismore difficult for hydrogen bonds to be bro­ken so water molecules can slide by oneanother freely.

The push and pull among water moleculesresults in them forming a tetrahedral (four­sided pyramid) shape when ice is formed.This shape is a result of three-dimensionalhydrogen bonding. Water molecules havegreater space between them when frozen thanwhen in liquid form. This is due to therepelling of like charges. The hydrogen bond­ing creates microscopic holes in ice much likeair holes in foam. This is why water is W1usualin its density. Most substances become moredense as they move from the liquid to solidstate. Ice (solid water) becomes less densethan water (liquid). This is why ice cubes floatin water. Water molecules are most dense at3.98°C (39.WF).

Hydrogen bonding can also occurbetween water and other polar molecules.Some food components that commonly formhydrogen bonds with water are sugars, stard1es,and proteins. Water's ability to hydrogenbond to other polar molecules helps sub­stances like sugar dissolve and stay distrib­uted in water. See 7-4.

Pressure, Temperature, and PhaseChanges

Imagine you could see a tall colullm ofatoms of air rising from the earth's surface.

147

•

7-4 The sugars in jelly, jam, syrups, and molassesstay distributed due to hydrogen bonds with watermolecules.

These atoms have a weight, whicl1 is deter­mined by the pull of the earth's gravity. Theforce of this weight pressing down on a sur­face is called atmospheric pressure (atm).

Gases in the air are pressing against yourbody all the time. Therefore, you are notlikely to be aware of atmospheric pressure.However, it is similar to the pressure thatwater puts on your body when you dive intoa pool. The weight of the water pushes in onyou all over your body. The deeper you go, thelarger this force, or pressure, is on the smfacearea of yom body.

Atmospheric pressure is measured basedon the average pressure of the atmosphere onan object at sea level. At sea level with stan­dard conditions, atmospheric pressme ismeasured as 1.0 atm, which equals 14.7pounds per square inch. What will happen toatmospheric pressure as you drive into themOW1tains or take off in an airplane? As alti­tude, or height above the earth increases, theCOlUllli1 of air above you becomes smaller.This means there is less atmospheric pressure.In Denver, Colorado, which is 1.61 km (1 mile)above sea level, the average pressure is 0.83atm. Likewise, at locations below sea level, thecolullm of air above you becomes larger andatmospheric pressure increases. At 0.09 km(282 feet) below sea level, Death Valley inCalifornia has an average pressure of 1.01 atm.See 7-5.

The boiling point of a substance is the pointat which it changes from a liquid into a gas.

148 Unit II Basic Chemistry

Atmospheric Pressure

7-5 The column of atmosphere pressing down on a surtace decreases as elevation above sea levelincreases.

the Dead Sea, which is 392 m (1,286 feet)below sea level, water boils at about 101°C(214°F). See 7-6.

Understanding the effects of atmosphericpressure on the boiling point of water can ben­efit all cooks. These concepts explain how apressure cooker works. Pressure cookers havea steam-tight seal. As steam is released insidethe sealed cooker, pressure builds. The waterin the cooker then has to have more energy,and thus a higher temperature, to boil. Thehigher temperatures cause foods inside apressure cooker to cook faster. For instance, at15 pounds of pressure above 1 atm, the boil­ing point of water will be 121°C (250°F). A3-pound pot roast cooked at this pressure willbe done in about 45 minutes. This roast wouldtake two to three hours to cook conventionally.

When water reaches its boiling point, watermolecules in the liquid are escaping into theair as steam.

As atmospheric pressure changes, the boil­ing point of water changes. When there is lessatmospheric pressure on the water, the mole­CLues need less energy to escape. When thereis more ahmospheric pressure on the water, themolecules need more energy to escape.Remember that temperature is a measure ofenergy. Therefore, at high altitudes, where theahmospheric pressure is lower, water wiD boilat lower temperatures. At low altitudes, wherethe atmospheric pressure is higher, water wiDboil at higher temperatures. At sea level, waterboils at lOOoe (212°F). At the top of MountMcKinley, which is 6,194 m (20,320 feet) abovesea level, water boils at about 78°C (173°F). At

Chapter 7 Water: The Universal Solvent

High-Altitude Baking

149

Lower atmospheric pressure can affect cooking procedures for people who live 1,000 m (3,300 feet) ormore above sea level. At this altitude, water boils at about 96°C (205°F). Therefore, foods simmered orboiled in water take longer to cook. This change in boiling point also has the following effects on bakedgoods:

.:. With less atmospheric pressure pushing down on the batter or dough, breads and cakes rise higherand have larger air cells. This can result in products that either become dried out and coarse intexture or collapse during baking.

