Lipids: Nature'sFlavor Enhancers

..........I

Chapter o

Fats and oils, which are known as lipids, servemany important functions in food preparation.

Objectives

After studying this chapter,you will be able to

describe the molecular structure of glyc­erides, phospholipids, and sterols.

define saturated, monounsaturated, andpolyunsaturated fatty acids.

list categories of lipids based on physicalstate and dietary sources.

relate physical characteristics of lipids totheir performance in foods.

examine the functions of lipids in foodpreparation.

analyze the nutritional impact of lipids in thediet.

216

Key Terms

lipid

glyceride

fatty acid

carboxyl group

monoglyceride

diglyceride

triglyceride

nonpolar

phospholipid

sterol

saturated

unsaturated

monOLnlsaturated

polyu nsa tura tedfat

oil

melting point

hydrogenation

marbling

solidification point

auto-oxidation

rancidity

antioxidant

smoke point

flash point

essential fatty acid

omega-3 fatty acid

plaque

atherosclerosis

lipoprotein

Chapter 10 Lipids: Nature's Flavor Enhancers

In today's society, much is written aboutthe dangers of fat in the diet. Many weight­loss plans recommend cutting fat and count­ing fat grams. It is true that too much fat canbe harmful. However, fat plays an importantrole both in food preparation and generalhealth.

In this chapter, you will examine the struc­ture of fat. You will Ieam how it functions infood preparation and why it is a vital part of ahealthful diet. As you study this chapter, it isimportant to remember that fat does have aplace in a healthful diet. The key is to keepeverything in balance.

Chemical Structure of Lipids

Lipids are a category of organic com­pounds that are insoluble in water and have agreasy feel. Fats, oils, shortening, grease, phos­pholipids, sterols, and cholesterol are all termsused for lipids and their related compounds.

Lipids, like carbohydrates, contain carbon,hydrogen, and oxygen. However, lipids differfrom carbohydrates in several ways. Lipidsare not polymers, they do not provide struc­ture to food products, and they cannot be dis­solved in water.

There are three general types of lipids infoods and the human body. These are triglyc­erides, phospholipids, and sterols. Each ofthese types has a unique chemical structure.

The GlyceridesMost lipid molecules found in foods and

the body have two basic parts. The base orcore of these lipids is a glycerol molecule.Molecules that have a glycerol base are calledglycerides. Glycerol has three hydroxylgroups that will react easily with other com­pounds. See 10-I.

The second part of most lipid molecules iscalled a fatty acid. Fatty acids are organicmolecules that consist of a carbon chain with acarboxyl group at one end. A carboxyl group isa carbon atom, two oxygen atoms, and ahydrogen atom. See 10-2. The carboxyl groupof a fatty acid will readily react with a hydroxylgroup of glycerol. The products of this reac­tion are a lipid and water.

217

Glycerol Molecule

H

H-C-(§)

H - 6---@~ hydroxylI Q/ groups

H-C~

10·1 A glycerol molecule forms the base of mostlipids found in foods and the body.

Carboxyl Group

C=OIoIH

10-2 The carboxyl group found at one end of afatty acid molecule allows the molecule to bondreadily with glycerol.

Because glycerol has three hydroxylgroups, it can join with one, two, or three fattyacids. A monoglyceride is a glycerol with onefatty acid attached at the site of a hydroxylgroup. A diglyceride is a glycerol with twofatty acids attached. Triglycerides have a fattyacid joined at each of the three hydroxyl sites.See 10-3.

Fatty acid chains vary in length from 4 to24 carbon atoms. There are 20 fatty acids thatcan combine with glycerol to form lipids.

Most of the lipids found in foods and inthe body are triglycerides. Triglycerides canhave three different fatty acids attached to theglycerol base. It is also possible for triglyc­erides to have two or three fatty acids that areidentical. With 20 fatty acids to choose from,many combinations of triglycerides are possi­ble. These options create triglyceride mole­cules with different characteristics. The chem­ical variations lead to differences in cookingperformance, shelf life, and nutritional value.See 10-4.

Fatty acids and glycerol are polar mole­cules. The bodies of fatty acid are nonpolar.However, the carboxyl groups on fatty acidsare slightly positive. The hydroxyl groups onglycerol are slightly negative. When fattyacids are mixed with glycerol, the polar ends

218 Unit III Organic Chemistry: The Macronutrients

Formation of a Triglyceride

H 0 H 0I II I II

H-C- OH+ HO-C--Rl H-C-O-C--Rl + H20

I ~ I ~H-C-OH+ HO-C--R2 --.. H-C-O-C--R2 + H20

I ~ I ~H-C- OH+ HO-C-- R3 H-C-O-C-- R3 + H20

Reactants: glycerol and3 fatty acids

Products: 1 triglyceride and3 water molecules

10-3 This chemical formula shows how fatty acids and glycerol combine to form lipids and water. Rl1 R21

and R3 represent the carbon chains that make up the center of the fatty acids.

combine. The newly formed lipids have twoends that are nonpola1~ or neutral in nature.Because fat molecules are nonpolar, they willnot dissolve in water.

Mono- and diglycerides are partially solu­ble in water because of their hydroxyl groups.Their fatty acids also make these moleculessoluble in fat. This dual solubility gives mono­and diglycerides an important function in thefood industry. They are often added toprocessed foods to keep mixtures of water andfats stable. Butter and margarine are examplesof foods to which mono- and diglycerides areoften added.

PhospholipidsA second basic type of lipids is the phos­

pholipids. A phospholipid is a glycerol basewith two fatty acids and a phosphorus-contain­ing acid attached. The fatty acids are soluble infats. The phosphorus-containing acid is solublein water. This allows phospholipids to mix withboth water-based and fat-based substances.

Phospholipids play important roles in thebody and in food products. In the body, cellmembranes contain lipids, but the fluids onboth sides of the membranes are water based.Phospholipids help carry fats back and forthacross cell membranes into the water-basedfluids. In food products, phospholipids helpfats stay mixed in water-based solutions.Phospholipids keep foods like mayonnaisefrom separating.

