Postharvest handling of

SweetpotatoeS

BrookeA.Edmunds|Graduate Research Assistant, Dept. of Plant Pathology, N.C. State University

MichaelD.Boyette|Professor, Dept. of Biological and Agricultural Engineering, N.C. State University

ChristopherA.Clark|Professor, Dept. of Plant Pathology and Crop Physiology, Louisiana State University

DonaldM.Ferrin|Assistant Professor, Dept. of Plant Pathology and Crop Physiology, Louisiana State University

TaraP.Smith|Assistant Professor, Sweet Potato Research Station, Louisiana State University

GeraldJ.Holmes|Associate Professor, Dept. of Plant Pathology, N.C. State University

The auThors graTefully acknowledge The following for review of This bulleTin: Charles Averre, Professor

Emeritus,Dept.ofPlantPathology,N.C.StateUniversity; WilliamBranch,ProfessorEmeritus,Dept.ofBiologicaland

Agricultural Engineering, Louisiana State University AgCenter; Abner Hammond, Professor, Dept. of Entomology,

LouisianaStateUniversityAgCenter;GeorgeKennedy,Professor,Dept.ofEntomology,N.C.StateUniversity;andJona-

thanSchultheis,Professor,Dept.ofHorticulture,N.C.StateUniversity.

ON thE COvER: (top) the process of harvesting sweetpotatoes involves digging the roots. here the soil is turned over using large disks that bring sweetpotato roots to the surface. (Bottom) high quality, fresh-market sweetpotatoes (cultivar Covington) cured, washed and ready for sale.

SwEEtPOtAtO AS ONE wORD. throughout this book, sweetpotato is deliberately spelled as one word unless directly quoting a source where it is spelled as two words (i.e., sweet potato). the one-word spelling was officially adopted by the National Sweetpotato Collaborators in 1989. Sweetpotato (Ipomoea batatas) must not be confused in the minds of shippers, distributors, warehouse workers, and above all consumers with the equally unique and distinctive potato (Solanum tuberosum) or the yam (Dioscorea sp.) which are also grown and marketed commercially in the United States.

TableofContents

Proper handling Pays 6

growing for improved Postharvest Quality 6

harvesting for Quality 7

The curing and storage facility 9

curing for Quality 11

BenefitsofCuring 11

ProblemsAssociatedwithImproperCuring 12

storing for Quality 12

ProblemsAssociatedwithImproperStorageConditions 12

Packing for Quality 14

reducing Packing damage 20

Packing line cleaning and sanitation 22

Product safety and certification Programs 25

Packaging and shipping for Quality 27

PackagingforQuality 27

ShippingforQuality 27

MarketLifeofPackedSweetpotatoes 29

key references 30

Appendix 1 32

Guide to Common Postharvest Diseases, Abiotic Damage and Insects

Appendix 2 40

U.S. Standards for Grades of Sweetpotatoes

Appendix 3 42

Construction Guidelines for Negative horizontal ventilation (Nhv) Curing and Storage Facilities

6

Proper Handling Pays

Theimportanceofproperhandlingofsweetpotatoes,

fromthefarmer’sfieldtotheconsumer’skitchen,cannotbe

overemphasized.Studiesshowthatsignificantpostharvest

lossesoccurbecauseofimproperhandlingandotherfactors.

OnaverageintheUnitedStates,20to25percentislostin

sweetpotatoesduringcuringandstorage,another5to15

percentislostduringshippingandretailing,andanaddi-

tional10to15percentislostaftersweetpotatoesreachthe

consumer.Intotal,poorhandlingpracticesmayresultinthe

lossofmorethanhalftheharvestedsweetpotatoesbefore

theyreachtheconsumer’stable.

Providingconsumerswithanacceptableproduct

(Figure1)demandsattentiontotheuniquepostharvest

requirementsofsweetpotatoes.Thispublicationhasbeen

preparedtoacquaintgrowers,packers,andshipperswiththe

mostcurrentinformationandrecommendationsforproper

postharvesthandlingofsweetpotatoes.Itincorporates

newinformationongoodagriculturalpractices(GAPs)and

packinglinesanitationandconfigurations,andtheresultsof

anin-depthpackinglinesurvey.Alsoincludedareplansand

operatingrecommendationsforamoderate-sizedsweet-

potatocuringandstoragefacilitywithnegativehorizontal

ventilation(NHV).Photographsofcommonpostharvest

diseases,abioticdamagenotcausedbydiseaseorganisms,

andinsectsareinAppendix1.

Growing for Improved Postharvest Quality

Successfulstoragestartswithhigh-qualityroots.Events

occurringduringthegrowingseasonmaylaternegatively

affectpostharvestquality.Somefactorssuchasweatherare

impossibletocontrol,whereasothers(suchasfertilization)

canbemanipulatedbyagrowertoensurethataquality

productgoesintostorage.

Theweatherduringthegrowingseason,especiallyjust

beforeandduringharvest,hasamajoreffectonpostharvest

quality.Anextendeddroughtfollowedbyheavyrainfre-

quentlyacceleratesgrowth,whichoftenproducesrootswith

Figure 2. Freshly harvested roots exude latex when cut. (PhOtO By G. hOLmES)

Figure 3. No latex exudation when cut: a symptom of chilling injury. (PhOtO By t. SmIth)

Figure 4. Proper cutting of slips is done above the soil line to avoid contact of the knife blade with soil. A contaminated blade may transfer disease organisms from the soil to the cut ends of slips. (PhOtO By G. hOLmES)

Figure 1. Proper postharvest handling is required to produce quality sweetpotatoes for retail markets. (PhOtO By G.hOLmES)

7

thin,delicateskinthatarepronetogrowthcracks(Figure

53).Besidesbeingunappealingtothecustomer,thesecracks

provideinfectionsitesforsoilbornepathogens.Additionally,

heavyrainsthatsaturatesoilformorethanafewhourscan

causerootasphyxiation.Water-saturatedsoilallowscarbon

dioxidetoaccumulateintheroots,aconditionthatmay

alsobeaccompaniedbyadepletionofoxygen.Asphyxiation

canhappenatanytime,butitismorelikelytooccurduring

warmperiods,especiallyifthevineshavebeenremoved

beforeharvest.Sweetpotatoesthathavebeenasphyxiated

mayappearhealthyforseveraldaysorweeks,butifinjury

wassevere,therootswilldieandbegindecomposingin

storage.Thefirstindicationofaproblemmayincludethe

lackofexudinglatexfromthevascularringofacutsweetpo-

tato(Figures2and3).Thesmellofalcohol,yeastor“decay,”

increasednumbersoffruitflies,andsecondarydiseasessuch

asbacterialorfungalinfectionsalsoappearduringstorageof

asphyxiatedsweetpotatoes.

Nitrogenfertilizationtimingandratesaffectpostharvest

quality.Whilethefinalstudiesarenotinyet,goodcultural

practicesdictatetheuseofnitrogenfertilizersearlyand

sparingly.Increasingyieldwithadditionalnitrogenfertil-

izermayresultinanabundanceoflarge,misshapedroots.

Researchoncalciumfertilizershasalsoproducedvariable

results.Somestudiesshowabeneficialeffectonskinquality

andappearance,whilemostshownoeffectonquality.

Fieldpracticescontrolsomepostharvestdiseases.

Fusariumrootrot,Fusariumsurfacerot,andblackrotare

justthreediseasesthatstartasinfectionsinthefieldbut

developsymptomsinstorage.Growerscanreducelosses

fromthesediseasesbyavoidingfieldswithaknownhis-

toryofdiseaseandbyusingslips(plantcuttingsusedas

transplants)thathavebeencutinsteadofpulled,which

avoidstransferringdiseasefromtheplantbedintothefield

(Figure4).Propercuringisalsoessentialtocontrollingmany

diseasesandisdiscussedinthecuringsectiononpage11.