•:. Liquids evaporate faster, resulting in dryer products or higher fat and sugar concentrations.

•:. Cakes can be underdone as a result of the lower boiling point.

To some extent, high-altitude baking requires trial and error. However, the following tips may help youconvert standard recipes for use at high altitudes:

.:. Use 5% more flour. This will slow the leavening action.

•:. Use up to 200/0 more water. This helps balance the rapid evaporation and drying.

•:. Increase the oven temperature by 25°F and reduce baking time by about 20%. This will speedcrust formation and help prevent overflow of batter.

•:. If increased oven temperatures will scorch the edges of the batter, try reducing sugar content by30 to 45 mL (2 to 3 tablespoons) per cup. Cook products at the recommended temperatures.

•:. Bake yeast breads just before they double in size or reduce yeast by 20%.

•:. If a recipe calls for whipped egg whites to be folded in, beat the egg whites just to the soft peakstage. This allows for the extra expansion of the egg whites and will keep them from bursting.

•:. Reduce the leavening agent (baking soda or baking powder) by 15 to 60%, depending on youraltitude.

7-6 Cooks need to make several adjustments when preparing foods at high altitudes.

Even if you do not have a pressure cooker,you can increase the boiling point of water.Placing a lid on a pot will increase the boilingpoint by as much as SoC to 6°C (lOop to 12°P).The heavier the lid is, the greater the increasewill be. The increase depends on the pressureneeded to push up the lid.

The freezing point of water is also affectedby atmospheric pressure. At sea level, waterfreezes at DoC. The changes in freezing pointare insignificant in most circumstances.

Impact of Impurities in WaterSo far, you have looked at the characteris­

tics and structure of pure water. It is importantto remember that water's structure makes itan excellent solvent. Because so many sub­stances dissolve in water, it is usually not inpure form. Anything that is added to watercauses it to be impure. These ilnpurities, orsubstances other than water, will affect theway water reacts.

Impurities are not necessarily unsafe. Saltand microorganisms are both impurities thatcould be found in water. Sharing a water glasswith someone who has been eating potatochips would transfer a harmless amount ofsalt. However, sharing a glass with someonewho is sick could transfer enough micro­organisms to cause illness.

Substances present in or added to watercan change water's physical and chemicalcharacteristics. Impurities can change water'sflavor, color, boiling point, freezing point, andhydrogen bonding. The amount of change willdepend on the amount and kind of impurity.

Hot tap water may contain more impur­ities than cold tap water. The heat can causeimpurities such as calcium and iron depositsin pipes and water heaters to dissolve into thewater. These impurities can produce unplea­sant flavors in foods and beverages. To avoidoff-flavors in hot beverages, prepare themwith cold tap water that you have heated.

f

150

Cooking TipPasta put in cold water or cooked at lowtemperatures will begin to dissolve into thewater before it is cooked. This causes thesurface of the pasta to become sticky andmushy. This can especially be a problemwhen cooking pasta at high altitudes,where the boiling point is below 100'C(212'F). Adding 5 mL (1 teaspoon) salt per1 L (1 quart) of cooking water will raise theboiling point O.5'C to 1.0'C (1 'F to 2'F).This higher temperature helps give pasta adesirable firm texture.

Functions of Water in FoodPreparation

VVater serves two main functions in thepreparation of food products. Water is animportant medium for transferring heat. It isalso a necessary ingredient for forming manyfood mixtures.

Heat MediumWater, in both its liquid and gaseous

states, is used to transfer heat energy intofoods. See 7-7. When water is heated, energyis transferred from the hea t source by bothconduction and convection. Pieces of foodsuspended in water have little effect on theboiling point of water. As a result, water boilsat a constant temperature. Gently boilingwater has the same temperature as rapidlyboiling ,·vater. However, energy is being trans­ferred at a lower rate with the gentle boil.

Whether you choose a rapid or gentle boilwill depend on the food you are cooking.Pasta, for example, needs a rapid boil. Thiswill keep the pasta in motion and prevent thenoodles from sticking together. Potatoes, onthe other hand, need a gentle boil. A gentleboil keeps the potatoes from banging into eachother in the cooking process. Potatoes thatbang about in the pan may break apart andbecome mushy on the surface.

Unit II Basic Chemistry

7-7 Steam from water is used to transfer the healneeded to cook these scallops.