SterolsThe third general type of lipids is the

sterols. The sterols are complicated moleculesderived or made from lipids. They includecholesterol, vitamin D, and the steroid hor­mones, including sex hormones. The mostfamiliar sterol, cholesterol, is a part of everycell in the human body. See 10-5.

Categories of Lipids

You need to understand the ways lipidsare categorized before you can understand theeffects of lipids in food mixtures. Lipids aregrouped according to molecular structure,physical state, and dietary sources.

Categories Based on MolecularStructure

One way to categorize lipids, or morespecifically fatty acids, is by how saturatedtheir carbon chains are with hydrogen atoms.In Chapter 4, you learned that each carbonatom is capable of forming four bonds. If twocarbon and six hydrogen atoms combine, allthe resulting bonds will be single bonds.

H HI I

H-C-C-HI IH H

Chapter'O Lipids: Nature's Flavor Enhancers 219

Co'mmon FCitty'Acids

Abbreviation* Name Structural Formula MeltingPoint (OC)

Food Sources

Saturated Fatty Acids

4:0

6:0

8:0

10:0

12:0

14:0

16:0

18:0

20:0

ButyricCaproic

Caprylic

CapricLaurie

MyristicPalmitic

Stearic

Eicosanoic

CH3(CH:J2COOHCH3(CH2)4COOH

CH3(CH2)6COOHCH3(CH2)sCOOH

CH3(CH2)10COOHCH3(CH2)12COOHCH3(CH2)14COOHCH3(CH2)16COOH

CH3(CH2)1SCOOH

-4.2

-3

16-16.5

31-32

44

54

63

70

74-76

buttermilk fats, ooconut

palm and coconut oils

nutmegmilkmilk, beef

10-4 These fatty acids commonly combine with glycerol to form a variety of triglyceride molecules.

*Abbreviation: Th'ls column is a shorthand description' of 'the fatty acid chains. The first number is the number of carbons in thefatty aCid. The second number is the number of double bonds.

Monounsaturated Fatty Acids

18:1 Oleic CH3(CH2hCH=CH(CH2hCOOH 14 milk, corn,cottonseed,peanuts, olives,sesame seeds, sun-flowers, canola oil,almonds, walnutscanola oil

22:1 Erucic CH3(CH2hCH=CH(CH2)11COOH

Polyunsaturated Fatty Acids

18:2 Linoleic CH3(CH2)4CH=CHCH2CH= -5 corn, cottonseed,CH(CH2hCOOH olives, peanuts,

sesame seeds,sun-flowers

18:3 Linolenic CH3(CH2)2CHCH2CH= -11 soybeans, wheatCHCH2CH=CH(CH2hCOOH germ

20:4 Arachidonic CH3(CH2)4CH=CHCH2CH= -49.5CHCH2CH=CHCH2CH=CH(CH2hCOOH

In the following examples, the carbon chainon the left contains the maximum number of

What happens if only four hydrogen atomsare available? The carbon atoms are still look­ing to form four bonds. A carbon atom canbond twice to another carbon atom. This willresult in the carbon atoms sharing two elec­trons. When two atoms form two bonds witheach other, the bond is called a double bond.

iliI

1(I

f!

I!j

r

III,I

Ii

li

Ii- -!Il

220 Unit III Organic Chemistry: The Macronutrients

H H HI I I ,f'0

-C-C=C-C-C-C=C-CI I I I I I I "0 - HH H H H H H H

hydrogen atoms. The one on the right doesnot. Notice that each double bond reduces thetotal numbel" of hydrogen atoms by two.

H H H H H H HI I I I I I I ,f'0

-C-C-C-C-C-C-C-CI I I I I I I "0 _ HH H H H H H H

Fatty acids that have one double bond in thecarbon chaul are called lJlOnOl/l1satlirated.Foods that are hjgh in monounsaturated fattyacids ulClude olive oil, almonds, walnuts, andcanola oil.

Polyllnsatllrated fatty acids have two ormore double bonds in the carbon chain. Eachdouble bond bends the fatty acid chain. Thebends make it difficult for the molecules topack together tightly. Safflower, sunflower,and corn oils are all high in polylmsaturatedfa tty acids.

A single triglyceride molecule can bemade of one saturated, one monounsaturated,and one polyunsaturated fatty acid. This mol­ecule would have characteristics of all threetypes of sa tura tion.

Iv[ost lipids contaul a combination of satu­rated, monounsaturated, and polYlU1saturat­ed fatty acids. The type of fatty acid present inthe largest amounts has the greatest effect onthe characteristics of the lipid. See 10-7.

Categories Based on Physicol StateOne of the easiest ways to categorize

lipids is by their physical state at room tem­perature. Lipids that are solid at room tem­peratme are commonly called fats. If lipids

Cholesterol

CH,

"CH -CH,-CH,I

CH, CH,

ICH/"

CH, CH,

CH,

HO

Fatty acids can be grouped by the numberof double bonds in their cal"bon chains. Fattyacids will have zero, one, or multiple doublebonds. The number of double bonds deter­mines how close the carbon chain is to con­taining the maximum number of hydrogenatoms.

The first group of fatty acids has no doublebonds present in the carbon chain. This meansthe carbon chaul contains the maximum num­ber of hydrogen atoms. When fatty acids havethe maximum number of hydrogen atoms,they are described as satllrated.

Butyric acid is a saturated fatty acid foundin butter. Stearic acid is a saturated fatty acidthat is a major component of beef fat. Noticethese examples of saturated fatty acids arefound in animal sources. Generally, lipidsfound in animal sources are high in saturatedfatty acids. See 10-6.

If a fatty acid does not contain all the hydro­gen atoms it could contain, it is lll/satl/rated.

,.

,"

10-5 Cholesterol, like other sterols, is a complexlipid molecule.

10-6 Cheese and pepperoni can make pizza atoad that is high in saturated fat.

Chapter 10 Lipids: Nature's Flavor Enhancers

Nutrition NewsWhat Are Trans-FattyAcids?