Harvesting for Quality

Sweetpotatoeshavethin,delicateskinthatiseasily

damagedbycutsandabrasions(Figure5).Roughhandling

duringharvestcancontributesignificantlytopostharvest

losses.Theselossesresultfromshrinkage(weightloss),

inferiorappearanceoftheroots,anddiseasesthatenter

throughdamagedskin.Plowingandhandharvestingor

harvestingwithamechanicaldiggerwillgivesatisfactory

resultsifdonecarefully.Mostgrowersharvestintoeither

20-bushelor40-bushel“double”woodenbins,although

some20-bushelplasticbinsareused.(Seepage44formore

detailsonpalletbindimensionsandcapacity.)Workers

shouldnotthroworstepontherootsinthebins(Figure6).

Palletbinsshouldneverbeoverfilled,asthispreventsproper

binstacking.Improperstackingwillinjuretheroots,notjust

ontop,butthroughoutthebin(Figure7).Overfillingcanalso

Figure 5. Skinning due to abrasions incurred during postharvest handling. (PhOtO By G. hOLmES)

Figure 6. Gentle handling during harvesting operations is critical to maintaining quality and reducing decay. (PhOtO By G. hOLmES)

Figure 7. Bins are often slightly overfilled initially so that as roots settle, the bin’s holding capacity remains maximized. however, if sufficient settling does not occur, overfilled bins will lead to tremendous injury when stacked. (PhOtO By G. hOLmES)

causestabilityproblemswhenstacking.Likewise,transport

overroughroadsorexcessmovementatthecuringand

storagefacilitycanresultinadditionaldamage.Although

promptandpropercuringcanhelphealinjuries,aninjured

sweetpotatowillneverregainitsoriginalappearance.

Afterrootsaredug,theyshouldbepromptlyloaded

andmovedtothestoragefacility.Otherwise,thereisariskof

injurybysunscaldorchilling,dependingonenvironmental

conditions.Sunscald(Figure8),aphysiologicalcondition

thatcausesadarkeningordeathoftheskin,mayresult

afterasfewas30minutesofexposuretobrightsunlight.

Ifsweetpotatoesareallowedtoremaininbrightsunfor

severalhours,eitherbeforetheyarepickeduporafterthey

areplacedinthepalletbin,theyarealmostsuretodevelop

sunscald.Sunscaldisunattractiveandcanbeasitefor

postharvestdecay.Somecultivarsofsweetpotatoesaremore

susceptibletosunscaldthanothers,anditismoreconspicu-

ousonlightorflesh-coloredcultivars.

Chillinginjurybecomesaconcernduringlate-season

harvests.Althoughsweetpotatoesfreezeatabout30°F(1°C)

andareimmediatelyruined,theyareinjuredattemperatures

below50°F(10°C).Theextentofthechillinginjuryisa

functionofboththetemperatureandlengthofexposure.For

example,onehourat40°F(4°C)mayproducethesamelevel

ofinjuryasfivehoursat45°F(7°C).Chillinginjuryisalso

cumulative;oneshortepisodebelow50°F(10°C)maynot

produceanynoticeableinjury,whereasmanyshortepisodes

maycausesignificantinjury.Unharvestedsweetpotatoes

maynotbeharmedbyafrost,dependingonthetemperature

ofthesoilsurroundingtheroots.Harvestassoonaspossible

afterfrosthaskilledthevinestoensurethatnoinjuryoccurs.

Neverleaveharvestedsweetpotatoesinthefieldovernight,

ascoolingmaycausesubstantialinjury.Damagecausedby

chillingmaynotappearformanyweeks—orevenseveral

months—afterthechillingoccurs.

Chillinginjuryisexpressedinmanywaysandcanbe

difficulttodiagnose.Themostcommonsymptomsare

surfacepitting,greatlyacceleratedrespiratoryactivity(dry

matterloss),andanincreaseinsusceptibilitytodecay

(especiallybluemoldcausedbyPenicilliumspp.SeeFigures

9through11).Othercommonsymptomsincludeinternal

breakdownandvoids,hardcore,failuretosprout,reduced

culinarycharacter(color,texture,taste,andaroma),and

discoloration(darkening)offleshwhenexposedtoair.If

chillingwassevere,therootsmaynotexudelatexwhencut

(Figures2and3),ordieandbegintodecomposeinstorage.

�

Figure 8. Sunscald (A) with deer feeding injury (B); undamaged root skin under the soil line (C). (PhOtO By G. hOLmES)

Figure 9. Surface pitting caused by chilling injury. (PhOtO By G. hOLmES)

Figure 10. Internal voids caused by dry matter loss. (PhOtO By G. hOLmES)

Figure 11. Secondary Penicillium mold invasion following chilling injury. (PhOtO By G. hOLmES)

b

a

c

The Curing and Storage Facility

Aproperlybuiltstoragefacilitymaintainsthetempera-

tureandhumidityrequiredforcuringandlong-termstorage

ofsweetpotatoes.“Commonstorage”inareaswithout

temperaturecontrolorassumingthattheambientcool

wintertemperaturesareadequatewillnotmaintainsweet-

potatoquality.Themosteffectivetypeofstoragefacilityuses

negativehorizontalventilation(NHV).

TheNHVsystemusesaslightnegativepressuretopull

theventilationairhorizontallypastthepalletbins.Fans

mountedinternallyalongthetopofaplenumwallonone

endoftheroomcreatethenegativepressure.Airfirstenters

themassofsweetpotatoesattheendoftheroomopposite

theplenumwall,throughductsformedbytheforkliftslotsat

thebottomofthepalletbins.Theairthenmoveshorizontally

throughthemassofsweetpotatoestowardopeningsinthe

plenumwall.Onceintheplenum,theairrisesandpasses

throughthefansandbackoutintotheroom,whereitmoves

horizontallyintheoppositedirectionbackoverthetopof

thestackedbins(Figures12and13).

9

Figure 12. Construction diagram for negative horizontal ventilation storage facility. (ILLUStRAtION By m. BOyEttE)

Figure 13. Loading pallet bins of sweetpotatoes into a new negative horizontal ventilation (Nhv) facility. (PhOtO By CCU INC.)

TheNHVsystemallowsgoodairmixing,sothereislittle

internalvariationintemperatureorhumiditythroughout

theroom.Further,becausetheairisinmotionandispassing

throughthemassofsweetpotatoes(norootismorethan

one-halfthedepthofapalletbin—approximately25inches

(63.5cm)—fromamovingstreamofair),thereisopportu-

nityforheattransfer.Goodheattransferisimportantfor

warmingthesweetpotatoesatthebeginningofthecuring

cycle,coolingthemattheend,andforremovingtheheatof

respirationthroughoutthestorageperiod.

Aseriesofmotorizeddampersislocatedontheexterior

wallacrosstheplenumfromthefans.Whilethesedampers

remainclosed,onlyinternalairiscirculatedthroughthe

palletbins.Thesedampersareopenedwhenoutsideairis

requiredforventilationorcooling.Airispulledintothese

openingsbecauseoftheslightnegativepressurethefans

createintheplenum.Thesizeandnumberofthesedampers

aredeterminedbythecapacityoftheroom.Whencorrectly

designed,approximatelyone-thirdoftheairpassingthrough

thefanswillbepulledfromoutside,withtheremainder

oftheairrecirculatedfromthestackofpalletbins.Theair

displacedbytheincomingairexitstheroomthroughgravity

shutterslocatednearfloorlevelattheendoftheroom

oppositetheplenum.

TheNHVsystemoffersanumberofimprovementsto

sweetpotatocuringandstoragefacilities:

• Airmovesefficientlyandconsistentlythroughthelarge

massofsweetpotatoes,providingventilationandheat

transfertominimizebothin-buildingvariationsand

fluctuationsbecauseofchangesinoutsidetemperature

andhumidity.Warmingsweetpotatoesatthebeginningof

curing,coolingattheend,removingheat,andwarming

seedsweetpotatoesforpre-sproutingbeforebeddingare

greatlyenhancedbyNHV’sefficientheattransfer.

• Becausetheairpassesthroughthemassofsweetpotatoes

horizontally,theNHVsystemdoesnotusefloortrenches.

Thefloorsmaybethestandardfourinchesofwelded

wiremeshorfiberglass-reinforcedconcreteoverapacked

grade.Thisfeaturesubstantiallyreducesthecostof

constructionandallowsNHVsystemstobeinstalledin

manyexistingstructures.

• Economicalstandardpole-typeorsteelcolumnandgirder

buildingsmaybeusedwithoutexpensivecustommodifi-

cationsortheneedforexcessivelywidespans.

