proteins: amino acids and peptides -...

Proteins: AminoAcids andPeptides

Proteins are complex molecules that makeup as much as 50% of the dry weight of livingcells. Proteins have a diverse nature. Thismakes it possible for them to play many rolesin living organisms as well as in food products.

The primary sources of dietary protein areeggs, dairy products, meat, poultry, and fish.Specific cooking principles must be followedwhen preparing these foods due to their high

Meat products such as these are commonsources of proteins in the diet.

Objectives

After studying this chapter,you will be able to

identify amino acid classifications based onnutritional use and chemical properties ofthe side chains.

describe the primary, secondary, and tertiarystructures of proteins.

list at least six factors that denature proteins.

state the fw1Ctions of protein in foodproduction.

apply basic principles of the chemistry ofprotein to cooking eggs, milk, and meatproducts.

compare the nutritional functions of proteinswith the functions of carbohydrates and fats.

Chapter

Key Terms

amino acid

amine group

peptide bond

polypeptide

essential amino acid

complete protein

incomplete protein

disulfide cross-link

hydrophobic

casein

whey

11

myoglobin

oxidation

reduction

denaturation

coagula tion

gluten

protein gel

albumin

collagen

aldehydes

Maillard reaction

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protein content. These foods provide the bodywith an important nutrient that serves manyfunctions.

The Structure of ProteinProteins are largely composed of the same

elements as carbohydrates and fats-carbon,hydrogen, and oxygen. In addition, proteinscontain nitrogen and usually sulfur. Some proteins also contain iron, copper, phosphorus, orzinc. Like carbohydrates, protein moleculesare made up of subunits. These subunits areorganic acids-acids that contain carbonatoms. All organic acids, like fatty acids, contain carboxyl groups (-COOH).

Amino AcidsThe organic acids in proteins are called

a11ti11.o acids. Amino acids have three basicparts to their structure. They have a side chainof carbon and hydrogen atoms, a carboxylgroup, and an amine group. The al1tine groupis one nitrogen and two hydrogen atomsbonded to a carbon atom. It can be representedby -NH2 or as the following chemical formula.

-N-H\H

There are 20 amino acids in the humanbody. As many as 150 more amino acids havebeen isolated in animals, plants, and singlecelled organisms. Generally, the amine andcarboxyl groups in an amino acid are bondedto the same carbon atom. Chemists use the letter R to represent the various carbon sidechains. Therefore, most amino acids can berepresented by the following formula:

carbon side chain~R 0I II

N-C-C-O-H/1 I t

H H Ht carboxyl group

amine group

Because of its polar nature, the carboxylgroup acts as an acid and the amine group asa base. The amine group from one amino acidwill readily combine with the carboxyl group

Unit III Organic Chemistry: The Macronutrients

of another. When two amino acids combine, awater molecule is released. The bond formedbetween the two amino acids is called apeptide bond.

peptide bond

R 01 R 0I II t I II

N-C-C-N-C-C-O-H/ I I I I

H H H H H

The new dipeptide is a protein moleculemade from two amino acids. It also has anamine group on one end and a carboxyl groupon the other.

Proteins are chains of many amino acidsbonded together. The shortest known proteinis a chain of 20 amino acids. Most proteinshave from 100 to 500 amino acids. Because ofthe many peptide bonds, proteins are alsocalled polypeptides. Polypeptides are molecules with many peptide bonds.

Amino Acids Essential for GoodNutrition

There are two ways to classify aminoacids. Amino acids can be classified based onnutritional use. They can also be classified asto the chemical nature of their side chains.

The 20 amino acids used in the humanbody are necessary for growth and body functions. The body can make many of theseamino acids as they are needed. It is not essential for them to be provided by the diet.However, 8 of the amino acids cannot be produced by the body. They are leucine,isoleucine, lysine, methionine, phenylalanine,threonine, tryptophan, and valine. Theseamino acids are classified as essential aminoacids. Essential a1nino acids are amino acidsthat must be supplied by foods in the diet. Thebody cannot grow new tissue or maintainhealth without these 8 amino acids. See 11-1.

A ninth amino acid, histidine, is alsoessential for infants and toddlers. The bodycannot make enough histidine to meet thedemands of the rapid growth that occurs inearly childhood. Extra histidine from complete proteins is needed daily.

Foods that contain aIlS essential amino acidsare called c0111plete proteins. Most complete

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Chapter 11 Proteins: Amino Acids and Peptides

Essential Amino Acids

243

11-1 Your diet must provide the essential amino acids to support growth and maintenance of body tissues.Scientists use symbols for the amino acids to diagram protein molecules.

proteins come from animal sources andinclude eggs, milk, fish, poultry, and meats.One plant source of protein-soybeans-isalso known for being quite high in quality.Before soybeans can support growth, theymust be heat processed for several hours. Thisdestroys toxic compounds that prevent use ofthe essential amino acid trypsin. Soybeans arelow in methionine, and most soy productshave methionine added to improve the qualityof the amino acids.

Other plant sources of protein come fromgrains and vegetables. These proteins tend tobe lower in quality and are called incompleteproteins. Incomplete proteins are short of oneor more of the essential amino acids needed forhuman growth. The amino acids that are shortare called limiting amino acids. For instance,lysine is often the limiting amino acid in cerealgrains. Tryptophan and threonine are also limiting amino acids in some cases.

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Amino Acid

Isoleucine

Leucine

Lysine

Methionine

Phenylalanine

Threonine

Tryptophan

Valine

Symbol

L

K

M

F

T

w

V

Structure of Side Chain

-CH-CH2 -CHaICHa

-CH2 -CH-CHaICHa

-CH-OHICHa

-CH-~

NIH

-CH-CH3

ICH3

Vegetarians must choose plant foods carefully to receive all 8 essential amino acidsnecessary for good health. Vegetarians cancombine some sources of incomplete proteinto form a complete source. Combininglegumes with grains, nuts, or seeds will generally create a complete source of protein. See11-2. Some combinations that contain adequateamounts of all 8 essential amino acids are.:. whole wheat bread and peanut butter.:. rice and red beans.:. refried beans and corn tortillas.:. hummus-a dip made with chickpeas

and sesame seeds

Classification of Amino Acids by SideChains

The second method for classifying aminoacids is based on the chemical properties ofthe side chains. The way the protein molecule

244

11-2 Although this meal does not include meat,the combination of red beans and rice provides allthe essential amino acids.

is shaped and how it functions depend on thepolarity of the side chains. Side chains can benonpolar, uncharged polar, positivelycharged, or negatively charged.