. -.

221

Research has indicated thattrans-fatty acids might be asharmful to your health as highly­saturated butterfat. Trans-fattyacids are found in many mar­garines. Until more research isdone, some health experts haverecommended eating solid mar­garines in moderation.

What is at the heart of theconcern is the shape of unsatu­rated fatty acids at the doublebond site. One of two things canhappen when a double bondforms in a fatty acid. As seen inthe figure to the right, cis-isomersbend or kink at the bond. Thetrans-isomers take a fairly linearshape. Isomers are two mole­cules that have the same molec­ular formula, but the atoms arearranged in different patterns.

Most unsaturated vegetableoils in nature have the

cis- formation. The bends andkinks make it difficult for the mol­ecules to move close togetherand solidify. Trans-fatty acids willsolidify at lower temperaturesbecause they are linear. Trans­fatty acids are a product ofhydrogenating vegetable oil.Originally it was thought thattrans-unsaturated fatty acids

would be nutritionally identical tothe cis-unsaturated fatty acids.Evidence indicates that the bodymay not be able to effectivelydigest the trans- form. These fattyacids could build up in blood ves­sels, increasing the risk of ather­osclerosis. More study needs tobe done to understand how thebody reacts to trans-fatty acids.

Trans-

•

are liquid at room temperature, they arecalled oils.

The numbers of hydrogen and carbonatoms on a fatty acid chain determine the tem­perature at which lipids liquefy. Double bondsin the fatty acid chain lower the temperatureat which a lipid will be liquid rather thansolid. Lipids that are liquid at room tempera­tme have one or more double bonds in thecarbon chain of the fatty acids.Polyunsaturated fatty acids become liquid atlower temperatures than saturated andmonounsatm"ated fatty acids. This characteris­tic is due to the bent shape of the chain and thelower number of hydrogen atoms.

Fats contain mostly sahu"ated fatly acids.Oils contain more monounsaturated andpolyw1saturated fatty acids. This means fatshave more hydrogen atoms than oils.Therefore, fats are denser and require moreenergy to liquefy. This causes the meltingpoint of fats to be higher than the meltingpoint of oils. Melting point is the temperatureat which a lipid is completely liquid.Although oils are liquid at room temperature,they will solidify if chilled to a low enoughtemperature.

Different types of fats and oils are saturat­ed to different degrees. Therefore, each type offat or oil has a different melting point. For

222 Unit III Organic Chemistry: The Macronutrients

Fatty Acids in Common Fats and Oils

Safflower oil 14

Walnut oil 28

Canola oil 61

Saturated Polyunsaturated

48

22

19

24

Peanut oil

Chicken fat

Margarine

Cottonseed oil

Soybean oil

Olive oil

Corn oil 25

Sunflower oil 20

, I

~.l

Lard

Shortening

Palm oil

47

47

39

'.1

Beef fat

Butterfat

Palm kernel oil

Coconut oil

10-7 The way fats and oils perform in cooking and food products is determined by the types of fatty acidsthey contain.

instance, oils high in polyunsaturated fattyacids will have lower melting points than oilshigh in monounsaturated fatty acids.

Oleic acid has one double bond in its car­bon chain and is therefore monounsaturated.It is used extensively in the production ofmargarine. Remember that double bonds

lower the melting point of lipids. Therefore,margarines with high oleic acid content willbe softer than butter, which contains more sat­urated fatty acids. These margarines will alsomelt a t lower tem pera tures than butter.Margarines have been developed with a widerange of melting points. Squeeze margarines

Chapter 10 lipids: Nature's Flavor Enhancers

stay fluid in the refrigerator. Baker's mar­garine, which is designed for making puffpastry, melts completely at 57°C (135°F).Butter melts at body temperature of 37°C(98°F).

Fats and oils usually come from differentsources. Fats usually come from animalsources. Examples include butterfat frommilk, lard from pigs, and tallow (used in can­dles) from animals such as sheep and cattle.Most oils come from plant sources. Examplesinclude corn, soybean, peanut, canola, andolive oils. See 10-8.

Hydrogenated Vegetable OilsHydrogenation is the process of adding

hydrogen atoms to an unsaturated lipid toincrease its saturation level. This process isused to make some oils solid at room temper­ature. The result is a product that has a highermelting point than tl1e oils. An example ismargarine made from 100% corn oil. Solidvegetable shortening is also made from hydro­genated vegetable oils.

Hydrogenation is done by bubblinghydrogen through liquid oil in the presence ofa nickel catalyst. The double bonds in the fattyacid cl1ains of the oil break. The chains pick upextra hydrogen atoms, becoming more satu­rated. The process can be stopped at anypoint. If oils were to be completely saturated,they would become too brittle for most uses ofsolid fats.

The most commonly hydrogenated oil issoybean. Cottonseed and palm oils are oftenadded in small amolli1ts.

223

Advantages of hydrogenated vegetableoils include

.:. longer shelf life tl1an oil or lard

.:. greater stability than lard

.:. lower production costs than lard

.:. faster dissolving and setting properties inchocolate production

Categories Based on Dietary SourcesAnotl1er way to group lipids is based on

the food sources from which they come.Triglycerides come from seven main groups ofdietary sources. Eacl1 group of triglycerideshas a similar molecular structure and physicalcharacteristics.

Milkfats come from the milk of cows,goa ts, and other mammals. Milkfa ts are highin palmitic, oleic, and stearic acids. The maindifference between milkfats and fats such aslard or tallow is the length of the fa tty acidmains. Most fatty acids in milk are shorterchains of 4 to 12 carbon atoms.

Lauric acids are the main component of agroup of lipids found in palms such ascoconut. Lauric acid makes up 40% to 50% ofaU the fatty acids in this lipid group. Lauricacid has a low melting point. These lipids arethe most saturated of the oils found in plants.