• Thesystemcanaccommodatealargevarietyofroom

sizes.Individualroomshavebeenbuiltassmallas20by

50feet(6.1by15.2m),with6,000-bushelcapacity,toas

largeas120by100feet(36.6by30.5m)with96,000-bushel

capacity.Thisflexibilityisparticularlyattractiveinthe

largerfacilitieswithlowerper-unitconstructionand

operatingcosts.Largerroomsmaymakeslightlymore

efficientuseofspacethansmallerroomsbutcanresult

inundercuredorovercuredrootsifnotfilledonatimely

basis.Smallerrooms,however,aremorequicklyemptied

andtakenoutofservice.

• TheNHVsystemmakesveryefficientuseoffloorspace.

Forthesystemtooperateproperly,thepalletbinsmust

beplacedtightlytogether,instraightrows,withaslittle

spacebetweenbinsaspossible.Thesystemhasworked

wellwithbinsstackedsix,seven,andeveneighthigh,but

itworksbestifallthebinsarestackedtothesameheight.

Itisalsobetterifallthepalletbinsarenearlythesamesize

andofthesameconstruction,asthisfacilitatesproper

stackingandminimizesairleakagebetweenbins.Someair

willinevitablyshortcircuitbetweenthebins,bothinthe

horizontalandverticaldirection,whichiswhythefansare

sizedtoaccommodatethisleakage.

• BecausethecostofoutfittingabuildingwithNHVis

modest,thereisnoadvantagetohavingseparaterooms

forcuringandstorage.Thismakesbetteruseofspaceand

eliminatestheneedtomovethesweetpotatoesattheend

ofcuring—anoperationthatistimeconsuming,expen-

sive,andinvariablyresultsindamagetotheroots.

• Eliminatingsubstantialvariationsintemperatureand

humidityinsidetheroomallowsformorepreciseand

sophisticatedcontrolsthathelpmaintainqualityand

reduceenergyusage.AlthoughmanyNHVfacilitiesare

successfullymanagedbyelectro-mechanicalcontrollers

(thermostats,relays,timers),thefullbenefitofNHV

technologyisrealizedbyusingprogrammablelogic

controllers(PLCs).Theseindustrial,computer-likedevices

maybeprogrammednotonlytocontrolthetemperature

andhumiditybutalsotomonitorandlimitenergyuse,

collectdata,soundalarms,andprovidesecurity.

• Theventilationfansmountedintheroofoftraditional

sweetpotatostoragefacilitiesareoftenasourceofleaks

andothermaintenanceproblems.InanNHVfacility,

airinletsandoutletsaremountedonthesidesofthe

building,whichmakesthemmucheasiertoinstalland

maintainandlesslikelytoleak.

• Variousspraysareeffectiveagainstinsectpests.Until

recently,however,itwasdifficulttoconsistentlydistribute

thematerialthroughoutthemassofsweetpotatoes.Even

infacilitieswithautomaticinsecticidedispersalsystems,

completecoverageisdifficult.Theuniformairmove-

mentinNHVfacilitieseffectivelydistributesinsecticides

throughouttheroomformaximumcoverage.

Formoreinformationonspecificconstructionguidelinesfor

theNHVsystem,seeAppendix3.

10

Curing for Quality

Aportionoftheannualsweetpotatocropisstill

marketedas“green,”althoughthepracticeisfadingfrom

favor.Greenrootsarewashed,graded,andpackedwithina

fewhoursordaysofharvestandshippedimmediatelyto

buyerswithoutcuring.Uncuredsweetpotatoesgenerally

lackthevisualappeal,shelflife,andculinarycharacterof

curedroots.

Mostsweetpotatogrowersandpackershaveinvested

inmoderncuringfacilitiesandconsiderpropercuring

anindispensablefirststepinaprocessthatallowsthe

industrytoprovideayear-roundsupplyofhigh-quality

sweetpotatoes.Successfulcuringrequiresrootstobeheld

atatemperatureof85°F(29°C)andarelativehumidityof85

to90percentwithproperventilationforthreetofivedays

immediatelyafterharvest.(Thedurationvariesdepending

ontherootpulptemperatureatharvest.Thegreaterthedif-

ferencebetweenrootpulptemperatureand85°F(29°C),the

longeritwilltaketocure.Seepage47formoreinformation.)

Adelayofasfewas12hoursbetweenharvestandcuringhas

beenshowntobedetrimentaltosuccessfulcuring.

Sweetpotatoesremainmetabolicallyactiveafter

harvest.Theyrespire,convertingstarchtosugarsthatare

metabolizedtoreleasecarbondioxideandwatervapor.

Sufficientmovementofair(ventilation)duringcuringis

essentialandhelpsdryrootsandanyadheringsoil,provides

properoxygenandcarbondioxideexchange,andisneces-

saryforgoodheattransferduringcuring.Aslittleasone-half

cubicfootofoutsideairperbushelperdayissufficientfor

properventilation.However,sweetpotatoesinjuredbyrough

handling,exposedtochilling,orharvestedfromwaterlogged

soilmayrequireasmuchas5to10cubicfeetofoutsideair

perbushelperday.

Thehumidityduringcuringshouldbeashighaspos-

sible(85to90percent)butnottothepointwherewatermay

beseenonthewalls,floors,bins,orespeciallythesweetpo-

tatoes.Allproperlydesignedcuringfacilitiesshouldhave

correctlysizedhumidificationequipmentandcontrols.The

costofthisequipmentiseasilyrecoveredinreducedweight

lossandbetterrootquality.Curingroomsshouldbeproperly

insulatedtoconserveenergyandreducecondensation.(See

Appendix3foradiscussionofinsulationmaterials.)

benefiTs of curing: 1.Curingenhancesculinarycharacteristics(eat-

ingquality).Asweetpotato’sculinarycharacteristicsare

acombinationofcolor,texture,taste,aroma,andfiber

content.Muchoftheculinarycharacterofanindividual

sweetpotatodependsonthecultivarand,toasmallerextent,

onculturalpracticesduringthegrowingseason.Someofthe

mostimportantculinarycharacteristics,however,arethe

resultofchemicalchangesthatoccurasaresultofcuring.

Propercuringhasbeenshowntoincreasethesensationof

moistnessandsweetness,enhancethearoma,anddecrease

starchcontentwhileincreasingsugars.

2.Curingaidsinwoundhealingandreduceslosses

duetoshrinkageanddisease.Whenrootsarewounded,the

exposedcellswillquicklydryanddie.Sweetpotatoeswill

naturallyexudestickylatexfrominjuries,particularlyatthe

endsofthesweetpotato(Figure14).Thismaterialmaydry

inafewhoursandappeartoclosethewound,butitactually

provideslittleprotectionfromdecayorganismsorweight

loss.Onlypropercuringcanresultin“true”woundhealing.

Undercuringconditions,thesweetpotatowilldepositalayer

ofmaterialunderthedeadcellsinthewoundedarea.This

barrierfurtherreducesmoisturelossandimpedesmicrobial

invasionofthetissue.Inthefinalstageofthisprocess,a

secondlayersimilartoundamagedskinisdepositedunder

thewoundinaprocessknownassuberization.

3.Curingsetstheskin.Freshlyharvestedsweetpotatoes

havethin,delicateskinthatiseasilybroken,scraped,or

otherwiseremoved(Figure5).Somecultivarsmaybewashed

andgradedwithoutseriousinjuryifitiscarefullydone

within24to48hoursofharvest.However,mostcultivars

requirecuringto“settheskin”becausetheskinquickly

becomestooloosetopermitsafehandling.Propercuring

afterharvestresultsinskinthatsetswithinfourtosixweeks.

Theexacttimerequiredforskinsetvariesconsiderably

acrosscultivars.Thefactorsinfluencingskinset,suchas

growingconditions,arenotwellunderstoodandarethe

subjectofongoingresearch.