Alanine, tryptophan, and leucine areexamples of amino acids with nonpolar sidechains. Nonpolar side chains are less solublein water. However, because they are nonpolar,they are attracted to other nonpolar compow1ds, SUd1 as lipids and cholesterol. Forinstance, you read in Chapter 10 aboutlipoproteins, which are clusters of lipid andprotein molecules. Some lipoproteins transport cholesterol in the blood. They are able todo this because the protein molecules in thelipoprotein clusters have nonpolar sidechains. These side chains are positionedtoward the outsides of the molecules, allowing them to attract cholesterol molecules.

A second group of amino acids has neutralpolar side chains that will form hydrogenbonds. These side chains are attracted to otherpolar molecules, such as water. The hydroxylgroup causes such a polar natw·e on serine,threonine, and tyrosine. The hydrogen bondingcan also form between sections of the proteinmolecule. These hydrogen bonds are partiallyresponsible for the shape a molecule will take.They are also partially responsible for some ofthe protein's physical properties and how proteins ftmction in food preparation.

A third group of amino acids has sidechains that have a positive or negative charge.


Positively charged amino acids, such as lysineand arginine, have a second amine group onthe molecule. Negatively charged aminoacids are aspartic and glutamic acid. Thepresence of positively and negatively chargedside chains enables some proteins to actas buffers.

Protein StructuresThe J1lunber of possible protein structures

is endless. However, all protein molecules arecomplex. This is due to the number of aminoacids and the order in which they combine. Itis also due to the interaction between the sidechains.

Primary StructureThe primary structure of a protein molecule

is the order the amino acids occur in the chain.Food scientists have "maps" of food proteins,such as casein from milk. These maps identifythe order of the amino acids. The primarystructure is a result of the chain of peptidebonds formed in making the protein molecule.

Secondary StructureThe seco/1dary structure of a protein mole

cule refers to the shape of sections of theamino acid chain. The secondary structure isdue to hydrogen bonding between aminoacids. These bonds cause bending of the molecule. The peptide bonds change directionbased on the types of amino acids that arejoined. Secondary structmes of protein occurin three patterns: helix, random coil, andpleated sheet.

A helix structure is a repeating coil. Thinkof the shape of a Slinky or a telephone cord.That is a helix shape. The amino acid sidechains are on the outside of the helix. As theamino acids twist around a central axis,hydrogen bonds form between amino acids onthe chain. These hydrogen bonds increase thestability of the molecule.

A random coil shape forms when some ofthe side d1ains prevent the setting up of ahelix. If you have ever played with a Slinky,you have probably had it eventually tangle.One of the sections would become twistedbackwards. This is similar to what happenswith random coils. It is like grasping andballing up a string. The string becomes twisted

Chapter 11 Proteins: Amino Acids and Peptides 245

fibrous

Molecular Interactions of ProteinsAs you have read, proteins have a com

plex nature, large size, and various sidechains. These factors allow many interactionswithin protein molecules and with other compounds. Understanding these interactions willhelp you wlderstand the many ways proteinscan function in food preparation.

A hydrogen bond can occur between thehydrogen atom of one side chain and thehydroxyl group of another. A hydrogen bondcan also occur beh¥een the oxygen atom inone peptide bond and the hydrogen atom ofanother peptide bond. The formation ofhydrogen bonds is basic to the stability of thesecondary and tertiary structures of proteinmolecules. Polar groups on the outside of protein molecules also allow protein molecules tohydrogen bond with water. This is why someproteins are water soluble. Albumin, the protein found in egg white, readily dissolves inwater. Water that is hydrogen bonded to protein is an example of bound water.

oII

R-C-N-RI~~ hydrogen bond between: two peptide bondsoII

R-C-N-RIH

Globular proteins do not tend to form linksthat will create a protein neh"ork. Hemoglobin, which carries oxygen in the blood, andlipoproteins, which carry cholesterol in thebloodstream, are globular proteins. Casein inmilk and albumin in egg white are otherexamples of globular proteins.

Fibrous proteins are usually made fromhelix-shaped strands. They are strong and arepart of connective tissues. Collagen, elastin,keratin, and myosin are examples of fibrousprotein. These proteins are fowld in musclefibers, ligaments, tendons, fingernails, andhair. Fibrous proteins tend to link to form anetwork of tissue.

Tertiary Structure

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globular

Protein StructuresPrimary Structure

~~jn~Cid~qu=ey

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~~~\ IVrandom coil

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11-3 Peptide bonds link amino acids into a chain,forming the primary structure of a proteinmolecule. Hydrogen bonds between the C=O andN-H groups of a peptide chain cause the chain tofold into a secondary structure. Hydrogen bondingamong sections of the side chains result in theformation of the tertiary structure.

Tertiary StructureThe tertiary structure of a protein molecule

refers to the three-dimensional structure of anentire amino acid chain. Think of the primarystructure as the individual fibers in a piece ofyarn. The secondary structure would be likethe fibers twisted together into a ply of yarn.The tertiary structure is like a balled up strandof yarn. The main tertiary structures are globular (balled up) and fibrous (strands). See 11-3.

and turned and folded in on itself with noset pattern.

The third shape common in protein molecules is the pleated sheet. This shape is muchlike the paper fans you may have made as achild. Side chains can be located both aboveand below the pleated sheet. This enables sidechains of one molecule to bond to side chainsin protein molecules above and below.

=

246

A second interaction that occurs betweenprotein molecules is disulfide cross-links.DisHlfide cross-links are covalent bonds thatform between two protein molecuJes at sidechains that contain sulfm. The more disulfidecross-links that are formed, the more stablethe protein will become.

A third molecular interaction of proteins isbetween nonpolar side chains. These interactions are called hydrophobic, or waterrepelling. These interactions occur betweenside chains with carbon rings.

