Chapter12fromUsingFoodScienceandTechnologytoImproveNutritionandPromote - PDF document

Chapter12fromUsingFoodScienceandTechnologytoImproveNutritionandPromoteNationalDevelopment, Robertson,G.L.&Lupien,J.R.(Eds),©InternationalUnionofFoodScience&Technology(2008) MinimizingPostharvestLossesinYam( Dioscoreaspp. ): TreatmentsandTechniques ZinashDeleboOSUNDEMScPhD DepartmentofAgriculturalandBioresourcesEngineering, FederalUniversityofTechnology, Minna,Nigeria Email: zinashdo@yahoo.com INTRODUCTION Yambelongstothegenus Dioscorea (Family Dioscoreaceae) andisthesecondmostimportanttropical rootcropinWestAfricaaftercassava.Besidestheirimportanceasafoodsource,yamsalsoplaya significantroleinthesocio-culturallivesofsomeproducingregionssuchasthecelebratedNewYam FestivalinWestAfrica.YamsoriginatedintheFarEastandspreadwestwards.Today,yamsare grownwidelythroughoutthetropics.WestandCentralAfricaaccountforabout94%ofworld production,Nigeriabeingthemajorproducer. Themostpopularandpreferredformofconsumingyamisthetuberform,eitherboiled,pounded, roastedorfried.Betterfinancialreturnsareobtainedbysellingtheyamsastubersratherthanas processedyamflour.Thus,farmersprefertostoremostoftheiryamsafterharvest.Methodsof storagevaryfromdelayedharvesting,storageinsimplepilesortrenchestostorageinbuildings speciallydesignedforthatpurpose,andapplicationofmoderntechniques. Causesofstoragelossesofyamtubersincludesprouting,transpiration,respiration,rotduetomould andbacteriosis,andattackbyinsects,nematodesandmammals.Sprouting,transpirationand respirationarephysiologicalactivitieswhichdependonthestorageenvironment(mainlytemperature andrelativehumidity).Thesephysiologicalchangesaffecttheinternalcompositionofthetuberand resultindestructionofediblematerialandchangesinnutritionalquality.Storagelossesinyamofthe orderof10-15%afterthefirstthreemonthsandapproaching50%aftersixmonthsstoragehavebeen reported. Anumberoftreatmentsandtechniqueshavebeendevelopedtoreducethesephysiologicalactivities andalsotoprotectthetuberfrompostharvestdiseases.Theseincludetreatmentwithchemicals, plantextracts,palmwineandgammairradiation;storagetechniquesusedincludecoldstorage, improvedundergroundstorageandimprovedyambarns.Thischapterdiscussesresearchintoyam postharvesthandlingaimedatimprovingtheavailabilityoftubersthroughouttheyear. YAMPRODUCTION Yam Dioscoreaspp. isanimportantfoodcropespeciallyintheyamzoneofWestAfrica.Althoughmore thansixhundredspeciesofthetuberexist,onlyafewareimportantasstaplefoodinthetropics. Theseincludewhiteyam (D.rotundata ),yellowyam( D.Cayenensis ),wateryam( D.alata ),trifoliate yam(D. dumetorum ),aerialyam( D.bulbifera )andChineseyam( D.esculenta )[1].WestAfrica accountsfor90-95%ofworldyamproductionwithNigeriathelargestsingleproducer.In2004, globalyamproductionwasabout47millionmetrictons(MT)with96%ofthiscomingfromAfrica. Nigeriaaloneaccountsforabout70percentofworldproduction[2].Itisthesecondmostimportant root/tubercropinAfricawithproductionreachingjustunderonethirdthelevelofcassava.Morethan 95percent(2.8millionha)ofthecurrentglobalareaunderyamcultivationisinsub-SaharanAfrica, wherethemeangrossyieldis10t/ha.InAsia,productionfor2004stoodat226,426MT[2]. Yamproductionisrelativelyexpensivecomparedwithotherrootandtubercrops;thisisattributedto costlyinputs,especiallylabourandplantingmaterial.Traditionally,yamsarepropagatedbywhole 