Cooking with SteamTo escape from the liquid to gaseous state,

water molecules have to absorb much energy.Experin1ents have shown that 9.7 kilocaloriesof energy are needed to turn 1 mole H,O at100°C (212°F) into 1 mole of steam. In Chapter 5,you learned that this energy, called latent heat,does not change the temperature. Therefore,steam that is not under pressure will have thesame temperature as boiling water.

Jt will actually take slightly longer to cookmany foods in steam than boiling water. Thisis because steam is a poor conductor of heat.However, the steam can cook the food fasterthan boiling water if pressure is added. This isbecause the temperature of steam under pres­sure can be higher than tl1e boiling point.Cooking occurs as latent heat is released intothe food when the steam condenses on thecooler food surface.

Steam cooking has two main advantages.Steamed foods are more flavorful than boiledfoods. Foods cooked in steam will also bemore nutritious than foods cooked in water.This is especially true when the cooking liquidis discarded. Fewer flavor compolU1ds andnutrients are dissolved into the steam thaninto boiling water.

Chapter 7 Water: The Universal Solvent

Cooking TipSteam eggs for an alternative to poaching.Lightly coat a custard cup with nonstickcooking spray. Crack an egg into the cus­tard cup, being careful not to break theyolk. Place the custard cup on a rack orsteamer in a saucepan with gently boilingwater in the bottom. Cover the saucepanand steam the egg for 4 minutes. Removethe lid and top the egg with a slice of low­fat cheese. Cover the saucepan and steamthe egg for one minute longer. Serve thesteamed egg on a toasted English muffin.

The Universal SolventWater is called the universal solvent

because it can dissolve so many substances.Most substances in food other than fats andoils are dissolved in a water base.Beverages, candies, baked goods, soups,

151

stews, casseroles, and sauces are all mix­tures of substances dissolved in a waterbase, 7-8. Gases, liquids, and solids can allbe fOlmd in water solutions. Understandinghow water works in solutions is helpfulwhen preparing many types of foods.

Gas-in-Water SolutionsCarbonated beverages are examples of

gas-in-water solutions. Carbon dioxide is thegas solute dissolved in water. The first carbon­ated beverage was created by nature whereunderground water passed through limestonelmder pressure. This process is now copied inhigh-pressure tanks above ground. Every day,beverage manufacturers produce millions ofliters of carbonated water for use in softdrinks.

Another gas that dissolves in water is oxy­gen. Hot water is able to hold less dissolvedoxygen than cold water. When water isboiled, dissolved oxygen escapes into the

Place '/, cup water in asaucepan. Place vegetables on asteamer and then place thesteamer in the saucepan. Coverwith a lid and heat on high untilsteam begins to escape.(Starting the food on high heatwill get the surface of the food to100°C as qUickly as possibie.)

Wait two minutes and turn theheat down to low. (It takes abouttwo minutes for the steam topush all the air out of the pan.)Time the cooking from this point.Do not lift the lid until cooking is

done. When you lift the lid, coolair rushes in and the cooking willstop. The air has to be pushedout of the pan again before cook­ing can resume.

Once you have turned downthe heat, all you have to do ismaintain the temperature. This iswhere you save energy. At thispoint, high temperatures wouldforce steam out of the pan. Morewater then has to be turned tosteam whose energy is lost whenit escapes. When the heat isturned down, only a little energy

is needed to maintain the tem­perature of the trapped steamand boiling water.

Steam at 100°C has moreenergy than air at 100°C. This isbecause of the latent heat neededto turn boiiing water into steam.Foods wiil cook faster if theheated ai r is forced out of thepan so only steam (vaporizedwater) remains. This means thefood needs to be in a pan with asnug lid.

7-8 Sauces are among the many food products made from substances dissolved in a water base.

152

atmosphere. Water that has been boiled andallowed to cool will have a slightly flat taste.This flat taste is due to the lower dissolvedoxygen levels.

Dissolved oxygen causes ice cubes to becloudy. To create clear ice, boil water for sev­eral minutes, cool, and then freeze. lee cubesmade this way will also last slightly longer.This is because once the oxygen moleculeshave been boiled out, the water molecules canpack together more tightly. This creates denserice cubes.

Cooking TipFreezing water does not cause it to lose itsability to dissolve substances. Freezingtemperatures merely slow the ability ofwater to dissolve substances. Even thecleanest freezer will have a "freezer odor"that will dissolve into ice over time. Thelonger the ice stays in the freezer, the moreobvious the taste change is. You canreduce this off-flavor by rinsing ice cubesin water for a few seconds before use. Thiswill melt the outer layer where most of theodor has been dissolved into the ice.