Remember, nonpolar side chains are notwater soluble. They do not form covalentbonds. However, nonpolar side chains canreact with lipids. Hydrophobic side chainsenable proteins to form lipoproteins, whichare an important part of cell walls. Proteinsthat are water soluble tend to have thehydrophobic side chains facing into the centerof the protein molecule.

An example of a hydrophobic protein iscaseiJl found in milk. This protein is vital tothe forming of curds in cheese making. Whena mixture of relU1in, salts, and acids are addedto milk, the globular casein w1tangles. Thenonpolar side chains of the casein bind withmilkfat, calcium, and one another to formcurds. See 11-4.

11-4 These cheese curds were formed when globular casein molecules untangled, allowing nonpolarside chains to bind wit!1 milkfat and one another.

.


A by-product of cheese production iswhey. Whey looks like a watery milk and ismainly composed of a group of water-solubleproteins, lactose, and minerals. It is used as anadditive in many commercially processedfoods. The water-soluble proteins in milk arecalled whey proteins. They form hydrogenbonds with water. Whey protein molecules donot change their shape in reaction to rennin,salts, and acids the way casein molecules do.

Color Changes of Protein PigmentsUnderstanding color changes in protein

pigments helps food scientists control colorchanges in meat during storage and processing. This is important because consumersbelieve that bright red color means meat isfresh. MyoglobiJl is the iron-containing protein pigment in muscle tissue that providesthe color. Myoglobin stores or holds onto oxygen in live an.imal tissue. \lVhen an oxygenmolecule is attached, myoglobin is a brightcherry-red color. When oxygen. is not present,the tissue becomes purplish in color. Afterprolonged exposure to oxygen, the myoglobinchanges to metmyoglobin which has a browncolor. The process of adding oxygen is areversible chemical reaction. This reversibleprocess of adding and removing oxygen to acompound is called oxidatioJl and reductio11.Oxidation adds oxygen and reductionremoves it. Hemoglobin is the other proteinthat uses the oxidation/reduction process todistribute oxygen in the body. Hemoglobin isfound in the blood and myoglobin is foul1d inmuscle tissue.

Once meat is bright red, it can be quicklywrapped il1 packaging that lin1its exposure tomore oxygen. Although the bright red colordoes not guarantee freslmess as compared to apurplish or brown color, consumers willchoose the bright red meat over other colors.Consumers commonly mistake red juice frommeat as blood. Red juice that pools arolmdmeat cuts is water with dissolved myoglobin,not blood that contains hemoglobin.

Nih·ites are added during curing processesto preserve meats. They also maintain a pinkor red color and are very stable. Nitrite combines with myoglobin to form nitric oxidemyoglobin. When cooked, this is converted to

-


nitrosohemochrome causing the pink or red ofcooked ham and bacon. When fresh or curedmeats develop green or yellow discolorations,it is a sign of bacterial growth.

Denaturation of Proteins

Protein is unique in that its shape can bechanged without changing the primary structure of the molecule. The secondary and tertiary structures of protein are fragile. Theycan be changed by physical or chemicalmeans. Any change of the shape of a proteinmolecule without breaking peptide bonds IS

called denaturation. Denaturation usuallyresults in a loosening or unfolding of the protein molecule.

Denaturation is sometimes reversible. Ifthe denaturation is slight, the protein willtend to return to its original shape. Proteinwill also tend to return to its original shape ifthe denaturation involves only hydrogenbond interactions. You can see an example ofthis by beating egg whites Lmtil frothy orfoamy. If you allow the egg whites to sit, theywill return to their liquid state.

Denaturation of proteins is usually notreversible. This is the case when denaturedproteins interact with other proteins whileunfolded. Breaking disulfide cross-Imks IS

also irreversible denaturation.Another type of permanent denaturation

is coagulation. Coagulation results when aliquid or semiliquid protein forms solid orsemisoft clots. One example of coagulation IS

milk curdling to form cheese. Cooked eggs areanother example of coagulation due to pennanent changes in disulfide links. Eggs are liquidbefore they are heated. After heating causeseggs to begin coagulating, they will ~ot returnto their liquid state. See 11-5. Gelatm, on theother hand, solidifies as it chills due to the formation of hydrogen bonds. When the gelatinis heated, the proteins return to aliquid state.

Denaturation will change many of thephysical characteristics of a protein. Ifhydrophobic side chains are exposed throughdenaturation, a protein's water solubility canbe reduced. This can lead to proteins forming

247

precipitates (solids) that can be separatedfrom a mixture.

Denaturation can alter the ability of a protein to bind water. Cooking meat causes theproteins to shrink, releasing water-based fluids and reducing the ability to hold water.Some cooking is needed to kill bacteria anddevelop flavors. However, too much cookingcauses too much coagulation. This results inprotein foods that are dry, tough, and rubbery.

Denaturing protein can also interfere withthe biological reactions of enzymes. You willread more about this in Chapter 12.

Methods of Denaturing ProteinProteins can be denatured by a number of

physical and chemical methods. Physical factors that denature proteins are hot and coldtemperatures, mechanical actions, soundwaves (including ultrasOlmd), pressure, andirradiation. Chemical factors that denatureproteins are pH changes (acid or alkali) andmineral salts. Because eacl1 protein is unique,the rate of the denaturing will vary greatly. TI1eamoLmt of denatming that occurs also varies.

Temperature ChangesHeat is the most common method of dena

turing proteins in food production. The amountof denaturation will depend on the temperature. Most chemical reactions double their ratewith every lOoC increase in temperature. Therate of increase for protein denaturation is 600

11-5 When eggs are denatured by heating, theywill permanently coagulate into a solid state.

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times for every IDcC increase in temperature.This is because it does not take very muchenergy to break the hydrogen bonds.

Denaturation will occur at a faster ratewhen the protein is wet, as in food mixtures.This is because of interactions between thewater molecules and the broken hydrogenbond sites.

Cold temperatw:es can also alter proteinstructures. This most often happens whenfoods are frozen. Milk tha t is frozen and thenthawed will have a curdled look due to thedenaturation of the milk proteins. Soy proteins and eggs are also likely to denature infreezing temperatures.

Mechanical ActionsMechanical actions such as beating,

rolling, and kneading can disrupt proteinstructures. Vigorous or prolonged actions willcause proteins to lock into new positions withother molecules. When bread dough is kneaded, proteins react with water and each other.They realign to form a viscoelastic (thick andstretchy) structure.