11

Figure 14. Latex stains on roots. (PhOtO By B. EDmUNDS)

Ifrootsmustbeshippedsoonafterharvest,thetime

requiredforskinsetmaybeshortenedsomewhatbyproper

curingatstandardconditionsfollowedbyseveralweeksat

properstoragetemperaturesbutatlessthan85percentrela-

tivehumiditywithgoodairventilation(asmuchas30to50

cubicfeetofventilationperdayperbushelmayberequired

inthiscircumstance).Thistreatmentmayallowrootstobe

shippedsoonerbutwillresultinincreasedweightloss,so

itisimportanttomovethisproductquicklytomarket.For

longerstorageperiods,followthecuringperiodwithnormal

storageconditions—55°F(13°C),85–90percentrelative

humidity,andadequateventilation.

Problems associaTed wiTh imProPer curing: Inadequateandexcessivecuringcanshortenshelflife,

increasesproutingduringstorage,andresultinexcessive

weightloss.Normalweightlossshouldnotexceed5to8

percentofthefreshlyharvestedweight.

Improperventilationduringcuringcanresultinan

extremelylowoxygen/highcarbondioxideenvironment.

Exposuretothisenvironmentforshortperiodshasbeen

showntoreducetheeffectivenessofcuring,shortenstorage

life,andalterthetasteofthesweetpotatoes,butthisproblem

isunlikelytooccurinaproperlyoperatedmodernfacility.

Curingatimpropertemperaturesorhumiditycan

reducequalityduringstorage.Researchhasshownthat

curingsweetpotatoesattemperaturesbelow75°F(24°C)

increasesweightlossanddecreasesstoragelife.Lowhumid-

ityalsoresultsininadequatehealingofwounds.

Curingthatcontinuesfortoolongcanresultin

widespreadsprouting(Figure15).Itisnotunusualtosee

short(lessthanone-fourthinch)sproutbudsonafewroots

towardtheendofcuring;however,widespreadsprouting

resultsinrapidweightloss.Thebestwaytominimizeweight

lossfromovercuringisnottoexceedtherecommended

threetofivedaysofcuringandtoreducethetemperature

to55to60°F(13°C)asquicklyaspossible.Maintainingthe

correctrelativehumidity(85to90percent)duringstorageis

alsocritical.

Storing for Quality

Thenextstepintheproductionofqualitysweetpotatoes

isstorageintheproperenvironment.Theprimarygoalof

storageistomaintainrootqualityandensureanadequate

supplythroughouttheyearbyminimizingbothphysiologi-

caldisordersanddiseasedevelopment.Currentexperience

showsthathigh-qualityrootsthatareproperlycuredand

held,undisturbed,underproperstorageconditions—55°F

(13°C),85to90percentrelativehumidity,withadequate

ventilation—remainmarketableforaslongas13months.

Thesestorageconditionswerefirstdeterminedin

the1920swithcultivarsgrownatthattime,andrecent

researchhasshownthattheseconditionsarestillvalidfor

moderncommercialcultivars.Itisimportanttomaintain

thetemperatureascloseaspossibleto55°F(13°C).Minor

fluctuationsofthreeorfourdegreesareexpected,butavoid

fluctuationsofmorethanfivedegrees.Fluctuationscan

occurwhenrootsarestoredincommonstorageorina

roomwithouttemperatureregulation.Fluctuationsofmore

thanfivedegreeswillleadtoprematurebreakdownofthe

sweetpotatoandexcessiveweightloss.

Higherrelativehumidity(greaterthan85percent)

wouldbeentirelysuitableforsweetpotatostorage.However,

fromapracticalstandpoint,veryhighhumidity(90to95

percent)isdifficulttomaintainconsistentlyandtomeasure

accurately.Additionally,veryhighhumiditywillcause

condensationtoformonthebuildingwallsorroof,causing

maintenanceproblemsandthewettingofbinsandroots,

whichpromotesdecay.Improperroominsulationcanalso

contributetocondensationproblems.

Problems associaTed wiTh imProPer sTorage condiTions: Improperstorageconditionscanincreasethedevelop-

mentofphysiologicaldisordersanddiseases.Physiological

disordersaretheresultofstressesrelatedtoexcessivelight,

heat,cold,andmoisture,orthemixofsurroundinggases

suchasoxygen,carbondioxide,andvariouspollutants.

Somedisorderscanbecausedbymechanicaldamage,

andallareabioticinorigin(notcausedbydiseaseorgan-

isms)andcannotbecontrolledbypostharvestpesticides.

However,manypostharvestdisorderscompromisethe

sweetpotato’snaturaldefenses,whichinturnincreases

susceptibilitytoinfectiouspostharvestdiseases.Insome

cases,physiologicaldisordersmayevenmimicinfectious

diseases.Commonphysiologicalproblemsresultingfrom

improperstorageconditionsincludeexcessivedrymatter

loss,sprouting,pithiness,hardcore,chillinginjury,and

moistureloss(Figures9through11).

Drymatterlossandpithiness.Sweetpotatoeslosedry

matterthroughnaturalrespiration.Respirationisachemical

processnecessaryforalllivingtissuewherebystarchesand

sugars(drymatter)areoxidizedtocarbondioxideandwater

vaporwiththeliberationofheat.Theheatgeneratedby

anindividualsweetpotatoisnegligible,butthecombined

outputofthousandsofbushelsinastoragefacilitycanraise

thetemperatureofsweetpotatoesone-fourthtoone-third

degreeperday.Thisheatmustbecontinuallyremovedfrom

12

thefacility,orthetemperaturewillriseaboveacceptable

levelsinashorttime.Astoragefacilitymusthaveprovisions

forcoolingwithoutsideairorarefrigerationsystem.

Althoughnotapparentexternally,significantdrymatter

lossmayresultinpithinesswiththeformationofmanysmall

voids(Figure10).Pithinessisverycommoninsweetpotatoes

heldforlongperiodsinpoorlycontrolledstoragefacilities.

Sproutinginstorage.Anothereffectofelevatedstorage

temperaturesissprouting(Figure15).Attemperaturesabove

60°F(16°C),sweetpotatoeswillsprout.Thelengthoftime

requiredforsproutingdependsonthetemperature.Itmay

takeamonthormoreforsproutstoshowat65°F(18°C),

butat75°F(24°C)andwarmer,sproutscandevelopinafew

weeks.Sproutingisalwaysaccompaniedbyrapidrespiration

andweightloss.Chemicalsproutinhibitorsarenotusedin

sweetpotatoesbecausepropertemperaturecontrolinhibits

sprouting.USDAstandards(seeAppendix2)listsprouts

overthree-fourthsofaninchlongasdefects.Sproutscanbe

manuallyremovedfromrootsduringthepackingprocess.

Chillinginjury.Chillinginjuryisrareinmodernstorage

facilities,butitcanoccurifrootsarekeptincommonareas

duringthewintermonths.Storagebelow50°F(10°C)can

resultinchillinginjurythatmaynotbeevidentuntilseveral

weekshavepassed(Figures9through11).

Excessiveshrinkage.Ifthehumidityislow,sweetpota-

toeswillloseweightasmoistureevaporatesfromthesurface

ofroots.Thisresultsinweightlossandmaycauseshriveling

oftheskin,especiallyatrootends(Figure16).Although

somemoisturelossispracticallyunavoidableduringcuring

andstorage,excessivewaterlossmaybeavoidedbymain-

taininghighrelativehumidityduringstorage.

13

Figure 15. Sprouting due to poor curing or storage conditions. Note that sprouts generally originate from the proximal end of the root (i.e., the end closest to the plant). (PhOtO By G. hOLmES)

Figure 16. weight loss is increased by skinned areas and leads to shriveling. (PhOtO By B. EDmUNDS)

Diseasedevelopmentinstorage.Byfar,postharvest

diseasesaccountforthegreatestlossinstoredsweetpota-

toes.Inextremeinstances,decaylossescanrunnearly100

percent.Theoccurrenceofpostharvestdiseasestendsto

varyfromyeartoyear.Outbreaksoccurwhenpathogensare

givenanopportunitytoproliferate.Manyofthediseasesthat

affectsweetpotatoesinstoragearefirstestablishedinthe

fieldoronplantingmaterialsuchasscurf(Figure51).Other

postharvestdiseaseorganismsarewind-orsoil-borneas

sporesandareessentiallyubiquitous(suchasRhizopus

softrot).