2 tubers(seedyam)orrelativelylargetuberpieces.Todayyammini-sets(orsmallpiecesofyam)are increasinglybeenusedasplantingmaterial[3].Thetubersareretainedfromtheharvestforthenext plantingseason.Thetubers(Figure1)whicharetheonlyedibleparthavebothatremendous capacitytostorefoodreservesandabilitytogrowintothedeeplayerofthesoil.Thetopgrowth consistsoftwiningvinesthatmaybeseveralmeterslongdependingonspeciesandgrowing conditions[Figure2]andrequiretrellisesoverwhichtoclimb. Figure1:YamtuberFigure2:Theyamplant Maturityassessmentiscriticaltoachievegoodqualityyam.Inthefieldamaturecropis distinguishablebycessationofvegetationgrowthandyellowingofleaves.Theperiodfromplanting orfieldemergencetomaturityisvariabledependingonthespecies.Tubersmaybeharvested6-10 monthsafteremergenceoronceat4-5monthsandagainat8-10months,dependingonspecies[4]. Theyielddependsonthesizeoftheseedpiece,speciesandenvironmentbutnormallyrangesfrom8- 50t/hain6-10months. IMPORTANCEOFYAM Yamisimportantbecauseofitsexcellenteatingquality;theyareapreferredfoodatsocialgatherings. Peopleconsumeyams,sweetinflavour,asacookedvegetablefriedorroasted.InWestAfricayamis oftenpoundedintoathickpasteafterboiling(poundedyam)andiseatenwithsoup[Figures3&4]. PoundedyamisthemostpopularfoodformofyaminWestAfricaandisoftenreservedforspecial occasionsintheurbanareas.Presently,wholeroastedyamhasbecomeapopularstreetorfastfoodin urbanareasthroughouttheWestAfricanyambelt[3].Virtuallyallproductionisusedforhumanfood. Inthemajoryamproducingcountries,averageconsumptionis0.5-1.0kgyamdaily[5].Yamisa preferredfoodandafoodsecuritycropinsomesub-SaharanAfricancountries.Unlikecassava,sweet potatoandaroids,yamtuberscanbestoredforperiodsofupto4oreven6monthsatambient temperatures. Sahore etal. ,[6]studiedchangesinnutritionalpropertiesofyam( Dioscoreaspp. ),greenplantain ( Musaspp. )andcassava( Manihotesculenta )duringstorageandshowedthatyamtubersunderwent onlyslightchangesoverafourweekperiodwhilecassavaandgreenplantaincouldbekeptonlyfora weekwithoutsignificantdeterioration.Thischaracteristiccontributestothesustainingoffoodsupply, especiallyinthedifficult(foodscarce)periodatthestartofthewetseason.Yamsarealsoprocessed intoyamchipsandflourthatisusedinthepreparationofapaste.Theproblemsencounteredby processorsincludealackofsuitablerawmaterials,dryingandstorageequipment,andpoorqualityof theprocessedproduct. 3 Figure3:PreparationofpoundedyamFigure4:Poundedyammealwithmeat andvegetables Apartfrombeingusedforfamilyfood,yamsarealsoacashcropforfarmers.Studiesshowthatyam isahighlyprofitablecropintheyamzoneconstitutinganaverage32%offarmers’grossincome derivedfromarablecrops[7].Figures5and6showatypicalyammarketinNigerStateofNigeria. YamalsoplaysasignificantroleinAfricansocio-culturaltraditions.Thecommercialimportanceofthe crophasnoterodeditstraditionalstatus.ItisanindispensablepartofthebridepriceandinEastern NigerianewyamfestivalsarecelebratedannuallyduringthemonthsofAugustandSeptember.In South-EasternNigeria,thecultivationandconsumptionofyamdatesbackseveralcenturies.Inthis area,yamisatotemofmasculinityandthecentreofannualharvestcelebrations;itisalsoacalendar croparoundwhichthefarmingseasonandtheannualfestivalrevolves. Figure5:YamcollectioncentreatGaratu,NigerStateofNigeria 4 Figure6:BuyingandsellingofyaminNigerStateofNigeria. NUTRITIONALVALUEOFYAMS