Unit II Basic Chemistry

Storage TipCarbon dioxide needs high pressure tostay dissolved or suspended in water. Softdrinks have to be sealed to keep the car­bon dioxide dissolved in the water. You cansqueeze excess air out of partially emptiedplastic bottles and then replace the cap.This will increase the storage time beforean opened soft drink goes flat.

Gas solutes such as carbon dioxide arereleased at faster rates as temperaturesclimb. Soft drinks stored at high tempera­tures will go flat faster. (This is especiallytrue if the containers have been opened.)Therefore, you should avoid leaving softdrinks in your car on a hot summer day.

Liquid-in-Water SolutionsIt is possible to have some liquid other

than water dissolved or stirred into water.Most alcohoUc beverages are examples of trueliquid-in-water solutions. Another example isthe solution of vinegar and water used to pre­serve pickled foods. See 7-9.

Chapter 7 Water: The Universal Solvent

Historical HighlightHow Soft Drinks GotStarted

153

Joseph Priestley was anEnglish chemist. He wanted toimitate the bUbbling water ofmineral springs. He successfullydissolved carbon dioxide in waterunder pressure in 1772. This firstartificial mineral water containedsodium salts (sodium bicarbonateor sodium carbonate). The car­bonation was formed by addingacid to the sodium salts. Thename "soda" came from sodiumsalts.

In 1806, the first artificial sodawater in the United States wasmade and bottled. This was done

by Benjamin Silliman, a chem­istry professor at Yale.

Lemon-flavored soda waterbecame popular after 1830. Theflavors of ginger ale and rootbeer followed later. Cola flavoreddrinks are the result of medicinalsyrups made from the Kola nut.These syrups were added tounflavored soda water to makethem easier to swallow.

In the 1850s, soft drink com­panies sold bottled, flavoredsoda water. Most soda water wassold by "soda fountains." Thesefood counters were usually located

in the local drug store.The name "soft drinks" was

given to distingUish these bever­ages from "hard" alcoholic drinks.The name "pop" came aboutbecause of the noise the earlybottles made when the lids wereremoved.

The average number of softdrinks consumed per person peryear went from 12 in 1900 toabout 585 in 1998. Today, car­bonated beverages are the drinkof choice for most people in theUnited States.

"

Glasllaus. Inc.

7-9 The liquid used to pickle many of these foodsis a solution of vinegar in water.

Many other liquid mixtures, SUdl as fruitjuice concentrates, are sold in water solutions.These are further diluted by adding morewater.

Solid-in-Water SolutionsWater is frequently the solvent for many

solids in food preparation. Whenever a sub­stance is dissolved in water, the dlemicalproperties of the solution will differ fromthose of pure water.

Salt and Sugar SolutiollsThe two most common solids used in a

water solution are salt and sugar. Both sub­stances cause water to freeze at a lower tem­perature and boil at a higher temperature thanpure water. The more salt or sugar in the solu­tion, the lower the freezing point and thehigher the boiling point will be.

The addition of salt to ice is what makeshomemade ice cream possible. Ice creamfreezes at a lower temperature than pure

~I

154

water. Placing the ice cream solution in a con­tainer of ice will cause the solution to becomecold. However, it will not freeze. Salt wateralso has a lower freezing point than purewater. When salt is sprinkled on the ice, twothings happen. First, the ice melts and com­bines with the salt to form salt water. Second,heat energy is pulled from the ice cream solu­tion and the ice, lowering the temperature.This transferred energy is needed for the saltcrystals to break their bonds and dissolve inthe water. The result is salt water with a tem­perature below O°C (32°F) and frozen ice cream.

As salt and sugar concentrations increase,the boiling point increases. High salt levels arenot desirable in most food products.Therefore, the amow1t of salt added will gen­erally increase the boiling point by only a fewdegrees Celsius. Salt is often added at a ratioof no more than 5 grams per liter of water.

Sugar is used in food products in mud1higher concentrations than salt. Sugar incandy syrups can have a ratio as high as fourparts sugar to one part water. This results in awide range of possible boiling points for sugarand water solutions. As the concentration ofsugar to water increases, so does the boilingpoint. The chemical interaction of sugar inwater causes the temperature to climb as thesolution is boiled. The next chapter will lookat how important the boiling temperature of asugar solution is to candy making. See 7-10.

7-10 The candy industry makes use of the char­acteristics of sugar and water solutions to developa wide range of sweet treats.

