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文献翻译

Howblanchinganddryingaffectthecolourandfunctionalcharacteristicsofyam（Dioscoreacayenensis-rotundata）flour

NoëlAkissoé,JosephHounhouigan,ChristianMestresb,MathurinNagoa

Abstract:

Colourandtextureareimportantfeaturesofamala,atraditionalthickpasteobtainedfromdryyamflour.Tubersfromeightyamcultivarswereblanchedat65°Cfor20minanddriedat40°Cfor5daystotestbiochemical,thermalandpastingchangesoccurringduringtraditionalyamflourprocessing.Blanchingreducedperoxidaseactivityanddryingreducedpolyphenoloxidaseactivity,buttotalphenolcontentandthebrownindexofflourandofamalaincreaseddramaticallyduringthelatteroperations.Thebrownindexofamalawassignificantlycorrelatedwiththetotalphenolcontentoftheflour（r=0.84）andtheperoxidaseactivityofthefreshtubers（r=0.75）.Amylosecontentandstarchgelatinizationenthalpyremainedstable.Forallcultivars,dryingsignificantlyincreasedtheonsetgelatinizationtemperature,suggestingtheoccurrenceofstarchannealing.Thelatterleadstoareductioninswellingpowerandsolubilityduringpasting,andhencetoalowerpasteviscosity.

@2003ElsevierScienceLtd.Allrightsreserved.

Keywords:

Yam;Phenols;Polyphenoloxidase;Peroxidase;Blanching;Viscosity

1Introduction

Yams（Dioscoreaspp.）areanimportantsourceofcarbohydrateformanypeopleofthesub-Sahararegion,especiallyintheyamzoneofwestAfrica.Toovercomethehighperishabilityoffreshyamtubers,duetotheirhighmoisturecontentandtheseasonalnatureoftheirproduction,insomewestAfricancountries（BeninandNigeria）yamsareprocessedintoflourusingawellestablishedmethod.Tubersarepeeled,sometimessliced,blanchedinhotwater（around65℃for15–50min）andsun-dried（Akissoé,Hounhouigan,Bricas,Vernier,Nago,&Olorunda,2001）.Theresultingdriedtubersorchipsarethenmilledintoflour,whichisusedtomakeathickpasteknownas‘‘amala’’withadifferentsensoryqualityfromthatofpoundedyam（Ajibola,Abonyi,&Onayemi,1988）.Themainqualityattributesofamalaaretextureandcolour（Hounhouigan,Kayode,Bricas,&Nago,2002）,whichmaybeaffectedbyyamcultivarcharacteristicsand/orbyprocessingconditions.

Thefleshoftheyamspeciesusediswhite,whereasthecolouroftheprocessedflourrangesfromcreamywhitetodarkbrown.Thediscolorationphenomenonhaslongbeenstudiedonfreshtubersandhasmainlybeenassociatedwithenzymaticbrowning,duetotheactionofpolyphenoloxidase（Almenteros&DelRosatio,1985;Ozo,Caygill,&Coursey,1984）andperoxidase（Asemota,Wellington,Odutuga,&Ahmad,1992）andtotheproductionofpolyphenolsandderivedproducts（Osagie&Opoku,1984）.Variousphenolicconstituentshavebeenreportedtoberesponsibleforthediscolorationofedibleyams:

catecholaminewasfirstsuspected（Franklin&Ruth,1972）,thencyanidin-3-glucoside（Imbert&Sea-forth,1968;Rasper&Coursey,1967）,whileOzoetal.（1984）reported（+）catecholandprocyanidinoligomersascontributingtothediscolorationofD.alata（Ozoetal.,1984）.Inthecaseofdriedyamflourandamala,Mestres,Dorthe,Akissoe,andHounhouigan（2002）reportedaclosepositivecorrelationbetweentheamalabrownindexandthetotalphenolcontentoftheflour.Izundu（1995）,ontheotherhand,pointedouttheroleofperoxidaseinamalabrowning.Theactualcontributionofpoly-phenoloxidaseandperoxidasetoamaladiscolorationis,however,stillunknown:

polyphenoloxidaseactivitycanbepartlyinactivatedduringblanchingat65°C（Ozo&Caygill,1985）,andpolyphenoloxidaseandperoxidaseactivitiesarenearlynilinyamflour（Mestresetal.,2002）.