Postharvestdiseasesmaybecausedbyfungi,bacteria,

orviruses,althoughfungiaremorecommoninsweetpota-

toes.Mostvirusesdonotcauseseriouspostharvestdiseases,

althoughsymptomsfromfieldinfectionsmaybefirst

noticedafterharvest(aswithrussetcrack)ormaydevelopin

storage(internalcork).Similarly,rootknotnematodesinfect

rootsinthefield,andtheresultingcrackingmaybenoticed

duringgradingandpacking(Figures54and55).

Controldependsonunderstandingdisease-causing

organisms,theconditionsthatpromotetheiroccurrence,

andthefactorsthataffecttheircapacitytocauselosses.

Additionally,followingapprovedculturalpracticesin

thefieldcansignificantlyreducemanyofthesediseases.

Sweetpotatoesshouldbeinspectedastheyareharvested.

Leaverootswithindicationsofestablisheddisease(lesions)

orobviousdefectssuchasgrowthcracksorexcessive

skinninginthefield.Gentlehandlingandminimizationof

environmentalstressescansubstantiallyreducethelevelof

postharvestdisease.Themanagementofspecificdiseasesis

discussedinAppendix1.

Packing for Quality

Thepackingofsweetpotatoesisanindustrialoperation

thatshouldbededicatedtodeliveringthehighestquality

producttotheconsumer.Thecurrentmarketdemands

uniformityinappearanceinbothcolorandsize(seecover

photobottom),whichnecessitateslongandcomplicated

packinglines.Unfortunately,longpackinglinescanincrease

theopportunityforskinning,bruises,cuts,andbrokenends

thatdetractfromappearanceandincreasethepossibilityfor

diseasedevelopment.

Ingeneral,goodpacking-linedesignstrikesabalance

betweengentle,yetefficient,handlingofthesweetpotatoes.

Indicationsoftheneedtoalteranexistingpackingline

includehighlaborandenergycosts,bottlenecks,congestion,

workercomplaints,accidents,andproductdamagesuchas

excessiveskinning,excessivelosstodisease,andlargepiles

ofbrokenendsonthefloorbelowproblemareas.

Anindustrysurveyofsweetpotatopackinghousesin

NorthCarolinaandLouisianafrom2004to2006revealed

similaritiesamonglayoutsandassociatedtroublespots

(Tables1through5).Table1describesthetypicalcom-

ponentsandlayoutsformid-andlarge-sizepackinglines

seeninbothstates.Aninstrumentedimpact-recording

device(SmartSpud,SensorWireless,Canada)wasusedon

packinglinesinbothstates.Thisdevicemeasurestheforce

ofimpacts(measuredasaunitofforcecalledag;1g=9.81

14

Figure 17. Instrumented impact recording device used to measure impacts on packing lines. Right: fresh sweetpotato; middle: molded urethane casing with accelerometer inside; left: handheld computer with antenna for receiving signal from accelerometer and recording impacts. (PhOtO By B. EDmUNDS)

Medium (Low Volume) Packing Line Large (High Volume) Packing Line

dump tank dump tank

wash/brush wash/brush

eliminator eliminator

grading grading

fungicide application wash/brush*

sizer fungicide application

grading sizer 1-expanding pitch type

box fill first grading

wax/brush*

sizer 2-electronic*

final grading*

box fill

Table 1.Typicalcomponentsandsequenceofcomponentsinmediumandlargesweetpotatopackinglines.

*Bold text indicates items that differ from medium packing lines.

meters/secondsquared)withhighernumbersindicating

areasordropsonpackinglineswherepotentiallydamaging

impactsareoccurring.Theoriginaldeviceusedbyresearch-

erswascalledaninstrumented spherebecauseofitsshape,

andamajorimprovementhasbeenthedevelopmentof

aurethaneorsiliconcasingthatmimicsthedimensions

ofthecommoditybeingtested.TheNorthCarolinaState

Universityengineeringdepartmentfabricatedacasingby

makingamoldofanactualsweetpotato(U.S.No.1grade,

Figure17).Thiscasingallowsformeasurementsthatrepro-

duceimpactsreceivedbysweetpotatoesonapackingline.

Impactsoccurringatspecificpointsonthepackinglinewill

bediscussedfurtherandaresummarizedinTables2and4.

DumpTank.Sweetpotatoesaregenerallydumpedinto

atankofwater(dumptank)eitherbyabinrotationdevice

onaforklift(Figure18)oranautomaticbinrotator(Figure

19).Becauseharvestedrootsgodirectlyintostoragewithout

washing,sweetpotatoesalwayshavesignificantamounts

ofsoiladheredtothesurface,evenwhenharvestedfrom

dry,sandysoil.Somedumptankshaveabibofsmallmetal

barsthatsifttheloosesoiloutastherootstumbleintothe

dumptank(Figure20).Thebibpreventssomeofthesoil

fromenteringthewater,buttheimpactoftherootsonthe

barsishighandisalsoamajorsourceofmechanicalinjury.

Skinningisparticularlyseverebecausedryrootsskinmuch

easierthanwetroots.Becausetheseverityofdamageis

directlyrelatedtothedistanceofthefall,automaticbin

rotationequipment(whichshortensthefalldistance)is

preferredtobinrotatorsonforklifts.Abetteroptionisan

automaticbinrotatorwithalid,whichgraduallyreleases

therootsintothedumptankandminimizesroot-to-root

impacts.Thebestoptionforminimizingrootinjuryisthe

“submergeddump”(Figure21).Thisrequiresspecialized

equipmentthatcompletelysubmergestheentirebininto

awatertank,allowingtherootstofloatoutandvirtually

eliminatingroot-to-rootimpacts.Submergeddumpshave

yettobeadoptedintheSoutheastmainlyduetotheexpense

associatedwiththeequipmentandthetransitiontoplastic

storagebins.(Woodenbinsquicklybreakdownwhenwet

andarepoorlysuitedforthistypeofdump.)

15

Figure 18. Dumping roots using a forklift-mounted bin rotator device. (PhOtO By G. hOLmES)

Figure 19. Dumping roots using an automatic bin rotator. (PhOtO By G. hOLmES)

Figure 20. Dump tank with metal bib to sift out soil. (PhOtO By G. hOLmES) Figure 21. In a submerged dump, the entire storage bin is placed in the dump tank, and roots float out. (PhOtO By G. hOLmES)

Thedumpoperatorcanhaveamajorinfluenceon

impacts.Operatorsmustbeabletoseeintothedumptank

sothattheycancarefullymonitorthedumpingoperation.

Sweetpotatoesshouldnotbedumpedintoanoverfilledtank,

whichcausesexcessiveimpacts.

Unloadingsweetpotatoesintothedumptankgenerates

alargeamountofdust.Inadditiontocreatingproblemsfor

workersandmachinery,thisdustusuallycontainsdecay-

causingsporesthatareareadysourceofcontamination

fornearbysweetpotatoes.Forthesereasons,ithasbecome

customarytopartitionorwalloffthedumptankfromthe

restofthepackingline.Additionally,ventilationfansshould

beinstallednearthedumptanktodrawthedustoutsidethe

buildingandawayfromtherestoftheline.Fansalsohelp

keepdustawayfromforkliftanddumpoperators.

Dumptanksvaryinsizebutmayholdseveralthousand

gallonsofwater.Aportionofthiswaterisflushedregularly,

alongwiththeheaviersoil,throughalarge,air-operated

slideorbutterflyvalvelocatedonthebottomofthetank.

Theaircylindermaybeswitchedonmanuallyoractivated

byasimplefloatswitchandtimercircuit.Manynewerdump

tanksalsohaveautomaticmonitorsthatadjustthewater

levelasneeded.

Toreducedecay,somepackerstreatthedumptank

waterwithantimicrobialagents.Sodiumhypochlorite(liquid

bleach)iscommonlyused.Unfortunately,bleachisquickly

deactivatedbylargeamountsofsoilindumptanksandmust

berechargedatregularintervals(dependentontheamount

ofsoilenteringthedumptank).Thegasesreleasedfrom

treateddumptankscanirritatetheskinandeyesofworkers

andcorrodemetalsurfaces.Veryhighconcentrationsof

bleachindumptanksmayalsoleadtorootbleaching.Other

treatmentsthathavebeenusedwithlimitedsuccessinclude

ozonationandcopperionization.Althoughthesetreatments

maykillpathogensinrelativelycleanwater,suspendedsoil

particlesdiminishtheirefficacy,makingthemimpracticalon

packinglines.Severalstudieshaveshownthatsanitizersare

noteffectiveforcontrollingRhizopussoftrot.

