浙大食化考研.docx
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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
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