Inaddition,changesinpastingproperties,occurringduringblanchinganddrying,havenotbeenfullyinvestigated,althoughtheyhavebeenidentifiedasanessentialandcriticalstageofyamchipprocessing（Akissoeetal.,2001;Ige&Akintunde,1981）.

Theaimofthisworkwastoassessthechangesincolourandpastingbehaviouroccurringduringblanchinganddrying,usingvariousyamcultivars,andtoinvestigatethephysicochemicalbasesforthesechanges.

2Materialandmethods

2.1Plantmaterial

EightcultivarsofDioscoreacayenensis-rotundata（Banioure,Deba,Gnidou,Kagourou,Porchekbim,Tam-sam,Terlunto,Yakarango）wereobtainedfromtheInternationalInstituteofTropicalAgriculture（IITA-Benin）.Accordingtofarmers,fiveofthem（Deba,Kagourou,Porchekbim,Tam-sam,Yakarango）areverysuitableforprocessingintodryyamchips,two（Banioure,Gnidou）areveryunsuitableandone（Ter-lunto）ismoderatelysuitable（Dansi,2001）.TheyamtuberswereharvestedinthefirsthalfofDecember2000andprocessed1weekafterharvesting.

2.2Experimentaldesign

Thetubers（5-7foreachcultivar）werehand-peeledinawaterbath（28-30°C）andslicedinto30mmthickcylindersusingacuttingbox.Onesampleofslices（referredtoasA）wasfreeze-driedandusedasacontroltoassessthephysicochemicalcharacteristicsoftherawmaterial.Theremainder（B）wasblanchedinathermostat-controlledwaterbathsetat65°Cfor20min,thenequilibratedinwaterat28-30°Cfor5min.Theblanchedslicesweredividedintotwogroups（B1andB2）.GroupB1wasfreeze-driedandusedtoassesstheeffectofblanching.GroupB2wasdriedinaforcedairovenat40°Cfor5daystoassesstheeffectofdrying.Eachgroup（A,B1,B2）wascrushedinamortar,thengroundinalaboratorycentrifugemill（Retsch,Haan,Germany）fittedwitha0.2mmscreen.Theflourwasthenstoredat4°Cuntilanalysis.

2.3Methods

2.3.1Physicochemicalcharacterizationoffreshtubers

Thefreshtuberswereweighed,theirdimensionsweremeasuredwithaVerniercaliperandtheirvolumewasdeterminedbymilletdisplacement.Ineachcase,ameanvaluewascalculatedforeighttubersofeachcultivar.Thedrymattercontentofthefreshtuberswasmeasuredafterdrying10gyampiecesat105°Cfor48h.Themeanvalueofthreetubersofeachcultivarwascalculated.

2.3.2Polyphenoloxidase（PPO）andperoxidase（POD）activityandtotalphenol（TP）content

PPO,PODandTPweredeterminedusingthemethodsdescribedbyMestresetal.（2002）,measuringtheoxygenconsumptionkineticat460nmwithcatecholassubstrateandthediscolorationkineticwithanopticaldensityat760nmafterreactionwithFolinreagent.

2.3.3HunterLabcolourcoordinates

Thecolourofthefloursandthepastes（obtainedwithaRapidViscoAnalyserasdescribedbelow）wasmeasuredusingaMinoltaCR-210portablechromameter（illuminantD65CIE1976）.TheHunterLabcolourcoordinatesystemL*,a*andb*valueswererecordedandthebrownindexwascalculatedas（100-L*）.