Vegetable butters come from the seeds oftropical plants. These lipids have at least oneunsaturated and one saturated fatty acid onevery molecule. Because the moleculararrangements are so sin1ilar, these lipids havea very narrow melting range. The most

10-8 Hamburger, milk, and butter are sources of animal fats in a typical U.S. diet. Sources of vegetableoils include rapeseeds (canola oil), corn, olives, safflowers, sunflowers, and soybeans.

! Ii/.

~ .J,

1':, '

li,J1rIh,al,

l!li Ii

224

important member of this group is cocoabutter, which is used frequently in candies.

Oleic-linoleic acids come from corn,peanuts, sunflowers, olives, cottonseeds, andsesame seeds. These lipids make up thelargest group of triglycerides. They containless than 20% saturated fatty acids.

Linolenic acid is found in large amounts insoybeans and wheat germ. \!\Theat germ needsto be refrigerated after opening. This isbecause linolenic acid reacts easily with oxy­gen during storage. This reaction causes flavorchanges that are often undesirable.Refrigeration will help prevent these unwant­ed flavor changes.

Animal fats are fOlU1d in meats and poul try.They contain large amounts of fully saturatedfatty acids. This gives them high meltingpoints. Animal fats are present as visible fatdeposits as well as specks and streaks scat­tered throughout muscle fibers. The specksor streaks of fat in muscle tissue are calledmarbling. Marbling is an indicator of flavorand tenderness. The more marbling a meatcut has, the more flavorful and tender it willbe. However, marbling also indicates a higherfa t content.

Marine oils come from fish. These oils con­tain large amounts of long-chain polyunsatu­rated fatty acids. These fatty acids have asmany as six double bonds each. The highdegree of lU1saturation makes these oils spoilor develop off flavors and odors very quickl),.This is why fish must be eaten or frozen with­in 24 hours. See 10-9.

Physical Characteristics of Lipids

Three physical characteristics impact theway lipids perform in food products. One isthe melting and solidification points of lipids.The second is the nonpolar nature of lipidmolecules. The third physical characteristic isthe tendency of lipids to react with oxygen.

Solidification point~

Unit III Organic Chemistry: The Macronutrients

10-9 Fish is a source of marine oils. which arehighly unsaturated.

Differing Melting and SolidificationPoints

Ice melts at DoC. Water freezes at DoC. Themelting and freezing points of wa ter are thesame temperature. Unlike water, lipids do nothave a specific melting point. This is becausemost lipids are mixtures of different kinds offatty acids. Because each fatty acid has a dif­ferent melting point, the lipids in a mixturewill melt at different temperatures.

The lipids in a mixture will also becomesolid at different temperatures. This results ina temperature range between the point atwhich all lipids are solid and all lipids are liq­uid. Lipids within this range are a mix of solidand liquid. The temperature at which all lipidsin a mixture are in a solid state is called thesolidification point. (Lipids are said to solidifyrather than freeze.) The solidification pointfor lipids is lower than the melting point. Thisis illustrated by the following diagram.

/ Melting point

all lipids are solid lipids are a mix ofsolid and liquid

all lipids are liquid

«

Chapter 10 Lipids: Nature's Flavor Enhancers

Lipids containing mostly saturated fattyacids have a higher melting point than lipidscontaining mostly unsaturated fatty acids.This is because the number of hydrogen atomsper carbon atom affects the melting point. Thenumber of carbon atoms in the fatty acid chainalso affects the melting point. The shorter thecllain of carbon atoms in the fatty acid is, thelower the melting point will be. For example,butter has high levels of butyric acid, whidlcontains 4 carbon atoms. Butter melts at alower temperature than beef fat. Beef fat ishigh in stearic acid, whim contains 18 carbonatoms. Both butyric and stearic acids are satu­rated. However, stearic acid's longer chain ofcarbon atoms results in a higher melting poin tthan butyric acid.

You may have noticed that vegetable oilbecomes cloudy when you refrigerate it. Thisis because some of the triglycerides have asolidification point that is higher than thetemperature in most refrigerators. The oilsused in salad dressing have been processed toprevent solidification at refrigerated tempera­tures. The vegetable oil is chilled until solidifi­cation of the higher-melting triglyceridesoccurs. The oil is then filtered to remove thesolid fat crystals. This is why commercialsalad dressings are easier to pom after chillingthan homemade ones.

225

Nonpolar MoleculesAs you recall from Chapter 7, water mole­

cules are polar. This means they have anunequal sharing of electrons. Lipid moleculeshave an equal or balanced sharing of elec­trons. This causes lipid molecules to be non­polar. Substances that are nonpolar dissolve orreadily mix with other substances that arenonpolar. Polar compowlds djssolve in orreadily rillx with other polar compowlds.However, polar and nonpolar molecules arenot attracted to each other. Therefore, waterand oil will not nUx.

Lipids are very large molecules. The vari­ety and shape of fatty acid chains createspaces between the parts of the molecules.These spaces cause lipids to be less dense thanwater. These spaces also prevent lipid mole­cules from packing together tightly. BecauseLipids are nonpolar, polar water moleculeswill not slip in and fill the spaces betweenlipid molecules. This results in the low densi­ty of lipids compared to water-based com­pOlLl1ds. This is why oil floats on water.

You can demonstrate tJus physical charac­teristic of lipids by combining vinegar and oil.The oil will rise to the top. For vinegar and oilto stay mixed in salad dressings, another sub­stance, called an emulsifier, must be added.This substance must have polar and nonpolarportions. See 10-10.

~-2~The next time you eat a piece

ot chocolate, savor the flavor byletting it melt in your mouth. Thereason chocolate melts in yourmouth is that fat in chocolate has

a very narrow melting range. Thisis because most of the lipids inchocolate have the same chemi­cal structure. The melting point ofthese lipids is close to body

temperature. As the fats melt,they release the chocolate flavor.The melting fat also gives choco­late candies their smooth mouthfeel.

226

10-10 Oil and vinegar dressing separatesbecause the tat molecules in oil are nonpolar andthe vinegar molecules are polar in nature.

