浙大食化考研.docx

浙大食化考研.docx

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浙大食化考研

Chapter4Lipids

4.1Introduction

Lipidsconsistofabroadgroupofcompoundsthataregenerallysolubleinorganicsolventsbutonlysparinglysolubleinwater.Theyaremajorcomponentsofadiposetissue,Andtogetherwithproteinsandcarbohydrates,theyconstitutetheprincipalstructuralcomponentsofalllivingcells.Glycerolestersoffattyacids,whichmakeupto99%ofthelipidsofplantandanimalorigin,havebeentraditionallycalledfatsandoils.Thisdistinction,basedsolelyonwhetherthematerialissolidorliquidatroomtemperature,isoflittlepracticalimportanceandthetwotermsareoftenusedinterchangeably.

Foodlipidsareeitherconsumedintheformof“visible”fats,whichhavebeenseparatedfromtheoriginalplantoranimalsources,suchasbutter,lard,andshortening,orasconstituentsofbasicfoods,suchasmilk,cheese,andmeat.Thelargestsupplyofvegetableoilcomesfromtheseedsofsoybean,cottonseed,andpeanut,andtheoil-bearingtreesofpalm,coconut,andolive.

Lipidsinfoodexhibituniquephysicalandchemicalproperties.Theircomposition,crystallinestructure,meltingproperties,andabilitytoassociatewithwaterandothernon-lipidmoleculesareespeciallyimportanttotheirfunctionalpropertiesinmanyfoods.Duringtheprocessing,storage,andhandlingoffoods,lipidsundergocomplexchemicalchangesandreactwithotherfoodconstituents,producingnumerouscompoundsbothdesirableanddeleterioustofoodquality.

Dietarylipidsplayanimportantroleonnutrition.Theysupplycaloriesandessentialfattyacids,actasvitamincarriers,andincreasethepalatabilityoffood,hutfordecadestheyhavebeenatthecenterofcontroversywithrespecttotoxicity,obesity,anddisease.

4.2DefinitionandClassificationofLipids

Theconsumerandtheprocessoroffoodsutilizesubstancesfromthenutrientgroupknownasfatsandoils.Fatsandoilsrepresentthemostprevalentsinglecategoryofaseriesofcompoundsknownaslipids.Theword“lipid”isdefinedinWebster’sunabridgeddictionaryas“anyofagroupsubstancethatingeneralaresolubleinether,chloroform,orothersolventsforfatsbutareonlysparinglysolubleinwater,thatwithproteinandcarbohydratesconstitutetheprincipalstructuralcomponentsoflivingcells,andrelatedandderivedcompounds,andsometimessteroidsandcarotenoids.”Thisdefinitiondescribesabroadgroupofsubstancesthathavesomepropertiesincommonandhavesomecompositionalsimilarities.

AclassificationoflipidsproposedbyBloorcontainsthefollowingelementswhichareusefulindistinguishingthemanylipidsubstances:

1.Simplelipids（neutrallipids）–Estersoffattyacidswithalcohols.

a.Fats:

estersoffattyacidswithglycerol

b.Waxes:

estersoffattyacidswithalcoholsotherthanglycerol

2.Compoundlipids–Compoundscontainingothergroupsinadditiontoanesterofafattyacidwithanalcohol.

a.Phospholipids（phosphatides）:

esterscontainingfattyacids,phosphoricacid,andothergroupsusuallycontainingnitrogen

b.Cerebrosides（glycolipids）:

compoundscontainingfattyacids,acarbohydrateandanitrogenmoiety,butnophosphoricacid

c.Othercompoundlipids:

sphingolipidsandsulfolipids

3.Derivedlipids-Substancesderivedfromneutrallipidsorcompoundlipidsandhavinggeneralpropertiesoflipids.

a.Fattyacids

b.Alcohols:

usuallynormalchainhigheralcoholsandsterols

c.Hydrocarbons

Foodsmaycontainanyorallofthesesubstancesbutthoseofgreatestconcernarethefatsorglyceridesandthephosphatides.Theterm“fatis”applicabletoalltriglyceridesregardlessofwhethertheyarenormallynonliquidorliquidatambienttemperatures.Liquidfatsarecommonlyreferredtoasoils.Suchoilsassoybeanoil,cottonseedoilandoliveoilareofplantorigin.Lardandtallowareexamplesofnonliquidfatsfromanimals,yetfatfromthehorseisliquidatambienttemperaturesandisreferredtoashorseoil.

Fatsandoilsalsocanbeclassedaccordingto“groupcharacteristics.”Fivewell-recognizedgroupsarethemilkfatgroup,thelauricacidgroup,theoleiclinoleicacidgroup,thelinolenicacidgroup,andtheanimaldepot-fatsgroup.

Thefourmajorclassesoflipidsarelistedinthetablebelow.

Characteristicsoflipids

LipidclassComments

WaxesEstersoflongchainalcoholsandfattyacids,foundascoatingsinplants

SterolsCholesterolisanexampleofasteroid.Itisneededforpropercellmembranefunction.