Unit II Basic Chemistry

Ten nl/(f CoffeeBecause water is a solvent, it will dissolve

flavor compounds from tea leaves and coffeegroW1ds. Changing the amount of tea leavesor coffee beans in proportion to the watervaries the strength of tea or coffee. Increasingthe brewing time will also increase thestrength of the t a or coffee flavor. However,increasing the brewing time will allow morepolyphenols to be dissolved in the water.

Some po!ypiIwo!s are compounds that cancreate a bitter aftertaste in tea or coffee. Thesepolyphenols can cause coffee prepared in apercolator to be more bitter than coffee pre­pared in a drip coffeemaker. This is becausepercolated coffee is brewed for 7 to 15 min­utes, which allows more polyphenols to beextracted.

Polyphenols are also present when tem­peratures used to brew tea and coffee arehigher than recommended. Proper brewingtemperatures for tea and coffee allow maxi­mlU11 flavor extraction without the develop­ment of a bitter aftertaste. The ideal brewingtemperature is just below the boiling point.

Water Content in Foods

It is important to understand water andhow it reacts during food preparation andstorage. This is because water is a major partof most foods. For instance, many fruits andvegetables are over 70% water. Even foodsthat are often considered dry contain water.Bread is 36% water, and popcorn contains 4%water. See 7-11.

VVater in food becomes part of the struc­ture of a food in three main ways. These arefree, bow1d, and as a hydrate.

When you cut into a grapefruit and juicesqUirts on your face, you have been hit withfree water. Free wnter is easily separated fromfood tissues. It can be pressed or squeezedfrom the food and often contains dissolvedcompounds. During processing, free waterwill easily boil or freeze. Free water also read­ily evaporates when foods are dried.

BOlllld wnter is tied to the structure oflarge molecules. Because of the small size andpolar nature of water molecules, they slideinto spaces between larger 11101ecules. V\'ater

Chapter 7 Water: The Universal Solvent 155

/\ .,./\ . i\ . /~ '!f.oq -\eiU - .ru -.:;;;;;~': . i;;ttr ~,,,'...,..... '<.,

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1 Item of InterestThe Solution to Brewing thePerfect Cup

Brewing Temperatures

different types of tea, Green teais not fermented or aged, Oolongtea is aged for a short time, andblack tea is aged for a longerperiod, As the tea leaves age,chemical changes occur, The dif­ferent compounds present as aresult of aging dissolve best atdifferent temperatures,

~ )...".

Whether your preference istea or coffee, the principles forsuccessful brewing are similar,The goal is to get the right flavorcompounds into the water andkeep unpleasant ones out of thesolution,

Drip coffeemakers releaseboiling water over coffeegrounds, By the time the waterhits the grounds, it has cooled tothe ideal temperature range, Thewater picks up coffee flavor com­pounds as it pours over thegrounds and drips to the potbelow, The coffeepot sits on awarmer to maintain the tempera­ture, Drink coffee immediatelyafter brewing for best flavor.Coffee flavor compounds are

extremely volatile and quicklyescape from the pot Extendedwarming causes coffee to developbitter flavors,

For tea, pour the boiling waterover the tea leaves or tea bags ina nonmetal teapot (Tea reactswith some metals reSUlting in ametallic taste to the tea,) Coverthe pot with a thick towel to helphold in the heat Let the potstand for 3 to 5 minutes,Maximum flavor is extracted in5 minutes, Longer brewing pullsbitter polyphenols into the tea, Toget a stronger tea, increase theamount of tea leaves not thebrewing time,

Recommended brewing tem­peratures vary somewhat for

Beverage

Coffee

Green teas

Oolong teas

Black teas

°C (OF)

85-96 (185-205)

82-93 (180-200)

91-99 (195-210)

93-99 (200-210)

Remove water from heat, then add coffee or tea,

The Procedure

Start with cool water not hot tap water,

Use freshly boiled water.

The Brewing Process

The Science

Hot water dissolves more metals and impuritiesfrom pipes and water heaters, This can increaseoff flavors from the water.

Extended boiling or boiling, cooling, and then boil­ing again causes a loss of dissolved oxygen,Water that is heated repeatedly tastes flat

The ideal brewing temperature is hot enough toextract the compounds responsible for coffee andtea flavor, If temperatures are too low, too fewcompounds are released to produce good bever­age flavor, If temperatures are 100 high, billerpolyphenols are released into the brew,

II!

I

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156

broccoli (cooked), 91 % water

cheese pizza (baked), 45% water

7-11 Water is a component of many food products.

Unit II Basic Chemistry

eggs (raw), 75% water

brownies, 10% water

hydrate of caffeine

The dot represents the hydrogen bondingbetween the two molecules. Hydrates canhave more than one molecule of waterattached to each molecule of the compound.