Tendency to DeteriorateAn important characteristic of lipids is

their tendency to react with oxygen. AlIto­oxidation is a complex chain reaction thatstarts when lipids are exposed to oxygen. Theoxygen will bind to the lipid molecules andthen to other compounds. Once started, auto­oxidation is like knocking over dominoes. It ishard to stop and spreads quickly.

Auto-oxidation causes lipids to deterio­rate. When oxygen is added to lipids, newcompounds are formed. These compoLmdshave an lmpleasant flavor and odor that isdescribed as rancid. Ra11cidity is a form offood spoilage, bu tit poses no short-termhealth risks. The main problem is the colorand flavor changes.

Unsaturated oils are more susceptible toauto-oxidation than saturated fats. This isbecause unsaturated oils contain doublebonds, which are weaker than single bonds.Oxygen can readily bind with lipid moleculesat the sites of the double bonds.

Unit III Organic Chemistry: The Macronutrients

Another type of deterioration occurs whentriglycerides are hydrolyzed. You recall fromChapter 8 that hydrolysis occurs when a largemolecule is divided into smaller pa.rts byadding water. When water is added to lipidmolecules, the molecules break apart into freefatty acids and glycerol. The shorter the car­bon chains are, the more likely the fatty acidsare to become rancid and develop off flavors.Butyric and caproic acids are short-chain fattyacids in butter. They are responsible for theunpleasant odor and flavor that developswhen butter becomes rancid. Long-chain fattyacids like stearic, palmitic, and oleic usuallydo not develop off flavors unless auto­oxidation occurs also.

Oxygen exposure can cause high-fat foodsto become rancid. To minimize oxygen expo­sure, SOine products are vaCUtlln sealed orflushed with nitrogen gas. Rancidity can alsobe prevented or slowed by adding antioxi­da.nts to lipids. A11tioxida11ts are compoundstha t will quickly react wi th oxygen to formnew substances. Antioxidants will react withthe oxygen before lipids do. Important dietaryantioxidants are vitamins A, C, and E.

Storage TipStore high-fat foods in a dark, oxygen-freeenvironment. This will slow the develop­ment of rancid flavors.

Functions of lipids in FoodPreparation

The structures and characteristics of thevarious types of fatty acids affect how they per­form in food products. Lipids serve six mainfunctions in cooking. Lipids act as heat medi­ums, tenderizers, aerators, flavor enhancers,lubricants, and as liquids in emulsions.

Transfer HeatLipids are an excellent heat medium. They

transfer heat from cooking utensils to foodquickly, evenly, and at very high tempera­tures. At normal ail' pressure, water boils at100°C and will not get any hotter no matterhow long you heat it. The temperature oflipids will continue to increase as heat is

Chapter 10 Lipids: Nature's Flavor Enhancers 227

10-11 Each time oil or fat is heated, the smokepoint drops. The higher the smoke point is, themore the oil or fat can be reused.

TenderizeFats are used to tenderize baked products.

Flour makes up the structure of most bakedgoods. The protein in flour has the tendencyto form long strands. The longer the strandsare, the tougher and chewier the food willbecome. Fat tenderizes by shortenu1g theselong strands. (This is why the solid white fatsold for baku1g is called sllOrlenillg.) Solid fatscoat the flour particles, maku1g the doughslippery. This prevents long protein strandsfrom forming.

The fat to flour ratio of a dough will deter­mine how flaky a baked product is. Biscuits

amount of food at a time. The more food thatis added to the hot oil, the more the tempera­ture of the oil will drop. If the oil drops below175°C (350°F), the food will become soggy.

After coating foods in batter for deep fry­ing, let them sit for 20 to 30 minutes. (Thisallows the starch in the batter to bind to thefood.) Fewer pieces of the batter will break offinto the oil and burn.

Do not salt food lU1til after deep frying.Salt pulls water to the surface of the food. Thiswill cause increased splattering when the foodis placed in hot oil. Salt also lowers the smokePOU1t of the oil and will, therefore, reduce thetime the oil can be used.

°C

266

257

246

227

216

210

191

185

177

Smoke Points of Fats and Oils

Lipid OF

Safflower oil 510

Soybean oil 495

Corn oil 475

Peanut oil 440

Sesame oil 420

Vegetable shortening 410

Olive oil 375

Lard 365

Butter 350

added. Because of this characteristic, lipidswill get hot enough to brown food. The exteri­or of the food will also develop a crisp texture.

Heat cannot be added to lipids indefinite­ly. Every lipid has a temperatme at which thefatty acids begin to break apart and producesmoke. This temperatme is called the smokepoint. As the fatty acids break down, theycombine with oxygen to form new com­pounds. These compounds have strong,unpleasant flavors. Once oil begins to smoke,breakdown has occurred and the oil should bediscarded. It will no longer fry food success­fully without creating lmdesirable flavor andcolor changes in the food. See 10-11.

Lard has a smoke point of 185°C (365° F).This is the same as the recommended temper­ature for deep frying. This means you have toheat lard to its smoke point when using it fordeep frying. Therefore, you can use it for deepfrying only one time for a short period.

If oil is heated long enough, it will becomehot enough to bmn. The flash point is thetemperature at which lipids will flame. If oil isheated without close monitoring, the temper­atme will rise w1til small flames appear acrossthe smface. This occurs at around 315°C(600°F).

Successful deep frying requires a hotenough temperature to cook the food all theway through without bumu1g the outside. Iffood is fried at temperatures below 175°C(350°F), the exterior does not brown fastenough. Excess oil will soak into the food. Theresult will be pale, soggy food that is extreme­ly high in fat content. If the oil reaches 205°C(400°F), the exterior of the food will begin tobum before the interior is done. The result willbe very dark colored food with a raw center.

Using an electric deep fryer can make suc­cessful deep frying easier and safer. Theseappliances are thermostatically controlled.The fryer will automatically turn off the heatwhen the oil reaches the set temperature. Thisreduces the risk of starting a kitchen fire.