The network of elastic protein strands thatgive bread dough its sh"Ucture is called glutel/.The protein in wheat flour is responsible forthis elasticity. Gluten changes its shape andstrengthens during the kneading process.Gluten enables yeast breads to rise withouttearing the dough. Gluten also enables breadsto solidify during baking to form the mainstructure of the bread. This is why breadflours need a higher protein content than allpurpose or cake flour. See 11-6.

In some food products, cal-e must be takento avoid denaturing the protein too much.When beaten, egg whites will trap air andbecome light and fluffy. If overbeaten, overcoagulation occurs and unattractive clumps form.

Other Physical Methods ofDenaturation

Several other physical methods can be usedto denature protein. One of these methodsinvolves the use of sound waves. It takes prolonged exposure to sound waves at high volumes to cause permanent denaturation.Another physical means of denaturation isirradiation. This will be discussed in detail inChapter 21.

Unit III Organic Chemistry; The Macronutrients

Chemical Methods of DenaturationSome of the methods used to denature

proteins involve chemical factors. One suchmethod is a change in pH. Exposure to acidsor alkalis can cause proteins to unfold. ThepH needed to denature proteins will vary.Each type of protein has a pH range in whichit is stable.

It is important for food scientists to knowwhat pH range results in denaturation foreach protein. The scientists can then put thisinformation to use in developing food products. For example, soy proteins will dissolveinto a sticky liquid in alkali. By forcing this liquid through tiny holes into an acid ba th, theprotein will coagulate into stringy fibers.These fibers resemble the texture of meat. Thedenatured soy protein is then used to makesimulated meat products.

11-6 A heavy-duty mixer with a dough hook canknead bread dough to develop the gluten neededto give bread its structure.

------------ A


Add vinegar or lemon juice to milk andnote the coagulation and curdling caused bythe low pH. Many dairy products, such assour cream, buttermilk, and yogurt, are theresult of acids denaturing the milk proteins.

Poached eggs are best when cooked inwater with a little vinegar added. The vinegar(acetic acid) coagulates the egg protein, whichhelps keep the egg compact. Try poachingeggs with and without vinegar and note theshape of the cooked eggs.

Another chemical method of denaturingproteins is exposing proteins to mineral saltsor metals. Sodium and potassium salts willreact to some extent with proteins, causingdenaturation to occur. This is why adding saltearly in the cooking process of high-proteinlegumes can cause some toughening of theproduct. Metals such as copper, iron, magnesium, and calcium will readily react with proteins. The presence of calcium is important inthe curdling process in cheese production.

Functions of Protein in Food

A full understanding of all the interactionsof protein in food production is not possible.The combination of so many complex proteinsin equally complex food mixtures makes identifying all the interactions and reactions difficult. However, it is possible to study the proteins involved in some simpler food mixtures.It is also possible to examine types of functions proteins perform in foods. Some of theseinvolve proteins being used as gelling agents,texturizers, emulsifiers, and foaming agents.Protein is also important in dough formationin baked products.

To determine how effectively proteins willwork in a given food product, food scientistsanalyze some important physical characteristics. Proteins have varying degrees of waterabsorption, solubility, and viscosity (ability toflow when poured). Each protein's viscosityand reaction to water can be altered throughdenaturation.

249

Form GelsIn Chapter 9, you studied the ability of

starches to form gels. You learned that gels canbe produced by cooking amylose in liquid tothe gelatinization point to make sauces. Gelscan also be made by combining pectin, sugar,and acid as in jellies. Proteins have the abilityto form gels, too. This is what happens whenthe protein gelatin is heated in water and thencooled. It is also what happens when a mixture of eggs, milk, and sugar is heated to makecustard.

A protein gel is a mixture of mostly fluidslocked in a tangled three-dimensional mesh.This mesh is made of denatured and coagulatedproteins. Protein gels contain long, thin, chainlike polymers of amino acids. The moleculesare cross-linked randomly to produce gelsthat behave like a rigid solid. Protein gels havetwo parts: the three dimensional molecularstructure and the liquid that is attracted to theproteins. The liquid keeps the proteins fromcollapsing, and the proteins keep theliquid from flowing away.

Unlike starch gels, gelatin has a narrowmelting and solidifying temperature range.Starches begin to gelatinize as they are cookedand form gels as they cool. Some protein gelsare very liquid when hot and thicken whenthey cool. The coagulation process is gradualand requires lower temperatures than starchgels. Like most starch gels, gelatin is softenedby acids and may develop syneresis (leakage)if cooked or stored too long.

To make a protein gel, plain gelatin is firstdissolved in cold water. If hot water is addedfirst, it will cause the gelatin to clump, makingit difficult to dissolve. The cold water causesthe gelatin molecules to swell. Boiling water isthen added to disperse the gelatin. Dispersionoccurs at 35°C (95°F). The gel forms when themixture is cooled to between 10°C (50°F) and16°C (61°F). See 11-7.

Gelatin dessert mixes are prepared by firstadding boiling water to dissolve the gelatin.Boiling water does not cause clumpingbecause the gelatin particles in dessert mixesare separated by sugar and flavoring.

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11-7 Gelatin dessert mixes are available in a rainbow of colors. Unflavored gelatin can be used tocreate your own flavor combinations.

Stiffness of protein gels increases withstanding at cool temperatures. Gelatin will setquickly when ice cubes are added to lower thetemperature of the dispersed mixture.However, a gelatin that is set qukkly will meltmore quickly at room temperature than a gelthat sets slowly.

Once gelatin is rehydrated, it must bekept refrigerated. Gelatin will lose its rigidstructure and become runny at room temperature. It will also be susceptible to bacterialgro""'th.

Protein gels are strengthened by severalfactors. The more gelatin a mixture contains,the firmer the gel will be. However, too muchgelatin causes the product to become gummy.Mineral salts will add strength to the gel byhelping to establish the cross-linkages in thestructure. Because of this, hard water or milkwill produce a firmer gel than distilled or softened water.