Washing.Mostsweetpotatopackinglinesuseawater

rinsesteptoremoveclingingsoil.Waterfall/curtainsand

normal-orhigh-pressurespraywashersmaybeused(Figure

22).High-pressurewashershavebecomepopularbecauseof

thedifficultyofremovingdarkersoilsfromsomesweetpo-

tatocultivars.Wateratpressuresashighas250poundsper

squareinch(psi)isdirectedbyspraynozzlesatthesurfaceof

sweetpotatoesastheytraveloverrotatingbrushes.

Thedumptankandspraywashercanuseseveral

thousandgallonsofwaterperhour.Evenifawellorother

sourcecansupplythisamountofwater,packingline

operatorsshouldconsiderdisposal:Thelesswaterapacking

lineuses,thelessthatneedsdisposal.Forthisreason,many

sweetpotatopackinglineshavescreensandtankstocollect

thewaterfromthewashstepanduseittosupplythedump

tank.Thissimpleplumbingarrangementcanreducewater

use(anddisposal)bytwo-thirdsormore.

Impactsaregenerallylowduringwashingbecausemost

packinglineswashoverabrushbed.Somebrushbedsare

immediatelyprecededbyametalincline,whichitselfcanbe

asourceofhighimpacts.

16

Figure 22. high-volume water rinse is used to clean roots. (PhOtO By B. EDmUNDS)

Figure 24. Proper lighting and height for grading table allows workers to ad-equately see and reach cull roots. (PhOtO By G. hOLmES)

Figure 23. Eliminator on a packing line. Roots smaller than 1.5 in (3.8 cm) in diameter fall through the bars. (PhOtO By G. hOLmES)

Eliminator.Mostpackinglineshaveoneeliminator.

(Lessthanfivepercenthavetwo.)Theeliminatorconsists

ofabedofparallelmetalrollersnormallyset1.5inches(3.8

cm)apart(Figure23).Theeliminatorquicklyremovestrash

andsmallunmarketableroots.Itisimportantthattheroller

spacingdoesnotgetdentedorbent,asthiswillincrease

thespacingandcausemarketablerootstobediscarded.

Impactscanbesevereifsweetpotatoesfalldirectlyontothe

rollers,ratherthanrollingdownaninclineontotheelimina-

tor,becausetherollersaresupportedbythechainguides.

Instrumenteddevicereadingsaveraged8.8gandranged

fromalowof0gwherethesweetpotatoesrolleddowna

gentleincline(totaldropof5inches)toahighof30.7gina

casewherethesweetpotatoesfellabout16inchesdirectly

ontotheeliminatorrollers.

Grading.Afterwashingandeliminationoftrash,

sweetpotatoesmoveontoatableforhandsortingand

removalofdecayedorotherwiseunmarketableroots.The

tablesaregenerallymadeofPVCrollersandshouldbeeasily

accessiblebyworkersfrombothsides(Figure24).Adequate

lightingisimportant,sothatdefectscanbeseeneasily.Table

heightshouldalsoallowworkerstoreachrootsinthemiddle

ofthetablecomfortably.Workerswhodirectlyhandleroots

shouldwearglovestoprotectrootsfromfingernailscratches

andhumanpathogens.Gloveswillalsoprotectworkersfrom

fungicidesorotherchemicalsusedonthepackingline.

Impactsonthegradinglinevary,dependingonheight

ofthedropandwhethertherewasaninclineorpadding.

Forexample,onepackinglinehadadropheightofonly3.5

inches,andtheimpactswereallbelowthethresholdrecorded

bytheinstrumenteddevice.Ontheotherextreme,another

packinglinehadrootsdropping12inchesdirectlyontothe

rollers,whichproducedamuchhigherimpact(23.5g).

Fungicideandotherdecaycontroltreatments.

Althougheveryeffortshouldbemadetopreventmechanical

injurytosweetpotatoesduringpacking,itisimpossibleto

avoidallinjuries.Decay-producingorganisms,especially

thosethatcausesoftrot(suchasRhizopus stolonifer),enter

throughinjuries.Bruisedorcrushedtissueoffersaparticu-

larlyfavorableplacefordecaytodevelop.Forthisreason,

mostsweetpotatoesnotdestinedforcanneriesorfurther

processingaretreatedwithanapprovedfungicide.

17

Figure 25. A dip tank can be used to apply fungicides (PhOtO By G. hOLmES)

Figure 27. Spray application over a brush bed is a common method of applying fungicides. (PhOtO By G. hOLmES)

Figure 26. Fungicides may also be applied by using a waterfall/curtain. (PhOtO By G. hOLmES)

Average Maximum Minimum

Number of drops 10 19 5

Number of turns 3 8 0

Cumulative impacts (g)* 118 302 31

Length of line (ft) 102 277 37

Speed of line (ft/min) 24 59 7

Table 2.Averagenumberofdropsandturns,cumulativeimpacts,andlengthofpackinglines,basedon24Louisianaand12NorthCarolinapackinglines.

*Cumulative impacts measured on all drops and turns on a packing line. Average of five runs with the impact recording device.

Fungicidemaybeappliedeitherbydippingtheroots

inatankofchemicalsuspension(Figure25),byusinga

waterfall/curtainapplication(Figure26),orbyspraying

thefungicideeitheraloneormixedinawaxsolutionasthe

sweetpotatoespassoverabrushrollerconveyor(Figure27).

Regardlessoftreatmentmethod,rootsmustbecompletely

coveredwiththefungicidesuspension.Notreatmentis100

percenteffective.Evenproperlytreatedrootsmaydevelop

significantdecay.Forspecificrecommendationsonthe

useoffungicides,refertothemanufacturer’slabelinstruc-

tions,stateExtensionmanuals(suchasNorth Carolina

Agricultural Chemicals ManualorLouisiana Plant Disease

Management Guide),orcontactyourlocalExtensioncenter

forguidelinesinyourstate.

Sizing.Sortingsweetpotatoesintouniformsizesisakey

functionofpacking.TheU.S.sweetpotatogradestandards

forbothfreshmarketandcanningwereupdatedin2005and

arefoundinAppendix2.Mostmid-sizesweetpotatopacking

linesemployexpandingpitchroller(EPR)sizers.Thesesizers

allowtherootstomovealongonaconveyorofrotatingand

ever-wideningrollers.Thesmallerrootsarethefirsttofall

betweenadjacentrollersandaredepositedonabeltmoving

perpendiculartothedirectionoftherollers(Figure28).

Largerrootsarecarriedfurtherbeforetheyaredepositedto

adifferentsectionofthebelt.EPRsizerssegregatesweet-

potatoesonlybydiameter.Sweetpotatoeswiththesame

diameterbutdifferentlengthswillbeplacedinthesame

category.Unlesstherearelargevariationsinrootlengths

(usuallyafunctionofcultivarandgrowingconditions),EPR

sizerssatisfymostcommercialU.S.graderequirements.

Asthereisanincreasingdemandformoreuniformity

bothindiameterandlength,high-volumepackershave

investedinelectronicsizersthatopticallyscanbothlength

anddiameterandsegregatesweetpotatoesintoprecise

grades.Ifoverlap(Figure29),abnormalshape,orcolor

preventsthesizerfromcategorizingarootproperly,itwill

traveltotheendofthesizerandgothrougha“returnloop”

toberesized.Asmuchas30percentofrootshavebeen

observedgoingthroughthisreturnloop.Withthisinmind,

considerationmustbegiventothedropheightsandspeed

oftheconveyorsonthesereturnloops,becauseskinning

1�

Figure 28. Roots falling out of an expanding pitch roller (EPR) sizer usually land on the root end. (PhOtO By G. hOLmES)

Figure 29. Overlapping roots (shown inside yellow circle) on an electronic sizer leads to placement on the return loop of the packing line. (PhOtO By G. hOLmES)

Table 3.Averagedecaycontrolproductuseandapplicationmethods,averagedover22Louisianaand21NorthCarolinapackinglines.