When the hydrate is heated, the water isdriven off. Compolmds that have the waterdriven off are allhydrolls, or free of water.

Food scientists know there is a relation­ship between water content and food perisha­bility. However, water content alone will notpredict how perishable a food is. Water that isbound to other compounds is not as available

...I..

molecules then form hydrogen bonds with thelarger molecules, such as protein and starchmolecules in fruits, meats, and vegetables.Bound water does not easily freeze or boil likefree water. It chemically reacts as part of thelarger molecule to which it is bound.

Much of the water in plant and animal tis­sue is inside the cells. Even when food tissueis cut or torn, the water does not flow out ofthe cells. This water is known as a hydrate. Ahydrate is any chemical compound that isloosely bound with water. Hydrates can haveone Or more water molecules bound to thecompound. For example, in its natural state,caffeine has one molecule of water attached tothe caffeine molecule. The molecular formulafor the hydrate of caffeine is written as follows:

c.,ffl'incmoliXull:'

watermol~><:ul('

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Chapter 7 Water: The Universal Solvent

to aid in food spoilage. Carrots and wholemilk are both 88% water. You could safely eata carrot that had been left at room temperaturein a lunch bag. A large amount of the water inthe carrot is bound to starch molecules. As aresult, carrots are solid. Milk is a liquid withmostly free water. Therefore, an uninsulatedcontainer of milk in the same lunch bag wouldno longer be safe. Bacteria may have multi­plied in the milk, causing it to spoil. The dif­ference is related to water's interaction withother compounds.

The U.S. federal government regulationson good food manufacturing practices includewater activity values. Water activity (Aw) isthe measure of the partial water pressure overa food. This is compared to the vapor pressure(gaseous water) over pure water at a giventemperature. Water activity takes into accountthe water available to support activity ofenzymes and growth of microorganisms.Water activity is measured from a to 1. Thehigher the number is, the more perishable thefood will be. Safe water activity levels havebeen identified for various processes, such asmaking concentrates and dehydrating fruitsand vegetables. See 7-12.

Water Activity of Common Foods

Foods

157

Functions of Water in the Body

Water is necessary for many body func­tions. Maintaining body temperature is onemain function of water. When your body tem­perature begins to rise, water is released to thesurface of your skin. You perspire. To evapo­rate, water needs energy. The nearest source ofenergy is you. The extra heat energy is pulledinto the water. As the water evaporates, itpulls the excess heat away from your body.

Transporting nutrients is a second func­tion of water in the body. Minerals and water­soluble vitamins must first dissolve in waterbefore they can be transported to where theyare needed. Water also helps transport wasteproducts out of the body.

A third function of water is to serve as areactant in metabolism. Metabolism is the com­bination of chemical and physical processesthat happen within the cells of the body.Chemical reactions are needed to produce,transfer, and store energy as well as to makenew cells. Many of these chemical reactionsrequire water as one of the reactants. Forexample, your body uses water to break downthe energy nutrients during digestion.

1.0-0.95

0.95-0.91

0.91-0.87

0.87-0.80

0.80-0.75

0.75-0.65

0.65-0.60

0.50

0.40

0.30

0.20

Canned fruits, vegetables, meat, fish, milk, juices

Hard cheeses, cured meat, some fruit juice concentrates, foods with 55°/c.sugar or 12°/c. salt content

Salami, sponge cakes, dry cheeses, margarine, foods with 65% sugar or 150/0salt content

Most fruit juice concentrates, sweetened condensed milk, chocolate syrup,maple syrup, flour, rice, fruitcake, country style ham, fondants (candy creamcenters)

Jam, marmalade, marshmallows

Rolled oats, fudge, jelly, molasses, dried fruits, nuts

Dried fruits with 15-20% moisture, toffee, caramel, honey

Pasta, spices

Whole egg powder

Cookies, crackers, bread crusts

Whole milk powder, dried vegetables with 5°/c. moisture, cornflakes

7-12 The water activity of a food affects the food's perishability. Foods with high water activity can bettersupport enzyme activity and microorganism growth.

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158

A fourth function of water is to becomepart of body tissue. The body is composed ofabout 60% water. The exact percentagedepends on the ratio of bone, muscle, andfat. All body fluids are also largely com­posed of water.

Meeting Your Body's Water NeedsMost people know they need six to eight

glasses of water a day. Many of you know thatyou can live much longer without food thanwithout water. This may lead you to wonderhow many people who "never" drink watercan survive? Think back to what you readabout water content in foods. Beverages aremostly water, as are soups and sauces. Yourbody can take water as well as nutrients andcalories from the many foods you eat. Yourbody gets the water it needs from the foodsyou eat and the beverages you drink. Yourbody also gets water as a by-product of chem­ical reactions.