Gels are weakened by acid, sugar, andfruit or vegetable pieces in the mixture.Gelatin can have up to 30 mL (2 tablespoons)of lemon juice added per 250 mL (1 cup) of liquid. This amount of acid will not keep a gelfrom forming. Sugar slows the gelling processas well as preventing some cross-linkagesfrom forming. This results in a weaker structure. Most gelatin mixes carefully balance thegelatin and sugar. Pieces of fruits or vegetables break the flow of the three-dimensional


gel structure. This mechanically interfereswith the gelling process. If too much fruit isadded, then gelatin concentrations must beincreased to compensate.

You probably are familiar with gelatin as ajiggly, cool, easy-to-prepare dessert or salad.However, gelatin is also used as an additive infood products for the following reasons:

.:. to provide structure and support

.:. to stabilize the foam in whipped products

.:. to thicken puddings and pies

.:. to control crystal growth in frozen foods

A protein gel can also be formed frommuscle tissue. Salt is added to destabilizesome of the proteins. The meat or meat piecesare then massaged or tumbled. The salt pullssome proteins into solution. The agitation ruptures some cells, increasing the protein available for gelatinization to occur. This process isused on some hams, chicken hot dogs,bologna-type sausages, and a Japanese fishsausage.

TexturizeThe texture or feel of protein can be cl1anged

from globular to fibrous by denaturation. Mostglobular proteins can be splm into fibers ifthere are few nonprotein compOLmds present.You have briefly read about this with reference to soy proteins being texhtrized for useas simula ted mea t products.

Another method of texhlrizing soy proteinfor use as meat substitutes involves heatcoagulation lmder pressure. High-protein soyflours are mixed with water, heated underpressure, and then extruded. Extrusioninvolves pushing the mixture through openings that shape the product. Flavoring andcoloring are added to make the product moreclosely resemble meat. These texhlrized proteins can be mixed with meat to extend orstretch its volume. Hamburgers made withthese texturized-protein extenders are lowerin cost and fat content.

Protein texturization is also used in developing process cheeses. Different naturalcheeses are mixed and then melted togetherby heating to about 71°C (160°F). The mixingand heating denature the proteins in cheese.


Item of InterestThe "Bare Bone" FactsAbout Gelatin

251

..1~The protein, gelatin, is made

trom collagen extracted from thebones and hides of animals.Collagen is a protein in connective tissue. It is extracted from theraw material, mixed with water,and processed to form gelatin.The gelatin is then purified,refined, and dried.

Pure gelatin is nearly flavorless and odorless. It may be

packaged and sold as unflavoredgelatin. It can also be combinedwith artificial flavors, colors, andsugar and sold as flavored gelatin.

Gelatin is used to thickenchilled pies, gelatin desserts, andice cream. Only 15 mL (1 tablespoon) of gelatin is needed tothicken 250 mL (1 cup) of liquid.

Gelatin is a rather insignificantsource of nutrients. It provides

only low-quality protein becauseit lacks the essential amino acidtryptophan. Most of the nutritionalvalue of gelatin salads anddesserts comes from addedingredients, such as fruils andvegetables.

When cooled, the process cheese has asmoother texture than the natural cheesesused in its production.

EmulsifyAn emulsion is a stable mjxture of a fat and

a water-based liqwd. Most stable emulsionshave three parts. One part is a nonpolar substance, like fat. A second part is a polar liquid,like water. The third part of an emulsion is anemulsifier, which can be a denatured protein.An emllisifier is a molecule that has a polar endand a nonpolar end, 11-8. Emulsions usuallyrequjre heat or mechanical action, such asbeating, to denature the protein and then formthe emulsion. Temporary emulsions of a fatand polar liqwd usually are created by beating or shaking without an emulsifier present.

Egg yolk is an excellent protei.n emulsifier.It is often used i.n home recipes for ice creamand mayOlmaise to keep the mixtures stable.

Other food product emulsions in whichproteins act to keep the fats and liquids dispersed include milk, cream, butter, andcheese. The casein in milk, with its looserandom coil structure, serves as a protein

emulsifier in these foods. Homogenized milkis a stable emulsion for two reasons. Homogen..ization forces rilllk through screens underhjgh pressure. Tills ruptures the membranesaround the fat globules, reducing the size ofthe globules in the milk. The pressure used inthe homogenization process also changes thestructure of the casein molecules. This structure change makes the casein better able tobond to the fa t.

Protein's ability to form emulsions cansometimes be a problem. For instance, soy,sunflower, and canola oil production involveseparating protein and oil emulsions withminimal damage to either by-product. However, grain proteins can make it difficult tofully extract the oils from the grain kernels.

Form FoamsIn a foam, gas is suspended in a Liqujd or

semisolid. The gas is usually air or carbondioxide surrounded by a film or bubble containing protein. In food production, foams aremade in three main ways: bubbling gasthrough a mixture, whipping or beating, anddepressuriza tion.

252 Unit III Organic Chemistry: The Macronutrients

Emulsifier

fatmolecules

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Develop GlutenAnother main nmction of protein in food

products is the development of gluten inbaked goods. Gluten is a strongly cohesiveand elastic protein. It is formed when wheatflour is combined with moisture and stirred orkneaded. The strength of gluten is partially aresult of disulfide cross-links that form duringmixing. As carbon dioxide is released in thedough, it forms tiny air pockets or bubbles.This causes the gluten structure to stretch.When baked, the gluten coagulates, formingthe light airy texture of bread and other bakedproducts.

The ability to form cohesive and elasticgluten is not present in other grains, such asrye and corn. This is why breads made withrye flour or cornmeal have a denser, heaviertexture. Recipes for such breads usually

bread, 11-9. All these products have a lighttexture due to gas being trapped WithUl theirstructures. One protein that is a good foamulgagent is albllmin, which is fOLmd in egg whitesand milk. Some caseins, whey, gelatin, glutenin, and soy protein are also good foamingagents. You will read about factors that affectthe stability of food foams in Chapter 22.

Mnpol'lr(lntl

bipolar proteinmolecule

water-basedmolecules

As a child, you probably practiced the firstmethod of forming foams. You would havedone this by inserting a sh·aw into a glass ofcold milk and blowing. The bubbles thatresulted were a foam formed by bubbling gasthrough a mixture. The elasticity of the proteinmolecules in milk makes this foaming actionpossible. nus method can create very largefoam volumes.