Percent of Packing houses

Louisiana North Carolina

DECAy CONtROL PRODUCt USED

Dicloran (Botran) 77-91* 81

Other (peracetic acid, bacteria-based biological control)

5-18* 9.5

None 5-9* 9.5

APPLICAtION mEthOD

Spray 50 32.5

Curtain 0 4.5

Dip 45 42

In wax 0 21

* Several packing lines used different products one time to another.

andbruisingcanbesignificant.Furthermore,asignificant

portionofrootsmaypassthroughareturnloopmorethan

once.Manyhigh-volumepackersemploybothanexpanding

pitchsizerandanelectronicsizertosizerootsefficientlyinto

thecorrectgrade.

Impactsdependontheheightofthedrop,whether

thereisaninclineontothesizer,andwhetherthereispad-

ding.Impactmeasurementswerequitelowonlineswhere

thesweetpotatoesrolleddownagraduallypaddedincline

ontothesizer,andwerehigheronafewlineswherethere

wasahighdropdirectlyontosizerbars.

Boxfill.Boxes(40and14lb)canbefilledbyhandin

smalloperations(Figure30).Mid-sizeandlargepacking

linesgenerallyfillboxesbyallowingtherootstofalloffa

conveyorintotheboxorbyusinganautomatedboxfill

system(Figure31).Automatedsystemsaremainlyusedwith

electronicsizers.Thesedevicesareabletocontrolthespeed

ofrootsenteringthebox,mayhaveahydraulictiltmecha-

nismthatautomaticallylowerstheboxasitfills,andwill

releaseboxeswhentheyhavefilledtothedesignatedweight.

Mostboxfillsareasimilarheight,soanyvariationin

impactmeasurement(Table4)isrelatedtohowfullthebox

wasatthetimeofmeasurement.Rootsdroppingintoan

emptyboxwillexperiencelargerimpactsthanthosedrop-

pingintoanalmostfullbox.Inthesurvey,higherimpacts

werefoundwithautomaticboxfillers,whichmaybedueto

thegreaterheightofthedroporthespeedoftheconveyor

fillingtheboxes.Impactsfromspecializedpacking(plasticor

netbaggingmachines)werealsomeasuredandcanbehigh

iflarge-distancedropsarepresent.

19

Figure 31. Automatic box fillers are integrated into electronic sizing equipment. (PhOtO By G. hOLmES)

Figure 30. Box fill can occur off the end of a conveyor. (PhOtO By t. SmIth)

Reducing Packing Damage

Mechanicaldamagetosweetpotatoesduringhandling

mayincludecuts,abrasions,andbruises,dependingonthe

physicsandconfigurationofthesurfacesinvolved.Asurvey

of46LouisianaandNorthCarolinasweetpotatopacking

linesfrom2004to2006providedvaluableinsightintoways

tolimitdamageduringpacking.Thisstudyconcludedthat

significantdifferencesexistamongvariouspackingline

configurationsandevenwithinthesamelineoperatedat

differentcapacitiesandspeeds.

Othersignificantfindingsindicatethatmostdam-

agetorootsoccursfromimpactsbetweenrootsandthe

varioussurfacesofthepackingline.Thissuggeststhat

cushioningthesesurfacesorotherwisereducingimpacts

maysignificantlyreducemechanicaldamagetotheroots.

Common“makedo”paddingmaterials,suchascarpet

andupholsteryfoam,performpoorlywhencomparedto

speciallyengineeredpaddingmaterialsbutare,nevertheless,

muchbetterthanbaremetalsurfaces.Useagood-quality

cushioningmaterialonallimpactsurfaces.Thebestmaterial

forcushioningisonethatabsorbstheenergyfromthefalling

root,soobtainsamplesfromthemanufacturerandtest

energyabsorbencybydroppingarootontothesamples.A

materialthathaslessbounceisabettermaterialforpadding

sweetpotatopackinglinesbecausetherootswon’trebound

andhitanotherpackinglinesurface.Theidealcushioning

materialshouldalsohaveatoughsurfacethatresistswear

anddoesn’tabsorbwateranddirt.Paddingmaterialshould

beeasytowashduringpackinglinecleanings.Carpet

materialsfailinthisregardbecausetheyabsorbwaterand

soilandaredifficulttoclean.

Keeppackinglinesaslevelasispractical.Packinglines

thatcontinuallyraiseandlowertherootsimpartpotential

energythatresultsinmechanicaldamage.Forexample,

whensweetpotatoesareelevatedbyabeltedconveyoror

othermeans,potentialenergyisimparted,whichispropor-

tionaltotheheightraised.Whensweetpotatoesareallowed

tofallorrollbacktothelowerlevel,thepotentialenergy

ischangedintokineticenergy(motion),whichmustbe

absorbedbytherootsorsomeimpactsurface.

20

Figure 32. Install velocity-reducing flaps to slow root speed over packingline drops. (PhOtO By G. hOLmES)

Whensweetpotatoesmustbeloweredfromonelevelto

another,dosogentlybyusinggenerousquantitiesofenergy-

absorbingblankets,strips(Figure32),orpaddedsurfaces.

Long,inclinedsurfaceswillreducevelocitybetterthannear-

verticalfalls.Conveyorbeltshaveminimalenergy-absorbing

ability.Whensweetpotatoesareallowedtodropontoabelt

supportedbysheetmetalorrollers(forexample,under

asizer),thelevelofbruisingisnearlythesameasifthebelt

werenotthere.Ifpossible,removethesupportstoallow

thebelttobesuspended,providinganenergy-absorbing

impactsurface.

Synchronizepackingandgradinglinecomponentsto

preventabruptchangesinvelocityordirectionoftheproduce.

Carefullyengineercrossconveyorstoincludecurvedand

paddedtransitionsthatallowagradualchangeindirection

andvelocity.Operatepackinglinesnofasterthannecessary

toreducerootdamageandwearonthecomponents.

Recommendationstoreducedamageonpackinglines

• Dumprootsslowlyintowater(notontoroots)inthe

dumptank.

• Usehigh-qualitypaddingonallimpactsurfaces.

• Uselonginclinestoreducedropheightsbetween

components(Figure33AthroughC).

• Reducethenumberofdropsandturns(Figure34).

• Reducetheoveralllengthofthepackingline.

• Removebeltsupports(iffeasible)toreduceimpact.

• Usedecelerationflapsandblanketstoreducethe

speedoverdrops.

• Instructworkerstohandlerootswithcare,andmoni-

torhandlingfrequently.

• Avoidabruptchangesindirectionandspeedofbelts.

Addpaddingifturnsareunavoidable.

• Reducepackinglinespeed.

21

Figure 33A-C. Avoid very steep drops onto unpadded hard surfaces. (PhOtOS By G. hOLmES)

Figure 34. Packing line turn without padding. Note worn paint where roots make contact with metal guides. (PhOtO By B. EDmUNDS)

a b c

Packing Line Cleaning and Sanitation

Nomatterhowcarefultheoperation,decay-producing

organismsenterthepackinghousewiththesweetpotatoes

andwillquicklycontaminateallworkingsurfaces.These

organismscanremainviableformanymonthsonstorage

bins,tankwalls,sortingbelts,rollers,andbrushes.Itispos-

sibleforsurfacestoremaincontaminatedfromonepacking

sessiontothenext,orinthecaseofstoragebins,fromone

yeartothenext.

Somepathogensthatcausesignificantpostharvest

diseasesofsweetpotato,suchasRhizopus,arecommon

everywhere,anditisnotpossibletoeliminatethemcom-

pletely.However,theamountofcontaminationisoften

importantindeterminingwhetherdiseasedevelops.Thus,

reducingtheamountofcontaminationisthegeneralaimof

cleaningandsanitationefforts.