When cooking at home, you may wish touse a number of guidelines food scientists fol­low for successful deep frying. Cut foods fordeep frying into small even pieces to decreasecooking time. Remove excess moisture fromfoods to reduce splattering. Cook only a small

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have a ratio of about one part fat for eightparts flour. This is just enough fat to givebiscuits the ability to pull apart in sheets ofmoist bread.

Pie crust recipes call for about one part fatfor every four parts flour. Too mud1 fat willcause the pie crust to fall apart. Too little fatwill cause the pie crust to be tough.

The first step in preparing biscuits and piecrust is cutting the fat into the flour. This stepcontinues lmtil the mixture resembles smallpeas or coarse crumbs. Cutting in fat distrib­utes it evenly throughout the dough.Ovennixing will cause the fat to soften to thepoint that it will begin to ding together. Ifworked too long, the fat-flour mixture wouldform a greasy ball. The dough would nolonger be suitable for biscuits or crust.

Puff pastry has a mixture of fat and flourlayered between thin sheets of yeast dough.The dough is kneaded to develop its structure,then it is rolled into a thin rectangle. A mixtureof two parts fat to one part floUl' is rolledbetween waxed paper and chilled. Once thismixture has solidified, it is placed on top ofthe yeast dough. The dough is then folded,sealed, and rolled thin. The dough is foldedand rolled w1til there are as many as fifty lay­ers of dough and fat. The flakiness of puff pas­try, croissants, and Danish result from the thinlayers of fat melting during cooking. Thedough browns and separates into the charac­teristic thin, tender, flaky sheets. See 10-12.

In ead1 of these food prod ucts, tenderizingis a result of the fat separating but not soakinginto the flour. The lipids that work best for thisfunction are the solid fats. The higher themelting point of the fat is, the longer the mix­ture can be worked without the fat meltinginto the flour. Butter, regular margarines, lard,and shortenings give the best performance.Whipped and liquid margarines will not workbecause of their high air and water contents.

Lipids tenderize other baked goods,including cakes, pancakes, muffins, and waf­fles. Fats aid in giving these products a fluffy,moist texture. The separating of the flourkeeps the prod ucts tender. Oil can be usedinstead of fat when the batter is a quick mixtype. This type of batter combines the liquidsin one bowl and the dry ingredients in anoth­er. The two mixtures are then blended.

Unit III Organic Chemistry: The Macronulrienls

10-12 Pie crust, biscuits, croissants, and pastriesmade from puff pastry are flaky and tenderbecause 01 the solid fat in the dough.

AerateAemtio/1 is the addition of air into a batter.

Saturated fats allow tiny air pockets to formwhen batters are sufficiently beaten. In orderfor aeration to occur, the fat must be able tohold its shape around the air pockets. This iswhy oils cannot be successfully substituted forfat in most cake recipes. Because oils are notsolid at room temperature, they cannot pro­vide the structure needed to trap air. The oilwill start to separate from the mixture beforecooking can stabilize the nonpolar moleculesthroughout the batter. The result is a grainiertextured cake.

Most conventional cake recipes call for thefat and sugar to be creamed (beaten together).The purpose of this step is to aerate the fat.The creaming process requires a fat that willsoften but not melt when beaten. Butterremains workable beh"een 18°C and 21°C(65°F and 70°F). At higher temperatures, thefat in butter becomes too liquid to supporttrapped air cells.

Timing the creaming step is important.Because beating increases friction, it increasesthe batter temperature. If the temperaturegoes too high, the fat will melt and thetrapped air will be lost. This results in a crum­bly, grainy texture.

Another example of aeration in lipids iswhipped margarines. Whipped margarineand butter products are made more spread­ilble by adding air. The fat is beaten untiltiny air pockets ilre trapped throughout.

Chapter 10 Lipids: Nature's Flavor Enhancers

Unwhipped butter and margarine are oftenpackaged as four quarter-pound sticks.Whipped butter and margarines come sixsticks to a pOlmd. The extra volume in thewhipped products is due to trapped air. Thatis why one stick of whipped butter calU10t besubstituted in recipes for one stick of butter.Whipped products can help lower fat in thediet. They make it seem as if you are usingmore butter on your food than you really are.

Enhance FlavorAn important function of fat in foods is

providing flavor. Much of the flavor in foodcomes from salt, sugar, and fa ts. Because theywant more flavor, most people in the UnitedStates get too much of these three ingredients.

Some of the fats in your diet are used asseasonings to flavor foods. People spread but­ter and margarine on bread and rolls mainlyfor flavor. Cooks add bacon fat to beans,soups, and sauces for the distinctive flavor itwill give the finished product. Chefs cookonions, garlic, celery, and peppers in fat as afirst step in preparing many sauces, soups,and casseroles, 10-13. Fat dissolves and dis­perses the flavor compounds from the vegeta­bles. The flavor will be sh'onger than if thevegetables were just sin1mered in a broth base.Olive oil provides flavor to salad dressings,and sesame oil enhances the taste of manyAsian dishes.

There are times when you want to taste theflavor of the main food prod uct and not thefat. When this is the case, it is best to use oilsthat have little or no flavor of their own.Cottonseed oil is one of the most flavorless ofthe oils. This is one reason why it is a favoriteof potato chip manufacturers. Vegetable short­ening, soybean oil, and canola oil are other rel­atively flavorless fats and oils.

LubricateLipids lubricate food components, making

it easier for them to slide over one another.This characteristic makes meat easier to chewas the fat content increases. Meat that is mar­bled has small flecks of fat evenly distributedtlu'oughout the muscle fibers. The even distri­bution of the fat creates a pleasant mouth feel.

229

10·13 Sauteing vegetables in fat brings out theirflavors.

Because lipids have a greasy textme, theyfeel slick or smooth to the tongue and palate.This characteristic causes many foods to seemmoister. This is one reason why butter, mar­garine, and mayOlmaise are popular spreadsfor breads and rolls. These spreads add to thefeeling of moistness without making thebread soggy.