PhospholipidsFormscellmembranes.Phospholipidsform“gums”incrudeoil.Usedasemulsifiersinfoodmanufacturing

TriacylglycerolEdibleoilorfat

VitaminsVitaminA,E,DandKaresometimesclassedaslipids

4.3Phospholipids

Allfatsandoilsandfat-containingfoodscontainanumberofphospholipids.Thelowestamountsofphospholipidarepresentinpureanimalfatssuchaslardandbeeftallow.Insomecrudevegetableoils,phospholipidsmaybepresentatlevelsof2to3percent.Fish,crustacea,andmolluskscontainapproximately0.7percentofphospholipidsinthemuscletissue.Phospholipidsaresurfaceactive,becausetheycontainalipophilicandhydrophilicportion.Sincetheycaneasilybehydrated,theycanberemovedfromfatsandoilsduringtherefiningprocess.Insomecasestheymayberemovedbyseparationoftwophases;forexample,ifbutterismeltedandfiltered,thepureoilthusobtainedisfreefromphospholipids.Thestructureofthemostimportantphospholipidsisgivenbelow.Afterrefiningofoils,neutralization,bleachinganddeodorization,thephospholipidcontentisreducedtovirtuallyzero.Thephospholipidsremovedfromsoybeanoilareusedasemulsifiersincertainfoods,suchaschocolate.Soybeanphospholipidscontainabout35percentlecithinand65percentcephalin.Thefattyacidcompositionofphospholipidsisusuallydifferentfromthatoftheoilinwhichtheyarepresent.Theacylgroupsareusuallymoreunsaturatedthanthoseofthetriglycerides.Phospholipidsofmanyvegetableoilscontaintwooleicacidresidues.Thephospholipidsofmilkdonotcontainshort-chainfattyacidsfoundinmilkfattriglycerides,andtheycontainmorelong-chainpolyunsaturatedfattyacidsthanthetriglycerides.

Thedistributionoffattyacidsinphospholipidsisnotrandom,withsaturatedfattyacidspreferentiallyoccupyingposition1andunsaturatedfattyacidsposition2.

StructureofPhospholipid

4.4AttributesofFoodLipids

Fatsandoilsarethemostconcentratedsourceoffoodenergy.Theyprovide9kcalofenergypergramwhichisapproximatelydoubletheenergyprovidedbyproteinsorcarbohydrates.Theyarecarriersoffat-solublevitamins,andtheycontributetofoodflavorandpalatabilityaswellastothefeelingofsatietyaftereating.

Lipidsintheformoftriglycerides,phospholipids,cholesterol,andcholesterolestersareimportanttothestructure,composition,andpermeabilityofmembranesandcellwalls.Theyperformafunctionofenergystorageinseeds,fruitsofplants,andanimals.Lipidsaremajorcomponentofadiposetissue,whichservesasthermalinsulationforthebody,asprotectionagainstshocktointernalorgans,andasacontributortobodyshape.

Fatsandoilsareusedasfryingfatsorcoolingoilswheretheirroleistoprovideacontrolledheat-exchangemediumaswellastocontributetocolorandflavor.Asshortenings,theyimparta“short”ortenderqualitytobakedgoodsthroughacombinationoflubricationandanabilitytoalterinteractionamongotherconstituents.Assaladoils,theycontributetomouthfeelandascarrierforflavors,andwhenemulsifiedwithotheringredientstheyperformthesamefunctionintheformofviscouspourabledressingorsemisolidfattyfoodsknownasmayonnaiseorsaladdressing.Margarinesareusedbothforbakingandcookingandastablespreads.Specificallyselectedormanufacturedfatsareusefulinconfections,especiallyasenrobingorcoatingagents.Thesefatsmusthaveashort-meltingrangeatbodytemperature.

Otherfattymaterials,suchasthemonoanddiglycerides,andcertainphospholipids,suchaslecithin,haveusefulrolesasemulsifiers.Monoanddiglyceridescontributetoshortingperformanceandactasstalinginhibitorsinbakeryproducts.Lecithinisusedasamoldreleaseagentinconfections,tocontrolfatbloominchocolatecandies,andasanantispatteringagentincookingmargarines.

Fatsandoilsareavailableinavarietyofforms.Butter,cookingoils,margarines,saladoils,andshorteningsareessentiallyall-lipidform.Saladdressingsandmayonnaisearecomposedofhighproportionsoffatsoroils.Ingestedfatsandoilsincludenotonlythosefromobvioussourcesbutalsothosefrominvisiblefatsources,suchascereals,cheese,eggs,fish,fruits,legumes,meat,milk,nuts,andvegetables.Thislattergroupconstitutesapproximately60%ofthedietaryfat.

Saladandcookingoilsarepreparedfromcottonseedoi1,soybeanoil,cornoil,peanutoil,saffloweroil,oliveoil,orsunflowerseedoil.Theseoilsareusuallyrefined,bleached,anddeodorized.Someoilsmaybelightlyhydrogenatedtoprovidespecialpropertiesandtoenhanceflavorstability.

Margarines,usedmostlyastablespreadsandtosomeextentascookingfats,arepreparedbyblending.suitablypreparedfatsandoils,withotheringredients,suchasmilksolids,salt,flavoringmaterials,andvitaminsAandD.Thefatcontentmustbeatleast80%.Vegetableoilsareusedpredominantlyformanufactureofmargarinealthoughsomeanimalfatsareused.Thefatsmaybesinglehydrogenatedfats,mixturesofhydrogenatedfats,orblendsofhydrogenatedfatsandunhydrogenatedoil.Specialmargarinesarepreparedwithincreasedlevelsofpolyunsatu