Yamsareanexcellentsourceofcarbohydrate,energy,vitamins(especiallyvitaminC),mineralsand protein.Somecultivarsofyamtuberhavebeenfoundtocontainproteinlevelsof3.2–13.9%ofdry weight.Ayammealcouldsupply100%oftheenergyandprotein,13%ofthecalciumand80%ofthe ironrequirementofanadultmale[8].Somefoodyamshavebeenshowntocontainphosphorousand vitaminssuchasthiamine,riboflavin,niacinandascorbicacid. Thechemicalcompositionofyamischaracterisedbyahighmoisturecontentanddrymatter.Thedry matteriscomposedmainlyofcarbohydrate,vitaminsaswellasproteinandminerals.Nutrient contentvarieswithspeciesandcookingprocedure.Cookingwiththepeelintacthelpsretainvitamins [5].Table1showstherangesofnutritionalcompositionforedibleyamsspecies. Table1:Nutritionalvalueofyam(Nutrientin100gofedibleportion) Nutrient Range Calories(kcal)71–135 Moisture(%)65–81 Protein(g)1.4–3.5 Fat(g)0.2–0.4 Carbohydrate(g)16.4–31.8 Fibre(g)0.40–10.0 Ash(g)0.6–1.7 Calcium(mg)12–69 Phosphorous(mg)17–61 Iron(mg)0.7–5.2 Sodium(mg)8.0–12.00 Potassium(mg)294–397.00 Thiamine(mg)0.01–0.11 Riboflavin(mg)0.01–0.04 Niacin(mg)0.30–0.80 Ascorbicacid(mg)4.00–18 Source(Osagie,1992) 5 Yamsmayalsocontainsmallquantitiesofpolyphenoliccompounds(e.g.tannins),alkaloidsand steroidderivatives.Thecarbohydratecontentoftheyamtuberrepresentsitsmajordrymatter componentandmaybeclassifiedasstarch,nonstarch,polysaccharidesandsugar.Starchinyam tubersisfrequentlyconvertedtosugarsprobablyasaresultofstressesexperiencedduringgrowth andstorage.Thesugarcontentisinfluencedbyvariety,locationandculturaltreatment.Thefree sugarsconsistmainlyofsucroseandglucose,withtheformerpredominating.Fructoseandmaltose havebeendetectedduringdormancy/sproutingperiods[9].Thepublishedproteincontentofyam tubersvariesveryconsiderablybetweenbothspeciesandcultivarsanddependsonvariousfactors includingculturalpractices,climateandedaphicfactorsunderwhichitwasgrown,itsmaturityat harvestandlengthofstoragetime.Highproteincontentischaracteristicofveryvigorousvarieties with D.alata tubershavingthehighestproteinlevelsamongtheedibleyams.Althoughtheprotein contentofyamislowerthanthatinmostcereals,yamcanprovidemoreproteinperhectareperyear thanmaize,rice,sorghumandsoybean(Idusogie(1971)asreferencedby[5]).However,thevalueof cookedyamasasourceofproteinislimitedbyitsbulk,thewatercontentbeingveryhigh. YAMSTORAGEANDSTORAGESTRUCTURES Yamisanannualcrop,soforittobeavailablethroughouttheyear,harvestedtubersmustbestored forsixtoeightmonthsbeforenewyamsareharvested.Thepossibilitytostorefreshyamtubersis decisivelyinfluencedbytheirdormancywhichoccursshortlyaftertheirphysiologicalmaturity (wiltingpoint).Duringdormancy,themetabolicfunctionofthetuberisreducedtoaminimum.It allowsthetuberasanorganofvegetativepropagationtoovercomeanunfavourableclimataticperiod. Consequently,varietiesofyamnativetoregionswithmarkedaridseasonshavealongerperiodof dormancythanthosewithashorterdryseason.Thedurationofnaturaldormancyfluctuates accordingtothevarietyofyambetweenfourandeighteenweeks[8].Duringthestorageperioda substantialamountofyamislost.Someoftheselossesareendogenous,i.e.physiological,andinclude transpiration,respirationandgermination.Otherlossesarecausedbyexogenousfactorslikeinsects, pests,nematodes,rodents,rotbacteriaandfungionthestoredproduct[10].Goodmanagementcan