Serve as Liquids in EmulsionsLipids are usually one of the two liquids in

an emulsion. An emulsion is a mixture thatcontains a lipid and a water-based liquid. Thismixture will not stay mixed together unless acompound is added that has a polar and anonpolar end. Examples of common fat-basedemulsions in food are mayonnaise, butter,milk, and bottled salad dressings. Phos­pholipids are compounds that help createemulsions. You will read more about thechemistry, application, and characteristics ofemulsions in Chapter 11.

230

Lipids in Your Diet

Most people in the United States consumetoo much saturated fat. On the other hand,some people try to eliminate all fats from theirdiets. See 10-14. Because lipids are an impor­tant part of a healthful diet, it is important tofind a balance. To correctly monitor lipids inyour diet, you need to understand the func­tions of lipids in your body. You also need tounderstand the role of cholesterol and its rela­tion to lipids.

Functions of Lipids in the BodyLipids have four important functions in

your body. The first is as a concentratedsource of energy. Lipids have 9 Calories pergram. That is more than twice the energy pro­vided by a gram of sugars or starches. Lipids

Unit III Organic Chemistry: The Macronutrients

take longer to digest than carbohydrates andgive a feeling of fullness longer. They helpprovide a steady supply of energy to yourbody between meals.

Your body stores fat not needed for energyin fat cells. Triglycerides are the body's stor­age form of fats. There are two kinds of fatcells: white fat cells and brown fat cells. Whitefat cells are mainly composed of one largedroplet of fat. The body tends to hold ontothese reserves. The cell expands as fat is addedand shrinks when fat is used. The brown fatcells contain many mitochondria. This is thepart of the cell in which the body producesenergy.

Two other functions of fats in the body arecell production and temperature regulation. Adiet that is 100% fat free for extended periodsis dangerous. Cell walls are made from a

Fat Consumption Trends in the United States (1977-1997)

Food Source Annual Increase Food Source Annual Decreaseper Person per Person

Baking and frying fats 5.81bs Table spreads 2.31bs

Salad and cooking oils 8.81bs Margarine 2.21bs

Cheese 171bs Cottage cheese 21bs

Fat free milk 2.7 gal Whole milk 11 gal

Poultry 311bs Beef, pork, and lamb 21 Ibs

Fish 31bs Eggs 72

Overall Changes

.:. People have decreased their total fat consumption by 80/0 of total calories.

•:. People are drinking 23% less milk and, eating 2.5 times more cheese.

•:. Annual per capita consumption of red meat has decreased by 21 pounds. However, total meat con­sumption has increased by 13 pounds per person annually.

•:. Calories consumed when eating out has almost doubled. The percentage of those calories from fathas decreased by 4%. The percentage of calories from fat is over 60/0 higher than that of home-pre­pared foods.

•:. The percentage of calories from fat in foods prepared at home has decreased by over 10%. However,this percentage is still 1.5% above the goal of 30% of total calories from fat.

Sanford, Scott and Allshouse, Jane. "Have We Turned the Corner on Fat Consumption?" Food Review, Sept.-Dec. 1998, pp. 12-18.

10-14 Health and nutrition findings toward the end of the twentieth century encouraged many people tochange their fat consumption patterns.

Chapter 10 Lipids: Nature's Flavor Enhancers

combination of lipids and protein. The bodyalso deposits fatty tissue around the vitalorgans to protect them from injury. Fatty tis­sue under the skin has two functions: to insu­late and to provide a reserve energy supply.Fat helps maintain your body temperature byacting as an insulator that holds in bodywarmth. Fat reserves in the body provideenergy when you consume too few calories ordeplete your glycogen stores through exercise.

The last function of fat is to help transportvitamins. Some vitamins are fat-soluble. Theyneed to combine with fat to be transported towhere they are needed in the body.

Essential Fatty AcidsFatty acids tha t cannot be produced by the

human body are called essential fatty acids.

231

The only hoVO fatty acids the body cannotmake are linoleic acid and linolenic acid. Bothof these fatty acids are polytUlsaturated andare found in most plant and fish oils. They areessential for growth and development.Linoleic acid is found ll1 large amounts incorn, cottonseed, and soybean oils. Chicken isanother good source of linoleic acid. Linolenicacid is found in canola oil, soybean oil, wal­nuts, and fish.

Omega-3 Fatty AcidsStudies have been done on Greenlanders

and Inuits. Researchers have tried to find outwhy these groups have low rates of heart dis­ease in spite of high-fat diets. Most of the fat intheir diets has been found to come from fish.Fish are very ltigh in the omega-3 fatty acids.

Nutrition NewsCarbohydrates vs Fats

Question: Which is more fat­tening, 100 calories of carbohy­drates or 100 calories of fat?

Recent research with rats hasrevealed that people may gainmore weight from fat caloriesthan from carbohydrate calories.

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The toast on the lell has 100 caloriesworth of fat from buller. The toast onthe right has 100 calories worth ofcarbohydrates from jam.

This is because fats require lessenergy than carbohydratesrequire to be converted for stor­age in the body.

When 100 calories of fat isdigested, the triglycerides breakinto fatty acids and glycerol.These parts pass through theintestinal wall into the blood­stream, where they can quicklyrecombine. The recombined fatgoes into fat storage cells if it isnot needed for energy. It takes 3of the 100 calories to digest thefat and then store it.

Carbohydrates that are notneeded for energy can also bestored as fat in the body.However, to change 100 caloriesof carbohydrates from starch into

fat, the body must first breakdown the starch. Once the starchis broken into individual sugarmolecules, the sugar is disman­tled. Then it must be assembledinto fatty acid chains. The fattyacids join to a glycerol and thenare stored as fat. This processuses 23 of the 100 calories.

if you eat 100 calories of fatyou do not need, 97 calories arestored as fat. If you eat 100 calo­ries of carbohydrates you do notneed, the body will first store itas glycogen. If glycogen storesare full, the carbohydrate ischanged to fat. Only 77 caloriesare left to be stored as fat.

Answer: Fat calories are morefattening!

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232

Omega-3 fatty acids have a double bondbetween the third and fourth carbon atomsfrom the end with the methyl group (CE3).