You should drink water rather than justgetting water as a part of other foods for twomain reasons. The first is that water by itselfhas no calories. If you drink four 12-ouncecans of regular soft drinks, you will have con­sumed about 600 calories worth of sugar. Thedrinks will make it harder for you to maintaina healthy weight. Their calories will replacecalories from foods that provide all the othernutrients you need for good health. See 7-13.

Although diet soft drinks provide few ifany calories, they are not a good substitute forwater either. Carbonated soft drinks are asource of phosphorus. Too much phosphorus

7-13 Plain water meets your body's fluid needswithout providing calories.

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Unit II Basic Chemistry

in the diet can keep the body from absorbingsome other nutrients, including calcium.Drinking plain water does not have such neg­ative effects on your body's nutrient balance.

There is a second reason not to rely onlyon foods for water. Many foods that are highin water are also high in either sugar or salt.Most people in the United States consume toomuch sugar and salt. A diet high in thesecomponents is a risk factor for many diseases.These diseases include obesity, heart disease,11.lgh blood pressure, and diabetes.

The Role of ThirstThirst is the one way your body lets you

know you need water. However, thirst is notan accurate warning system. Yow' body needswater before you feel thirsty and after yourthirst appears to be quenched. It seems thatthirst does not occur until water supplies arealready short. There is a delay between theneed for water and the feeling of thirst.

Water is critical to almost all body hmc­tions. A shortage of water can lower energy lev­els, reduce coordination, and begin to damagebody tissue. It is best to conSLU11e water regu­larly tlu'oughout the day before t1Urst begins.

Ice water relieves thirst faster tl1an warmwater. Most of you know that ice water feelsmore refreshing. It is a faster thirst quencherbecause it cools the stomach. This causes thestomach to const.rict and forces the water intothe bloodstream at a faster rate.

Should you choose water or a sweetenedbeverage when thirsty? Water is better thansoda or fruit drinks. This is especially truewhen you are taking part in moderate physi­cal activity lasting less than an hour. In thenext chapter, you will see how water is neces­sary to digest sugar. If you consume sweet­ened soft drinks, part of the water will be tiedup digesting the sugar. This makes less fluidavailable for other uses. Your thirst will oftenreturn in as little as 30 minutes when you con­sume a sweetened soft drink.

If you will be physically active for morethan an hour, you may wish to choose a sportsdrink. These drinks are specially designed tomeet your body's fluid needs during extendedphysical activity. They provide smalleramounts of sugar than soda or fruit drinks.

iil

III~1- -.!IIII

Chapter 7 Water: The Universal Solvent

This amount of sugar can be absorbed by thebody and provide a source of energy duringactivity. Sports drinks also supply smallamounts of sodium and potassium that yourbody loses through sweat. The sodium helpsyour body retain fluids, and the appealingtaste of these beverages may encourage you todrink more.

You have learned that water has a lowerboiling point at higher altitudes because ofreduced atmospheric pressure. This reducedpressure also affects your water needs. Morewater evaporates from skin and throughbreath at high altitudes, where atmosphericpressure is lower. This affects water needs forairplane passengers as well as people inmountainous areas. The higher the altitude,the more water you will need to drink to stayhydrated, or full of water.

A Safe Water Supply

Water is the most valuable naturalresource. All life depends on its availabilityand safety. Nature has been recycling waterlong before people named the concept. Rainwashes and waters the earth. The rainbecomes part of rivers, streams, and lakes orfilters through the soil to the undergroundwater table. Collected rainwater, groundwaterfrom lakes and streams, and wells that tapunderground water are plentiful in the UnitedStates. It is easy to forget that one person'swastewater later becomes another person'sdrinking water. See 7-14.

For years, people assumed they coulddump waste in large bodies of water withoutany side effects. Following are some of theproblems created by water that has been con­taminated or polluted by this practice:•:. TyphOid is caused by bacteria that spread

from human and animal feces into watersupplies. This disease is common in heavilypopulated areas without proper sanitation.

.:. The Mediterranean Sea along the Rivierais toxic to most life-forms due to factoryand human wastes in the water.

.:. In the 1970s, dumping from factoriescaused mercury buildup in rivers andoceans. This endangered swordfish andtuna supplies.