easilycontroltheexogenouslossfactorswhiletheenvironmentcontrolsothersourcesofloss. YamstoragestructuresandmethodswereevaluatedintheformerBendelStateofNigeria[11]inthe middlebeltofNigeria[12;13]andinWesternNigeria[14].Thisworkshowedthatthestorage structuresuseddependontheconstructionmaterialavailable,amountoftuberproduced,prevailing climaticconditionofthearea,purposeofyamtuberstorage,socio-culturalaspectsofstorageandthe resourcesofthefarmer,inparticulartheavailabilityoflabourandcapital.Inthehumidforestzone yamisstoredinayambarnwhichistheprincipaltraditionalyamstoragestructureinthemajor producingareas.Barnsinthehumidforestzoneareusuallylocatedundertheshadeandconstructed soastofacilitateadequateventilationwhileprotectingtubersfromflooding,directsunlightandinsect attack.Thereareseveraldesigns,buttheyallconsistofaverticalwoodenframeworktowhichthe tubersareindividuallyattached[4;11].Tubersaretiedtoaropeandhungonhorizontalpoles1-2m high(Figure7);barnsupto4mhigharenotuncommon.Dependingonthequantityoftubertobe stored,framescanbe2mormoreinlength.Theropesareusuallyfibrous;insouth-easternNigeria theyaremadefromtheraffiaobtainedfromthetoppartofthepalmwinetree.Manyfarmershave permanentbarnsthatneedannualmaintenanceduringtheyear’sharvest.Inthesesituations,the verticalpostsareoftenmadefromgrowingtreeswhicharetrimmedperiodically.Palmfrondsand othermaterialsareusedtoprovideshade.Thevegetativegrowthontheverticaltreesalsoshadesthe tubersfromexcessivesolarheatandrain[4]. 6 Figure7:Typicalyambarninthehumidforestzone(individualyamstiedtolivepoles) TheyambarnintheGuineaSavannahzoneisconstructedfromguineacornstalk,sticks,grassand yamvines.Theyamsareheapedatdifferentpositionsinthebarn[12;13].Suchbarnsare constructedeveryyearandaresituatednearthehouseunderatreetoprotectthetuberfrom excessiveheat(Figures8and9).Attheendofthestorageperiodthebarnisburntdownandin December/Januaryanewstructureisbuiltforthenextharvest[13].Unlikethehumidforestwhereit isimportantthattheyamsareseparatedtoavoidrotting,indrierareassuchasNigerStatetheyams canbestackedintopilesinthebarn[10;13].Attheonsetoftherainyseasontheyamsaretransferred toamudhutorguineacornstoragerhombutoprotectthemfromtherain.Anotheryamstorage structurefoundinthesavannaregionistheyamhouseoryamcrib[4;10]. “Yamhouses”have thatchedroofsandwoodenfloors,andthewallsaresometimesmadesimplyoutofbamboo.Theyare raisedwelloffthegroundwithratguardsfittedtothepillars.Yamtubersarestackedcarefullyinside thecrib(Figure10).Yamisalsostoredundergroundintrenchorclampsilos.Inbothmethodsapitis excavatedandlinedwithstraworsimilarmaterial[15].Thetubersarethenstoredonthelayerof straweitherhorizontallyontopofeachotherorwiththetipverticallydownwardsbesideeachother. Theyamsarethencoveredwithstraworsimilarmaterials;insomecasesalayerofearthisalso added. Figure8:TheyambarnintheGuineaSavannazone 7 Figure9:TheinteriorviewofatypicalyamstorageinthesouthernGuineaSavanna zone(piledintoheapsinayambarn) Figure10:AtraditionalyamhouseusedintheSavannaregion (redrawnfromWilson,1980) QUALITYCHANGESOFYAMTUBERDURINGSTORAGE Causesofstoragelossesofyamtubersincludesprouting,transpiration,respiration,rotduetomould andbacteriosis,insects,nematodesandmammals[16].Sprouting,transpirationandrespirationare physiologicalactivitieswhichdependonthestorageenvironment,mainlytemperatureandrelative