Chemical reactions and rancidity of fats

Rancidification is the process of complete or incomplete oxidation or hydrolysis of fats and oils when exposed to air, light, or moisture or by bacterial action, resulting in unpleasant taste and odor. Specifically, it is the hydrolysis or autoxidation of fats into short-chain aldehydes and ketones, which are objectionable in taste and odor. When these processes occur in food, undesirable odors and flavors can result.

1. Chemical reactions &
Rancidity of Fats

2. Saponification
•Saponificationisaprocessthatinvolvestheconversionoffat,oil,or
lipid,intosoapandalcoholbytheactionofheatinthepresenceof
aqueousalkali(e.g.NaOH).
•Soapsaresaltsoffattyacids.

3. Rancidity
•Itreferstothespoilageofafoodinsuchawaythatitbecomesundesirable(and
usuallyunsafe)forconsumption.
•Whenpeoplesaythatafoodhas"gonebad,"whatthey'reusuallytalkingabout
israncidity.
•Itistheprocessofcompleteorincompleteoxidationorhydrolysisoffatsandoils,
whenexposedtoairorlightormoistureorenzymes,resultinginunpleasanttaste
andodor.

4. Oil Reversion
•Itisthebreakdown(oxidation)ofanessentialfattyacidlinolenicacid,foundin
certainvegetableoils,leadingtoanundesirableflavorchangepriortothestartof
actualrancidity.
•Therefore,beforetheactualdecayoftheoilonecan"taste"thedifferenceandpredict
thattheoilmayrunbadinfuture.
•Areversedoil/fatoncedetectedcanhelpsaveourselvesfromconsumingapotentially
badfat/oil.
•That'sbecausearancidoilcanhavedamaginghealtheffectssuchascancer,heart
diseaseetc.
•ExampleisreversionofSoybeanoil.Soybeanoilhasatriglyceridecompositionrich
inmonounsaturated(23%oleicacid)andpolyunsaturatedfattyacids(57%linoleic
acid;7%linolenicacid).
•Thelinolenicacidcontainsthreeunsaturatedbondandwhichcanbeeasilyundergone
oxidationprocess.Becauseofthepresenceofhighcontentoflinolenicandlinolenic
acid,oilundergoesoxidativedegradationanddevelopscolorchangeandfishyor
paintysmell.Thisprocessiscalledoilreversion.

5. Mechanism of Oil Reversion
•Oxidationistheprimarymechanisminvolvedintheoilreversion.
•Whenitcomestooxidation,linolenicacidisabouttentimesmore
vulnerablethanlinoleicacidandaboutonehundredtimesmorethan
oleicacid.
•Theoxidationofdoublebondsisaradical-drivenprocess.

6. Radical reactions typically have three steps
1.Theinitiationreaction,whereanenergysource(heat;light)generates
aradicalonthefattyacid.
2.Apropagationstepwithoxygengivingrisetoperoxides,whichreact
withmoreunsaturatedfattyacid,creatingnewradicals.
3.Aterminationreaction,wheretworadicalsinteractforminganew
singlebond.

7. Types of Rancidity
1.OxidativeRancidity
•Thespecifictypesofrancidityinvolvingoxygendamagetofoodsarecalled
“oxidativerancidity.”
•Duringtheprocess,oxygenmoleculesinteractwiththestructureoftheoil
anddamageitsnaturalstructureinawaythatcanchangeitsodor,itstaste,
anditssafetyforconsumption,
•i.e.fatisoxidizedanddecomposesintocompoundswithshortercarbon
chainssuchasfattyacids,aldehydes,andketonesallofwhicharevolatile
andcontributetotheunpleasantodorofrancidfats.
•Oxidativerancidityleadstotheformationofbothunpalatableandtoxic
compounds.

8. Types of Rancidity
2.HydrolyticRancidity:
•Fatsareenzymaticallyhydrolyzed,thereleaseoffreefattyacidsfrom
glycerides,causesomerancidodor.Thisprocessiscalledhydroiytic
rancidity.
•Hydrolysiswillsplitfattyacidchainsawayfromtheglycerol
backboneinglycerides.
•Thesefreefattyacidscanthenundergofurtherauto-oxidationleadsto
oxidativerancidity.