The second and most common method offorming foams is whipping or beating the protein mixture. Whipping gives a more uniformdispersion of gas than blowing air through amixture. Small Lmiform bubbles tend to bemore stable. Stiffly beaten egg whites are anexcellent example of this method of makinga foaln.

The third method of forming foams uses asudden release in pressure, as in aerosol cans.The release in pressure causes air spaces torapidly expand. Dissolved air and liquid arereleased as a foam. Whipped cream in cans isthe main example of this method. Anotherexample of this method can be seen by opening a warm 2-liter bottle of root beer.

Food foams include meringue, foamcakes, marshmallows, whipped cream,whipped toppings, ice cream, souffles, and

11-8 Protein emulsifiers are bipolar molecules. They stabilize mixtures by attracting water-based molecules to the polar end and fat molecules to the nonpolar end.


11-9 Whipped topping, angel food cake, marshmallows, and fruit-filled meringue are examples of proteinfoams.

require at least as much wheat flour as flour ormeal from other grains.

The addition of other proteins to breaddoughs can increase the nutritional value.However, other proteins can also interferewith the development of the gluten network.Cold milk contains globular proteins thatwill interfere with gluten formation. Scaldedmilk has been denatured and does not interfere with the gluten structure. This is whymost yeast bread recipes with milk call forscalded milk.

Cooking High-Protein Foods

High-protein foods include eggs, milkproducts, meat, poultry, and fish. All thesefoods are damaged by cooking temperaturesthat are too high or cooking periods that aretoo long. This is because of the rapid denaturation of protein when heated. The proteinmolecules tend to shrink and lose water. Toomuch heat will result in a dry, rubbery, toughproduct.

Principles of Storing and Cooking EggsTwo factors cause the deterioration of eggs

in storage. The first is the loss of cal'bon dioxide through the eggshell. As carbon dioxidemoves thmugh the shell, the pH of the eggchanges from neutral to basic. This causes theproteins to break apart.

The second factor that causes the deterioration of eggs in storage is part of the watermoving into the egg yolk. This stretches andweakens the membrane surrounding the yolk.This is why it is harder to separate older eggs.It is also harder to turn a fl'ied egg withoutbreaking the yolk if the egg is not fresh.

Many egg producers apply a special sprayto eggs to reduce the loss of carbon dioxideand moisture through the shell. This sprayhelps eggs maintain quality for a prolongedshelf life. See 11-10.

Lengthy storage is not the only factor thatwill affect the outcome of cooked eggs. Eggwhites are composed of protein, water,riboflavin, niacin, magnesium, and potassium.Albumin, the protein in egg white, is easilydenatured by heat. If eggs are heated at hightemperatures or for long periods, the coagulation will be more extensive. This will result ina firm to tough egg white. Low temperaturesand short cooking times will. allow the eggwhite to coagulate while remaining soft andtender.

Principles of Cooking MilkMilk-based products include white sauces,

cheese sauces, puddings, and Cl'eam soups.Two common problems can occur whenpreparing such products. The first problem iscurdling. Curdling occurs when acid causes

(I t

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254

USDA

11-10 This equipment is used to dry and then oileggs. The oil slows the loss of carbon dioxide andpreserves the quality of protein in the egg white.


the globular casein protein molecules in milkto unfold and stick together. Curdling can beavoided by combining the acid with starchbefore the milk is added. Cream of tomatosoup is an example.

The second common problem that canoccur when cooking with milk is scorching.Scorclling occurs when the protein clumpsformed by the heat sink and burn to the pan.Whey proteins will begin to coagulate at 66°C(150°F). Constant stirring helps preventscorching by keeping the whey proteins fromsinking. Scord1ing can also be prevented bycooking milk-based products in a double boiler.A double boiler suspends the product overboiling water and steam. This keeps the tempera tme of the product lower than if it were ina pan in direct contact with the heat source.The lower temperature wiU help prevent themilk proteins from sticking and burning tothe pan.

Casein will not coagulate unless concentrations are high or certain salts or acids arepresent. Pretreatment is necessary to preventcoagulation of casein in evaporated milk.Preheating alters the calcium salts that triggercoagulation. Stabilizers are then added justbefore concentration.

Technology Tidbit"Eggs"actly Stable

....

Hens lay fewer eggs duringthe winter months than they doduring other times of the year. Tohelp stabilize the egg supplythroughout the year, the foodindustry uses cold storage technology to slow egg deterioration.

Before eggs are placed in coldstorage, they are dipped in a thincoating of mineral oil. This is

done within 12 hours of laying.The coating reduces the loss ofmoisture and carbon dioxidethrough the porous eggshell. Thishelps stabilize the pH of the eggand maintain freshness.

The coated eggs are placed ina controlled atmosphere of carbon dioxide or ozone. Thisatmosphere must have 85% to

90% humidity and be at -1 SC toO'C (29'F to 32'F). This atmosphere will maintain egg freshness for up to 6 months.

Commercial cold storagecosts money. This accounts forthe rise in egg prices seen insome areas each year betweenChristmas and Easter.


Food FeatureIs My Chicken Done?

255

When you see a dark reddishstain near a chicken bone, youmay wonder if the chicken isthoroughly cooked. The only reliable way to test the doneness ofcooked poultry is with a meatthermometer. Insert the probe ofthe thermometer into the thickestpart of the thigh of a whole bird.Insert the probe into the thickestpart of individual poultry pieces.Make sure the probe does nottouch bone. Whole birds andthighs should reach an internaltemperature of ao°c (180'F).Breast pieces should reach an

internal temperature of 75'C(170'F).

When eating out, you are notlikely to have the opportunity tocheck cooked poultry with a meatthermometer. If you are in doubtabout the doneness of poultry insuch a situation, check the colorof the meat juices. Press thepoultry gently. If the juices fromthe meat are clear, the poultry isdone. A light pink color on theoutside portions of poultry is usually from a chemical reactioncaused by smoking the meat.However, if the inner meat and

meat juices have a pink tinge,the poultry may be undercooked.You can request further cooking.