humidity[16].Thesephysiologicalchangesaffecttheinternalcompositionofthetuberandresultin destructionofediblematerial,whichundernormalstorageconditionscanoftenreach10%after3 months,andupto25%after5monthsofstorage[16]. Investigationsonthebiochemicalchangesinstoredyamtubershaveshownthatchangesinstarch, sugars,andproteintakeplaceduringlong-termstorage[17;18].Astudyofyamtuber( D.dumetorum ) 8 storedunderambientandcoldroomconditionsshowedarapiddropinmoistureandstarchcontent andanincreaseinthetotalalcohol-solublesugarsandreducingsugarsafter72hoursofstorage[17]. Therateofdecreaseinmoistureandstarchcontentandtherateofincreaseinsugarlevelwerehigher intubersstoredatroomtemperaturethanthosestoredundercoldroomconditions.Asimilartrend wasobservedfor Drotundata cv.Osheiand D.dumetorum cvJakiriafter110daysofstorageunder ambientconditions[18];weightlossesreached31%inOsheitubersand35%inJakiridueto sproutinganddehydration.Starchcontentdecreasedbyapproximately3.5-4.5g/100gwhilesugar andfibrevaluesincreasedslightlyinbothcultivars. Astudyofthephysical,chemicalandsensorychangesoccurringinwhiteyams (Dioscorearotundata) andyellowyams (Dioscoreacayenensis) storedfor150daysintraditionalbarnsshowedlossesin moisture,drymatter,crudeproteinandascorbicacidafter120daysofstorage[19].Sensory evaluationratedthestoredtubershigherthanthefreshtubers.Asimilarstudy[20]reporteda17- 22%reductioninweight,30-50%reductionincrudeproteinand38-49%increaseinsugarcontent fortwocultivarsofwhiteyams( Drotundata) storedinabarn.Generally,instoredtubersthereis reductioninweight,crudeprotein,starchandmineralcontentwhilethesugarandfibrecontents increase[21]. METHODSOFIMPROVINGYAMSTORAGE Yamstorageproblemsareessentiallyofthreekinds:(a)directdamagebydiseases,pestand nematodesinthefieldandinstorage;(b)sproutinglosses;and(c)respiratoryandevaporationlosses. Allthesereducetheoverallquantityandqualityofthetuberswithfoodreservesbeingincreasingly depletedbyoneormoreofthesecauses.Innormalconditions,tubersofmanyclonesremained dormantfor10-12weeksbeforesproutingstarted.Researchershaveworkedonanumberof methodsandtechniquestoimproveyamstorage.Differentintensitiesofgammairradiationoffered sometechnicaladvantagesforstoringyamtuberforfreshconsumption[22;23;24].Irradiationwas aneffectivetreatmentfortheinhibitionofsproutingofyam( D.rotundata )forastorageperiodofsix monthsatdosesofbetween7.5and15kradwithoutinducingadversechangesinacceptabilityor physiologicalproperties[22].Anaveragedoseof120Gyandadoserateof114Gy/hrwereappliedto thecultivar D.rotundatacvAsana andirradiatedandnon-irradiatedtubersstoredfor6monthsside bysideusingtwodifferenttypesofstorage,viz.barnandstorageontheground;resultsshowedthat irradiationreducedsproutinginbothstoragetypes[23].However,rottingincreasedwithstorage timeandtherewaslessrottingintheyamsstoredinthebarnthanthosestoredontheground.Itwas alsoobservedthattherewaslessrottinginthenon-irradiatedyamsstoredonthegroundthanthe irradiatedones.Aftersixmonthsofstoragefoodproductsmadefromirradiatedyamswerejudgedto bebetterinqualitythanthosemadefromthenon-irradiatedones.Differencesinvarietalresponsesto gammairradiationhavebeenreported[24]. Somechemicalcompoundshavebeenusedtoprolongdormancyandretardsprouting[25-29]. Gibberillicacid(GA)whenappliedtotuberssoonafterharvestwasabletoextenddormancyby9-11 weeksfor D.rotundata andby13weeksfor