2.3.4Pastingbehaviour

PastingpropertiesweredeterminedusingaRapidViscoAnalyser（RVA,NewportScientific,Narrabeen,Australia）on8%drymattersuspension.Thesuspensionwasheatedfrom,35°Cto95°Catarateof6°Cmin",maintainedat95°Cfor4minthencooledto50°Catthesamerate.Viscosityatthestartofthe95°Cplateau（V95b）,viscosityattheendofthe95°Cplateau（V95e）andendviscosityaftercoolingto50°C（V50）weremeasured.

2.3.5Swellingpowerandsolubility

TheswellingpowerandsolubilityproceduresdescribedbyMestres,Nago,Akissoe;,andMatencio（1997）weremodified,withadrymatterconcentrationof4%（wb;1.2gofdrymatterdispersedindistilledwatertogiveatotalmassof28g）beingused.Thesuspensionwasheatedfrom35°Cto95°Catarateof6°Cmin"1andheldat95°Cfor1minusingtheRVA.Theheatedsuspensionwasthencentrifugedat3000gfor15minatambienttemperature.

2.3.6Thermalbehaviour

AmylosecontentandstarchgelatinizationpropertiesweredeterminedbydifferentialscanningcalorimetryonaPerkinElmerDSC7device（Perkin-Elmer,Norwalk,USA）,asdescribedbyMestres,Matencio,Pons,Yajid,andFliedel（1996）andMestres,Boungou,Akissoe:

andZakhia,（2000）.

2.4Statisticalanalysis

AnalysisofvarianceandcorrelationandregressionanalyseswereperformedusingStatitcfsoftware（ITCF,Boigneville,France）.

3Results

3.1Physicochemicalcharacterizationoffreshtubers

Drymattercontentrangedfrom24.9to32.1%（wb,Table1）.Therewasgreatheterogeneityintheweightandsizeofthefreshtubers:

volumerangedfrom202mlforTamsamto955mlforGnidou.Itshouldbenotedthatthelowest-ratedcultivars（BanioureandGnidou）hadthelargesttubers,atleasttwicethesizeoftheothers.

Table1Physicochemicalcharacteristicsofyamcultivars

Yamcultivars

（localname）

Drymatter

（%wb）

Weight

（g）

Volume（ml）

Length（cm）

Head-diameter（cm）

Tail-diameter（cm）

Banioure

27.4

725

830

34.3

6.3

5.9

Deba

26.3

381

385

23.0

4.3

4.1

Gnidou

28.3

1150

955

31.7

6.3

6.2

Kagourou

32.1

468

464

24.3

4.8

4.1

Porchekbim

30.3

313

324

20.7

4.8

3.8

Tam-sam

24.9

182

202

19.7

2.9

3.3

Terlounto

29.6

454

439

22.0

5.5

5.0

Yakarango

26.5

338

366

17.5

5.2

4.8

3.2Contrastingbehaviourofpolyphenoloxidaseandperoxidaseactivitiesandtotalphenolsduringyamchipprocessing

Polyphenoloxidase（PPO）andperoxidase（POD）activitiesandtotalphenol（TP）contentvariedtoalargeextentwithcultivarandunitoperation（Tables2and3）.FreshGnidouandBaniourecultivars（cvs）showedcontrastingbehaviourforPPOandPODactivities:

Table2Effectofprocessingstageandyamcultivar

onpolyphenoloxidaseactivity（μmolO2min"-1g-1）

Yamcultivars

（localname）

Freeze-dried

Oven-dried

Freshslices

Blanchedslices

Blanchedslices

Banioure

10.4

10.2

1.2

Deba

32.2

19.8

0.6

Gnidou

72.6

51.4

3.7

Kagourou

35.7

24.7

2.5

Porchekbim

59.5

42.5

4.5

Tam-sam

13.6

11.0

1