Even poultry that is thoroughlycooked according to a meat thermometer may have a dark reddish stain near a bone. This iscaused by hemoglobin (bloodprotein) seeping out of the bonemarrow during cooking. A darkreddish stain is more likely tohappen in poultry that has beenfrozen. Such stains may alsooccur in poultry from very youngbirds. However, the stain is notrelated to doneness.

Cooking TipBaked custard is a sweetened milk and eggmixture that will curdle if overheated. Youcan use a technique that applies a principlesimilar to a double boiler to keep bakedcustard smooth. In the oven, place the panfilled with custard into another pan, whichis partially filled with water. Water maintains a temperature of 100'e during heating. Therefore, it will protect the custardfrom the hotter temperatures of the oven.

Principles 01 Cooking MealMost meats contain three categories of

proteins: muscle fibers, connective tissue,and myoglobin (deep red pigment). One ofthe goals in cooking mea t is to soften theconnective tissue to make the meat more tender. Collagen is a protein in connective tissue. It begins to soften and break down intogelatin when cooked in moist heat. Collagen

in connective tissue of young animals (veal,lamb, and pork) does not begin to soften illltilit reaches 50°C (122°F). Collagen of older animals (beef and mutton) does not begin to softenuntil it reaches 60°C (140°F). Unfortwlately, asthe internal temperature of meat reaches 50°C(122°F) during cooking, muscle fibers willstart to toughen. This means the heat neededto soften connective tissue toughens musclefibers.

Roasts with much connective tissue needto be well done to allow enough time to soften COJUlective tissue. Cooking a roast in 60°C(l40°F) liquid would require five to six hoursof cooking to soften connective tissue. Thistime and tempera ture combina tion makes themeat vulnerable to bacterial contanlination.BoiJing the roast for one hour will soften connective tissue without risk of bacterial contamination. However, muscle fibers willtoughen considerably. The best balance seemsto be simmering the roast in 86°C to 93°C

256 Unit III Organic Chemistry: The Macronutrients

Daily Protein Needs

mglkg BodyAge Weight

Recommended Temperatures forCooking Meat and Poultry

11-11 Checking internal temperatures with a foodthermometer will ensure that meat and poultryare cooked to the desired degree of doneness.

71°C (160°F)

63°C (145°F)

71 °C (160°F)

77°C (170°F)

71°C (160°F)

77°C (170°F)

71 °C (160°F)

71°C (160°F)

60°C (140°F)

InternalTemperature

82°C (180°F)

74°C (165°F)

82°C (180°F)

POUltry

Chicken

Ground poultry

Turkey

Pork

Fresh, medium

Fresh, well done

Ground pork

Ham, fresh

Ham, precooked

Beef and Lamb

Ground beef or lamb

Medium rare

Medium

Well done

Product

o[0] II

CH3CH20H ., CH3CH

The Maillard ReactionWhen amino acids in grains and meats

are heated at high temperatures, a threephase chemical reaction occurs that causes achange in color and flavor. One step in thisprocess is the oxidation or dehydrogenationof alcohols to compounds known as aldehydes.An aldehyde is an alcohol that has beendehydrogenated.

(180°F to 200°F) liquid for at least two to threehours. This time and temperature combination provides enough heat to soften connective tissue without excessive toughening ofthe muscle fiber. (Cuts of meat larger than 3 to4 kg [7 to 9 pounds] will take longer.)Foodborne illness is not a risk if the meat iskept hot (above 60°C [140°F]) until it is served.

Meat with little connective tissue can beprepared with dry heat and shorter cookingtimes. Cooking time with dry heat methods isdetermined by the internal temperature of themeat. See 11-11.

The reaction between proteins and carbohydrates that causes food to brown whencooked is called the Maillard reaction. Thechemist Louis Maillard first identified thisreaction.

The Nutritional Contributions ofProteins

Protein is one of the energy nutrients.However, this is not its most important function. Protein is needed for growth and repairof body tissue. Protein is also needed forfighting disease, fluid and electrolyte balance,pH balance, and regulating body functions.See 11-12.

Infants (0 to 6 months) 2,000

Infants (6 to 12 months) 1,500

Children (1 through 6 years) 1,200

Children (7 through 14 years) 1,000

Adolescents (15 through 18 years)males 900females 800

Adults (19 years and over) 800

Pregnant women need an extra 109 of proteinper day, and lactating women need an extra 15 g.

Support Growth and RepairThe most important function of protein in

the body is to provide nitrogen and aminoacids for growth and repair. Every cell in thebody contains protein. As many as 10,000 proteins have been identified in a single human

11-12 People in every age group need high-qualityproteins from animal sources or complementaryplant sources.


Myoglobin is a protein thatholds oxygen in muscle tissue.Myoglobin gives meat its color. Itis also responsible for the colorchanges that take place in meatduring cooking. Fresh meat has ared color because myoglobin isred when exposed to oxygen.Pork is lighter than beef becauseit contains less myoglobin. Whenmeat is cooked, the heat causesmyoglobin molecules to lose anelectron. The result is a colorchange to brown.

Have you ever wondered whysome pieces of chicken are whitemeat and some are dark? Thecolor of chicken and turkey meatis related to the type of muscle

fibers in the meat. The type ofmuscle fiber is based on theenergy source used most oftenby those muscles. Breast andwing muscle fibers are designedfor rapid sudden use. They burnglycogen for energy, which doesnot require oxygen. Legs andthighs have muscles designed forduration, or slow steady use.These muscles burn fat as wellas glycogen. Oxygen must bepresent for fat to be turned intoenergy.

Like beef and pork, chickenand turkey meat contain the protein myoglobin. Myoglobin holdsoxygen and is brown to red incolor, depending on the amount

of oxygen present. Becausebreast and wing muscles do notrequire oxygen to produce energy, breast and wing meat arelight in color. Because leg andthigh muscles need oxygen toturn fat into energy, leg and thighmeat have a darker color. Fatstored in these muscle tissuesalso gives dark meat a higher fatcontent than white meat.

Duck and goose meat is alldark because these birds fly forlong distances at a time duringmigration. This requires the fatand oxygen stores of dark meat.This is why duck and goose arefattier than chicken and turkey.

cell. Protein is used to make muscle fibers,connective tissue, cell walls, and red andwhite blood cells. Hair cells and nails also contain large amounts of protein. Whenever thebody is injured, tmder stress, or ill, the needfor protein increases.