D.alata speciesofyamtuber[25;26]. D.alata tubers treatedwithGAatthebeginningofstoragegerminated4weekslaterthancontrols,theeffectbeing lesswhentreatmentsweregivenlater;treatmentsgivenafterthreemonthsofstorageweretoolateto inhibitgermination[27].Chloroisopropylphenylcarbamate(CIPC)solutionandpowderwhichis successfullyusedtoinhibitdormancyonpotatotubersdidnothaveanyeffecton D.rotundata [28]. Theeffectofmaleichydrazideincontrollingsproutinginyamshowedthatsoakingthetubersof D. esculenta and D.rotundata in1000ppmsolutionsfortenhoursbeforestoragereducedtherateof sproutingby16%and8%in D.esculenta and D.rotundata respectively[29].However,others[22] reportedthatmaleichydrazidewasnoteffectiveininhibitingsproutingin D.rotundata tubers.Other chemicalsusedinyamstorageincludecommercialwax,lime,benlateandcaptan.Theobserved effectsofchemicalsonthestoragelifeofyamtubershavebeensummarised,withtherebeingspecies andcultivardifferencesintheresponseofyamstochemicaltreatments[5;30]. Plantextractshavebeenusedtoimprovethequalityofstoredyamtubers[31-33].Theeffectofneem barkwaterextract,neembarkslurryandneemleafslurrytreatmentsonthequalityofstoredyam 9 showedthatsproutingwasdelayedbyonemonthinallneem-treatedtubers[32].Rottingwasalso delayedbythreemonthsintuberstreatedwithneembarkextract;asimilarresultwasobservedwhen usingneembarkextractandneemleafslurryforsprouting[33].However,theneemtreatmentsin thiscasedidnothaveanyeffectinreducingordelayingrotting.Theeffectoflimeandneemwoodash treatmentinthreedifferentcultivarsofbruised D.rotundata tuber s showedthatlimewasmore effectiveincontrollingrotinstoredyamtubersthanneemwoodash[31].Anothermeansof controllingrotandinhibitingsproutinginyamtubersistheuseofpalmwine;farmersclaimthat tuberstreatedwithpalmwineshowlessrotbutthisclaimisyettobesubstantiated. Theuseofimprovedyamstoragestructureshasbeenreportedbysomeauthors[1;14].The improvedbarnisrectangularinshapewithvaryingdimensions.Theflooriscementedandraised abovethegroundleveltopreventpestsfromgainingaccesstothebarn.Thewallsareconstructedof plasteredconcreteonemeterabovethegroundandtherestisprefabricatedchickenwiremeshjoined togetherbywelding[14].Theroofiseithercorrugatedaluminiumsheet,raffiamatsorgrass.Inside thebuildingareanumberofwoodenshelveswhichhavecompartmentsandtheyamsarearrangedon theseshelves.Theadvantageofthisstructureisthatplacingthetubersontheshelfrequirelesstime andlabour,providesadequateventilation(throughthewiremesh)andthespacebetweenthewall andtheshelvesfacilitatefreemovementduringinspectionofthetubers[1;14].Studieswere undertakentoobtainquantitativedataontheweightlossofstoredyamtubersasaffectedbyair temperature,relativehumidity,lengthofstorageonsproutingandtuberweight[34-36].Yamtubers storedforsixmonthsinatraditionalyambarn,apitstructurewithcrossandverticalventilationviaa centrallylocatedchimneyandanotherpitstructurewithoutachimneyshowedthatthepsychrometric airconditionswerecharacteristicallydifferentinthethreestructures[34].Averageairtemperature was25°C,27°Cand35°Cforpitwithchimney,pitwithoutchimneyandbarnrespectivelyandthe humiditywashighestinthepitwithchimneyandlowestinthebarn.Thelowtemperatureinthepits wasduetoshieldingfromsolarradiationandthecoolingeffectoftheshadedsoilmasssurrounding thepitstructure.Thelowtemperatureinthepitwithachimneyresultedindelayedsproutingand