159

Steps· in Wastewater Treatment

1. Remove suspended solids in the waterusing filters and gravity to screen or settleout large particles.

2. Remove soluble organic matter by firstallOWing microbes to feed on the matter stillpresent in the water.

3. Aerate the water. Adding oxygen kills someof the harmful bacteria.

4. Add a disinfectant to kill harmful bacteria.Common disinfectants are chlorine andfluorine.

5. Use advanced specialized treatments, suchas adding activated carbon to absorbcoloring agents, odors, herbicides, andpesticides. The carbon is periodicallyregenerated by heating to temperatures ofBOO°C (1652°F). These high temperaturesvaporize all but the carbon.

7-14 Local communities in the United States care­fUlly treat wastewater to make it safe for drinking.

.:. Fish in Lake Erie began to die as a resultof factory and human waste disposalbuilding to toxic levels. In the 1970s, thetoxic buildup was so bad that some peo­ple declared Lake Erie to be "dead."

.:. On June 21, 1995, in North Carolina,dams holding 35 million gallons of hogsewage broke, polluting rivers anddestroying fish populations.

A contaminant or pollutant can be any­thing that makes a substance impure orunsuitable. Anything that causes water to beunsafe for use is a water contaminant. Some ofthe more common sources of contaminants areanimal and human wastes, chemicals, andgarbage.

Biological PollutantsBiological pollutants include bacteria,

protozoa, viruses, and organic wastes. Manymicrobes that break down waste products arenot harmful by themselves. In large volumes,however, their oxygen needs can depleteoxygen levels in rivers and lakes. This cancause fish to suffocate. Large volumes ofdecaying organic wastes will also reduceoxygen levels in water supplies. The main

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1(1

Ii-I

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160

sources of organic wastes include human andanimal sewage, wood and paper mills, andfood processing plants. Another problem isorganic wastes may leave particles in thewater that can coat or clog machinery.

Chemical Contaminants in WaterThere are three main kinds of chemical

contaminants found in water: metal ions,acids, and toxic substances.

Metal IonsA temporary water hardlless is caused by the

presence of calcium and magnesium ions.These ions dissolve in rainwater as it soaksinto soil. The ions combine with bicarbonatesin the water to form salts. These salts are easi­ly removed through heating. However, theycan coat pipes, valves, and cooking equip­ment. They can interfere with the transfer ofheat and add to the difficulty of cleaningequipment. They provide a place for bacteriato multiply and reduce the effectiveness ofsoap. Hard water has also been found totoughen the textures of fruits and vegetablesduring processing.

When sulfur and chloride compoundscOInbine \vith calciu111 and 1l1agnesiull1, wateris called permal1e11t1y hard. These ions must beremoved through an ion exchange method.Permanently hard water is passed over mate­rial that contains loosely bound sodium andhydrogen cations. The exchangel~ called awater softener, gives up the sodium and hydro­gen and collects the calcium and magnesiumcompounds.

AcidsAcids that dissolve in the water supply

can change the pH of water. Acid rain is anexample of this kind of pollutant. Burnedpetroleum products give off carbon monoxideand carbon dioxide. Carbon monoxide is anexample of a nonmetal pollutant. Carbondioxide forms weak carbonic acid (H,CO,).The more carbon dioxide that dissolves in therain, the lower the pH of the rainwater. Carbondioxide is not the only source of acid rain.Other acids from manufactured sourcesinclude sulfur oxides (SO,), nitrogen oxides(NO,), and hydrochloric acid (HCI). Low(acidic) pH levels can kill plant and animal life.

Unit II Basic Chemistry

Physical Contaminants in WaterAn often overlooked source of pollution is

garbage and litter. Many people thoughtlesslytoss garbage vvhen picnicking, boating, andfishing. People often drop soiled diapers,cans, bottles, and plastics with no thought oftheir potential harm. Fish have died becauseof swallowing plastics. Swimmers have beencut on discarded bottles and cans. Litter canharbor bacteria, cause physical harm, andbreak down into toxins that enter the watersupply. See 7-15.

Water Contaminants and theBeverage Industry

Consumers expect a particular soft drinkto taste the same whether they are in Tokyo,London, or New York City. Physical, biologi­cal, and chemical contaminants vary from oneregion to another. These contaminants canchange the flavor of the water. Water process­ing plants generally remove only harmful con­taminants. Therefore, most bottling compa­nies have established water standards fortheir products. To make a uniform product,these companies treat water beyond the levelof treatment provided by water processingplants. Beverage bottlers remove contami­nants from their water sources that produceunwanted flavors before adding soft drinksyrups.

USDA

7-15 This worker is testing the contaminant levelsof water to be sure the water will support thegrowth of shrimp.