9. Factors Affecting Rancidity and Reversion
1.Oxidation:
•Primarycauseofrancidity.
•Oxidationprimarilyoccurswithunsaturatedfatsbyafreeradical-
mediatedprocess.
•Thesechemicalprocessescangeneratehighlyreactivemoleculesin
rancidfoodsandoils,whichareresponsibleforproducingunpleasant
andnoxiousodorsandflavors.
•Thisprocessiscalledauto-oxidationoroxidativerancidity.

10. 2.Hydrolysis:
•Triglyceridesreactwithwaterunderappropriateconditiontoform
diglyceridesandfreefattyacidresidues.
•Diglycerideslatercombinewithwatertoformmonoglyceridesand
fattyacids.
•Finallythemonoglyceridescompletelyhydrolysedtoformglycerol
andfattyacids.
•Thisprocessiscalledhydrolyticrancidity.

11. 3.PresenceofMicroorganisms–MicrobialLipase:
•Certainmicroorganismscanproducethehydrolyticenzymecalled
lipase,whichdirectlyinterferesthehydrolysisoftriglyceridesand
produceglycerolandfattyacids.
•Thesefattyacidsundergoauto-oxidationtoformrancid.
•ThemicrobiallipaserequiressuitablepHandotherconditionsforits
activityuponfatsandoil.

12. 4.PresenceofUnsaturationinFattyAcidChain:
•Whenafattysubstanceisexposedtoair,itsunsaturatedcomponents
areconvertedintohydroperoxides,whichbreakdownintovolatile
aldehydes,esters,alcohols,ketones,andhydrocarbons,someofwhich
havedisagreeableodours.
•Butterbecomesrancidandliberatesvolatileandfoul-smellingacids,
particularlybutyricacid.

13. 5.Polyunsaturation:
•Themorepolyunsaturatedafatis,thefasteritwillgorancid.
Vegetableoilshavetobecomeseveraltimesmorerancidthananimal
fats.
•Presenceofpolyunsaturationinoilsandfatsmakesthemmore
susceptibletoranciditythanmonosaturatedandothertypesof
saturatedfattyacids.

14. 6.ChemicalStructureofOilsandFats:
•Ifoilsandfatsarechemicallymorecomplexandconsistsmore
numberofdoublebond,morenumberofcarboxylorhydroxylgroups,
thenthechancesofbecomerancidishigh.
•Thedoublebondsfoundinfatsandoilsplayaroleinauto-oxidation.
Oilswithahighdegreeofunsaturationaremostsusceptibletoauto-
oxidation.

15. 7.TemperatureandpH:
•Thesearetheimportantfactorwhichinfluencesthefooditemsrichin
fatandoilsbecomerancid.
•SuitabletemperatureandalkalinepHarerequiredforthehydrolytic
actionofmicrobiallipase.
•TemperatureandpHindirectlyinfluencetheauto-oxidationand
hydrolysis.

16. 8.HeatandLight:
•Presenceofheatandlightacceleratetherateofreactionoffatswith
oxygen,i.e.,heatacceleratesauto-oxidation.
•Heatandlightactastheenergysourcefortheproductionoffree
radicalinrancidityandreversionofoilsandfats.

17. Prevention of Rancidity
1.AdditionofAntioxidants:
Antioxidantsretardoxidationprimarilybyeitherhydrogenorelectron
donation.Naturalantioxidantsincludeflavonoids,polyphenols,ascorbicacid
(vitaminC)andtocopherols(vitaminE).
2.AdditionofSequesteringAgents:
Sequesteringagentsbindmetals,thuspreventingthemfromcatalyzingauto-
oxidation.ExamplesofsequesteringagentsincludeEDTA(ethylenediamine
tetraaceticacid)andcitricacid.
3.ProperStorageofFatsandOilFood:
Anothermethodforpreventingrancidityoffoodistheproperstorage,
keepingawayfromtheactionofoxygen.Rancidificationcanbedecreasedby
storingfatsandoilsinacool,darkplacewithlittleexposuretooxygenorfree
radicals,sinceheatandlightacceleratetherateofreactionoffatswith
oxygen.