Further study is needed to understandhow omega-3 fatty acids work in reducingheart disease. However, research indicatesthese fatty acids help lower triglyceride levelsin the blood and slow the growth of plaque inthe arteries.

Fatty fish are high in two kinds of omega-3fatty acids, eicosapentaenoic acid (EPA), anddocosahexaenoic acid (DHA). Recent researchon fatty acids from fish sources seems to indi­cate that EPA and DHA strengthen brain-cellmembranes improving cell-to-cell commw1i­cation. They may also reduce joint inflamma­tion and prevent heartbeat irregularities andmental decline.

Researchers are now recommending twoto four meals (eight ounces) of fatty fish perweek. Fish that have pink or red flesh arehigher in omega-3 fatty acids than fish withwhite flesh. Albacore tuna, salmon, lake trout,and sardines are good sou.rces, 10-15. Canolaoil, flaxseed, and walnuts are good sources ofalpha-linolenic acid that can become omega-3fatty acid in the body.

The Role of CholesterolCholesterol is used to build cell mem­

branes. Up to 25% of all cell walls are choles­terol. Cholesterol is a rigid molecule thathelps solidify cell walls. Because cholesterol

10-15 Sardines, salmon, and tuna provideomega-3 fatty acids.

Unit III Organic Chemistry: The Macronutrients

is insoluble in water, it adds stability to thecell's structure. The body makes cholesterolfrom lipids.

Too much cholesterol in the blood resultsin lipids and cholesterol being deposited onartery walls. These deposits are called plaque.Because cholesterol is rigid and insoluble inwater, these deposits reduce the elasticity ofartery walls. This hardening of the arteries isknown as athel'Osclerosis. Blood pressureclimbs as the heart works harder to forceblood through the narrowing arteries. Thisdisease is the leading cause of heart attacksand stroke.

Cholesterol is transported throughoutthe body by lipoproteins. Lipoproteins areclusters of lipid and protein molecules. Low­density lipoproteins (LOU carry cholesterolfrom the liver. High-density lipoproteins (HOUfind unneeded cl10lesterol and return it to theliver. HDL and LDL work together to keepcholesterol levels in balance. Too much LDLcan clog arteries, increasing the risk for heartattack or stroke. High levels of HDL appear toprotect against heart attack. An optimal levelfor LDL is less than 100 mg/dL and greaterthan 40 mg/dL for HDL.

The average adult consumes 300 mg ofcl10lesterol a day. The liver makes another1,000 mg from fats. Although high cholesterolintake is not wise, it is not the only problem. Ahigher level of LDL means a higher risk ofdeveloping atherosclerosis. To a small degree,you can lower LDL through your diet.However, the most successful means of chang­ing HDL and LDL levels appears to be regularexercise and maiJ1tainjng a healthy weight.

A number of tips can help people limit thefats and cholesterol in their diets. It is impor­tant to monitor overall fat intake, not just cho­lesterol. This is because saturated fats increasethe liver's production of LDLs. High-fat dietscan cause buildups of plaque to begin duringthe teen years.

Choosing lowfat and fat free foods is oneway to limit fat intake. Food scientists havebeen busy developing hW1dreds of new prod­ucts. The scientists are tTying to meet the con­sumer demand for 100"fat versions of high-fatfavorites.

Chapter 10 lipids: Nature's Flavor Enhancers 233

Technology TidbitMeasuring Fat Content

Sandra E. Kays, a researchchemist for the USDA in Athens,Georgia, her graduate studentLaura L. Vines, and her col­leagues have developed a way tomeasure fat content in cerealproducts with near infrared

reflectance readings. Traditionalmethods of measuring fat con­tent require 10 hours and the useof chemical solvents. When per­fected, this method will require10 minutes and a spectrometerwith a spectral resolution of

10 nm. The FDA and Food Safetyand Inspection Service must cer­tify the technique, and near-IR­reflectance models will have tobe developed for eachapplication.

Some vegetable oils have "cholesterol free"on their labels. Such labels are intended tomake products appear more healthful.However, all vegetable oils are d10lesterol free.Cholesterol is found only in animal sources,sum as meats, dairy products, and egg yolks.

Keep in mind that many molesterol-freefoods are lugh in fat. Remember the body canmange fat into cholesterol. Beware of foodswhose label reads "pure vegetable oiL" Manyof these foods contain lugh levels of coconutand palm oils. These tropical oils are more eco­nOnUcal than other vegetable oils, but they arelughly saturated. This makes them as likely toralse blood cholesterol levels as animal fats.

Health TipA high-fiber diet can help trap dietary cho­lesterol and move it through the body. Thiskeeps the digestive tract from absorbingsome of the cholesterol from foods.

Unsaturated OilsHeart disease has been found to be low

among people in the Mediterranean region.Their diets are not low in fat. However, mostof the fat consumed is from olive oil. Thevery low levels of saturated fatty acids inolive oil are an important factor. The most

important factor, however, appears to be thehigh number of monounsaturated fatty acids.

Polyw1saturated oils lower LOL however,they also lower the beneficial HOL.Monounsaturated oils lower LOL withoutlowering HOL. This is why some health pro­fessionals recommend olive and canola oils asthe preferred oils in a healthful diet.

Olive oil has two disadvantages. It has adistinctive flavor that is popular in salads butmay not be suitable in all foods. It also has alow smoke point. This makes it a poor choicefor deep frying. However, it can be used suc­cessfully for stir-frying because of the shortcooking time.

Dietary RecommendationsWhat is the balance when it comes to fat in

the diet? The recommended guidelines are20%-35% of calories should come from fat.Children and adolescents typically require thehigher percentage of calories in their dietsfrom fat. Adults whose body mass index (BMl)is in the low to nud 20's should stay w1der 30%of their calories from fat. People who are over­weight should keep their fat intake around20%. Most fats in the diet should corne fromsources of polyw1saturated and monounsatu­rated fatty acids. A fat-free diet does not pro­vide the essential fatty acids needed for newcell growth and other body functions.