reducedweightlosscomparedtotheotherstructures.Similarresultswereobtainedbyothers[35; 36].Inadditiontothelowtemperatureinthepits,thediurnaltemperaturevariationwasalsolow comparedtothebarn. Inanotherstudythestorageenvironmentwasmodifiedbyventilatingthebarn[21;37].Theeffectof intermittentandcontinuousairflow(achievedbyplacingastandingfaninthebarn)onweightloss andsproutingofwhiteyamtuberswasasignificantreductioninsproutingandweightlossoftubers whensuppliedwithairflow[37].Asimilarresultforsproutingandweightlosswasobservedwhen intermittent(sixhourly)airflowwasincorporatedinaconventionalyambarn[21].Attheendofthe storageperiod,thepercentageofrottingwassignificantlylowerinthebarnwithintermittentairflow (lessthan2%)comparedtothebarnwithnoairflowwhere12%ofthetubersweredecayed[21]. Regularremovalofsproutsasameansofreducingweightlossinstoredtubershasbeenstudied[20; 27;38].Byremovingthegrowingshootsweekly,weightlossofstoredyamtuberscanbereducedand usefulstoragelifeincreased[27].Iftubersarestoredonshelveswhereaircirculatesfreelyaround them,thenthenewsproutscanbeeasilyseenandrapidlyremoved.Thistechniquewillextendthe storagelifebyupto8months.Monthlyremovalofsproutsreducedfreshweightlossduring5months storageby11%forcultivarsof D.alata and D.rotundata tubers[38].Thereweredifferencesinweight lossduetothesproutcuttingtechnique;cuttingofthesproutatthebaseratherthan1cmfromthe basewasrecommended.Table2showsthepercentageweightlossfortuberswithandwithout sproutsremovedaftereighteenweeksofstorageinthebarnandimprovedpitstoragefortwo cultivars( giwa and asuba )of D.rotundata tubers[36];theweightlossfortuberswithsprouts removedislowerthanwhenthesproutwasnotremoved. 10 Table2:Weightlossfortuberswithandwithoutsproutsafter18weeksofstorage (Osunde2003) Weightloss% Treatment GiwaAsuba BarnPitBarnPit Sproutremoved22202018 Sproutnotremoved26.324.322.722.7 Storageofyamtubersatarelativehumidityof80%andtemperatureof16°Creducesmoistureloss anddelayssprouting.Adesuye[14]statedthat15°Cisasafetemperatureforstorageofyamandit inhibitssproutingforsixmonths.However,changingthetemperaturetoinfluencedormancyis limitedbecausethetissueisdestroyedwhenthetemperaturefallsbelow15°C.Inaddition, influencingthestorageclimatebyexternalenergy(refrigeration)isrestrictedeconomicallyduetothe highenergycost. CONCLUSION Yamsarethemostnourishingplantsinthedietofmanyinhabitantsofinter-tropicalregions,tosuch anextentthattheirveryexistenceiscentredonthiscrop.Theaverageinhabitantoftheyamzone consumesbetweenhalfandonekilogramofyamdaily,representingabouthalfoftheirtotalcalorific intake.Yamsaregenerallyabundantandsoldcheaplyatharvesttime,butlater(especiallyduringthe plantingseason)theybecomescarceandexpensive.Ifyamcouldbestoredwithoutheavyloss, suppliescouldbecomesteadier,priceswouldfluctuatelessandfarmerswouldbeencouragedtogrow thembybeingassuredofasteadierincome. Thischapterhaspresentedanumberoftechniquesandtreatmentstoimprovethequalityand quantityofstoredyamtuber.However,thereisaneedforfurtherresearchintothepostharvest storageofyamtubertobetterunderstanddifferencesinvarietalresponsestotreatments.Such researchshouldbeconductedinthefarmer’sfieldwithhisactivecooperation,sothatalargepartof theresearchmaybedirectlylinkedtohisneeds,andmaythusaffecthiminthemostactiveway. 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