Most body cells are replaced within aseven-year period. Cells lining the intestinaltract must be replaced every three days. Bloodcells must be replaced every tlu-ee to fourmonths. Therefore, even adults need a dailysupply of protein to replace worn out cells.

Fight DiseaseA second key ftmction of protein in the

body is to help fight disease. Antibodies areproteins designed to attack foreign substancesthat enter the body. Whenever the body is

exposed to germs, it manufactures antibodiesdesigned to destroy those specific germs.Amazingly, tI,e body remembers and produces more of those antibodies tI1e next time itis exposed to the same germs.

Maintain Fluid and Mineral BalanceAnother ftmction of proteins in the body is

performed by proteins in cell walls. These proteins help control the movement of water andminerals in and out of the cells. Too muchfluid in the cells will cause cells to rupture. Ifcells contain too little fluid, they will die.Maintaining the right mineral balance isimportant for the nerves, brain, and musclesto function proper!y.

4'

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258

Maintain pH BalanceProteins in the blood perform another

important function in the body. Normal bodyprocesses result in the production of acids andbases. The blood must carry these acids andbases to the liver and kidneys to be processedor excreted. The pH of the blood must bemaintained between a pH of 7.4 and 7.6. If theblood pH changes too much, it can causecoma or death.

Proteins in the blood control this important pH balance. These proteins are buffersthat pick up acids or bases when there are toomany. These proteins can also release acidsand bases when the blood level drops too low.The polar side chains of the amino acids maketh.is possible. The carboxyl group on an aminoacid acts as an acid and the amine group actsas a base.

Control Bodily FunctionsProteins playa role in controlling many

bodily functions. They do this by being a part


of hormones and enzymes. Hormones are animportant part of many body processes. Forinstance, the hormone insulin is involved inregulating glucose levels in the blood.Hormones control growth, regulate the reproductive system, and maintain other criticalbody functions. Enzymes are a necessary partof the many chemical reactions that occurwithin the body. Chapter 12 discusses theimportance of these proteins.

Provide EnergyThe body does not store extra protein or

turn it into muscle. (This is contrary to whatyou might read in some ads for body-buildingprotein and amino acid supplements.) Whenyou consume more protein than your bodyneeds, your body can change the amino acidsinto an energy source. Your body does this byremoving the NH, and C=O. The H, isturned into ammonia. The C=O becomes partof a compound known as ketones.

I J

"''.I',.,I i:

Recent ResearchBrowned Foods andCarcinogens

A new health concern in thenews is the formation of possiblecarcinogens in baked and grilledprotein-based foods. One of thecompounds is called aery/amide.Acrylamide is a compoundformed from glucose, fructose,and the amino acid asparagines.It is formed from the Maillardbrowning reaction. It forms infoods that are cooked at hightemperatures. For example, thebrowner the crust on toast orFrench fries, the greater theacrylamide content.

It was first identified in food inApril 2002 by a group of Swedishscientists. So far scientists havelearned that it is harmful to thereproducfive and developmentprocesses of rats and mice. Moreresearch is needed to determineif the low levels in the food supply are harmful to humans.

Recommendations are tomaintain a balanced diet and.:. avoid overcooking or using

extremely high temperaturesin cooking,

.:. fry foods to a light ratherthan dark golden brown,

.:. scrape dark crumbs off toastand baked items,

.:. soak and rinse potatoesbefore making homemadeFrench fries.

MAcrylamide: Putting the CurrentFindings into Perspective," Food Insight:Current Topics in Food Safety andNutrition, IFIG Foundation, May/June 2004


Both ammonia and ketones put a strain onthe kidneys. A person must consume largevolumes of water to flush these substancesfrom the body. This is why high-protein, lowcarbohydrate diets can be dangerous.

Future Protein NeedsWhen researchers compare present food

production to world popula tion projections,they predict there will be food shortages in thefuture. New sources of protein must be foundif there is to be enough for everyone. Food scientists worldwide are working on severalsolutions to meet future protein needs.

One area of research is developing grainsthat yield higher levels of protein. Triticale is across of wheat and rye that has more proteillthan any variety of wheat. Triticale hasimproved cereal production ill many developing countries. Amaranth is a traditional Aztecgrain with high-quality proteill. It has beenfound to grow in areas with very low raillfall.See 11-13. More work is needed to find tasty,high-proteil1 grains that will grow withoutirrigation or fertilization.

Another area of research that may increasehigh-protein grain sources is biotechnology.Researchers are working at altering the geneticstructure of plants. They hope to changeincomplete protein sources to complete proteins. Care must be taken and extensive testmg done to ensure that unwanted changes donot occur as well.

Health ConcernsFood allergies are the imnllme system's

reaction to a protein m food that is mistakenfor a harmful substance. Allergies to nearly175 different foods have been docmnented.Symptoms can occur within seconds or maytake as much as 72 hours to occur. Symptomscan vary drama ticaUy from one person toanother and one food to the next. Symptomsillclude but are not limited to hives, itdlillg,tiJlgling, swellillg, red and watery eyes, SillUSdrainage, edema, swollen jomts, vomitmg,diarrhea, coughing, wheezing, dizzmess, andanaphylaxis. Anaphylaxis is a severe lifethreatening allergic reaction where swellingcloses air passages, suffocating the victim. Aslittle as one-fifth teaspoon of an allergen can

259

Agricultural Research Service, USDA

11-13 Food scientists are studying the proteinvalue of amaranth and other grains in an effort toaddress the world hunger problem.

cause death. The most common food allergensare peanuts, tree nuts, dairy, soy, wheat, eggs,fish, and shellfish.

Food sensitivities are allergic-like responsesto nonproteiJl substances m food. They can bejust as dangerous to the person who is susceptible and can have the same symptoms as atrue allergy. Reactions to the additives sodiumnitrite and nitrate are a common example.

Concerns for the food mdustry il1Cludeaccurate ingredient labeling to alert consumers of allergens, sharmg equipment ill themanufacturillg process that can result inundeclared residues of allergens, and the useof processed ingredients that come from common allergens. For example, hydrolyzed vegetable protein could come from soy or wheatthat are common food allergens.

proteins: amino acids and peptides -...

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