Reducingtheintroductionofpathogensontothe

packingline.Onmostsweetpotatopackinglines,rootsare

dumpedintoatankofwaterandthenfedontotherestofthe

line.Thisprocessoftenspreadsdisease-causingmicroorgan-

isms.Thus,reducingtheamountofdiseasethatdevelops

instoragewillreducethenumberofdiseasedrootsthat

enterthedumptankandcontaminatethepackingline.The

followingareafewimportantconsiderationsforreducing

diseasedevelopmentinstorage:

22

Table 4. Impactmeasurementsassociatedwithcommondropsonsweetpotatopackinglines,averagedover23Louisianaand12NorthCarolinapackinglines.*

*Average of five passes with the instrumented impact recording device**EPR=expanding pitch rollers

Components/Transfer Point AverageCumulative Impact (g) Maximum Minimum Number

Surveyed

DUmP

By hand 6.8 7.1 5.1 1

Onto bars (dry dump) 13.0 17.3 8.7 2

Into tank w/ metal lid 13.0 22.9 5.4 3

Directly into tank 12.8 36.1 0 29

mISCELLANEOUS

Onto eliminator (metal bars) 8.8 30.7 0 27

Onto grading line (PvC bars) 8.3 29.2 0 40

Onto brushes 8.2 35.9 0 58

Onto conveyors (plate supported) 9.6 35.4 0 34

SIZERS

Onto EPR** sizer 17.7 30.6 4.5 29

Out of EPR sizer (onto conveyor) 14.1 34.2 4.6 29

Onto drop-roller sizer 19.2 25.7 10.1 1

Out of drop-roller sizer (onto conveyor) 5.8 13.3 0 1

Electronic sizer 21.6 53.8 4.6 5

BOX FILL

By hand or falling off conveyor 12.4 39.5 0 30

Automated box filler 30.3 72.5 4.2 6

• Extremefieldconditionsatharvest:Extremeenvi-

ronmentalconditionsatharvestcanincreasedisease

developmentinstorage.Verydrysoilincreasesskinningof

theroots,aprobleminitself,andcanalsofavorFusarium

rootrot.Theotherextreme,floodedsoils,cancontribute

tothecomplexconditionofsouringthatleadstosimul-

taneousincreasesinmanydiseases,includingRhizopus

softrot,bacterialsoftrot,Fusariumrootrot,andsourrot.

Floodingalsoresultsinmudadheringtothesweetpota-

toesandbeingcarriedintostorage.Anythingthatcanbe

donetoavoidextremelydryorwetconditionsatharvest

willreduceproblemsinstorage.

• Sanitationofstoragebins:Binsthatcanholdinexcessof

20bushelsofsweetpotatoesarelikelytohaveatleastafew

rootsthatdevelopdiseaseduringalongstorageperiod.

Assumethattheyareallcontaminatedafteruse.Cleaning

andsanitizingthebinsaftereachuseshouldbearegular

practice.SpottsandCervantes(1994)studieddisinfesta-

tionofwoodandplasticsurfacesusedinbinsforstoring

pears.Theyfoundthatsteamwasthemosteffectivein

reducingpopulationsoffungithatinfectpears.Chlorine

compounds,sodiumorthophenylphenate(SOPP),and

quaternaryammoniumcompoundswerealsoeffective

butwerelessconsistentthansteam.Mostoftheseagents

weresimilarlyeffectiveonwoodandplasticexceptsodium

hypochlorite,whichwaslesseffectiveonwood.Leaving

thebinsoutsideinthesummer,wherethesurfacesthat

contactsweetpotatoesareexposedtodirectsunlight,may

alsohelpreducemicrobialcontamination.

• Reducingdiseasedevelopmentinstorage:Following

therecommendationsforpropercuringandstorageof

rootsremainsoneofthemosteffectivewaysofreducing

diseasesinstorage.Seethe“CuringforQuality”and

“StoringforQuality”sectionsforspecificguidelines.

AsinglerootaffectedwithRhizopussoftrotcan

producemillionsoffungalspores.Likewise,asingleroot

affectedwithbacterialsoftrotcanreleasemillionsof

bacterialcells.SincebothRhizopussoftrotandbacterialsoft

rotcancompletelyrotasweetpotatoinafewdays,itmakes

sensetoremoveallrootsthatfalloffthepackinglinefrom

thepackingareaattheendofeachday.Otherwise,they

willquicklybecomeasourceofinoculumthatcaninfect

healthyroots.

CleaningandSanitation.Nomatterhowrigorousyour

disease-controleffortsare,somediseasedrootswillgointo

thepackingoperation,andtherewillbeaneedtocleanand

23

Table 5. Percentofpackinglinesthatuseatechniqueorhaveeachtypeofcomponent,basedon25Louisianaand21NorthCarolinapackinglines.

Percent of Packing houses

Louisiana North Carolina

DUmP

By hand 0 9.5

Onto bars then into dump tank 0-8* 9.5

Directly into dump tank 84-92* 81

Onto rollers (dry dump) 8 0

mISCELLANEOUS COmPONENtS

Eliminator 56 75

waxer 0 57

Drying fans 0 43

SIZERS

No sizing equipment 12 9.5

1-2 EPR** sizers only 84 62

Electronic sizer only 0 19

Drop-roller sizer only 4 0

Both EPR sizer and electronic sizer 0 9.5

BOX FILLERS

Automatic box fill equipment 0 28.5

Box fill from off end of conveyor 96-100*** 62

Box fill by hand picking 0-4*** 9.5

* two packing lines have movable sets of rollers that are used only when roots are muddy; otherwise, roots are dumped directly into the dump tank.**EPR=Expanding pitch rollers*** One line hand picks sometimes and lets them fall from end of conveyor other times.

cleaning vs. saniTaTioncleaning: the removal of debris from packing line surfaces using physical methods such as high-pressure water sprays or a detergent.

saniTaTion: the reduction of the number of microorganisms on packing line surfaces to levels that do not cause disease problems.

sanitizethepackingline.Regardlessofwhatsurfaceisbeing

sanitizedorwhatagentisusedforsanitizing,itwillbemore

effectiveifthesurfaceiscleanedfirst.Somemicroorganisms

formbiofilms(amicroscopiclayerofbacteria)thatare

particularlydifficulttoremove.Removalofbiofilmsrequires

detergentandpressure,eitherfromscrubbingorfroma

pressurewasher.Inordertoproperlysanitizesurfaces,the

followingsequenceshouldbefollowed:

Nonporousandexposedsurfacesmayberelativelyeasy

tocleanandsanitize,butothercomponentsofthepacking

linemayrequirespecialattention.Ofparticularconcernare

thebrushbeds.Smallbitsofdebrisandtissuebrokenfrom

sweetpotatorootsmaybecomelodgedatthebaseofthe

brushbristlesandsupportthegrowthofmicroorganisms.

High-pressurehosesorapressurespraywillflushdebrisout

ofthebrushes.

Therearenouniformstandardsinthesweetpotato

industryforsanitizingpackinglines,andtheeffectofsanita-

tiononpostpackingdiseasedevelopmentisnotknown.

Somepackershavereliedonchlorine,primarilysodiumor

calciumhypochlorite,withoutunderstandingthefactors

thatinfluenceitsefficacy.Inwater,themostcommonly

usedformsofchlorinethatareusedforsanitizing(sodium

hypochlorite,calciumhypochlorite,orchlorinegas)allform

hypochlorousacid.Hypochlorousacidisthecomponent

thatkillsmicroorganismsandisreferredtoasavailableor

activechlorine.AthighpH,hypochlorousaciddissociatesto

formahypochloriteion,whichisnoteffectiveasasanitizer,

andchlorinesolutionswithapHabove8arenoteffectivein

killingpathogens.BelowpH6,chlorinesolutionsarehighly

corrosive,andtheactivityisrapidlylostduetovolatilityof

elementalchlorine.Thevolatilechlorinecanalsoirritate

workers.Thus,inordertohaveaneffectivesanitizingsolu-

tion,itisnecessarytomeasurebothchlorinecontentandpH

andmaintainthepHbetween6.5and7.5.

Organicmatterinactivateshypochlorousacidand

reducestheamountofavailablechlorine.Eventhoughthe

chlorinecombinedwiththeorganicmatterisinactivated,

itisstillmeasuredby“totalchlorine”testkits.Thismakesit

difficulttomaintainaneffectivechlorinesolutioninadump

tankwherelargevolumesofsoilandorganicmaterialenter.

Chlorinemustbeaddedfrequentlytoreplacethechlorine

lost.Forproduceharvestedaboveground,checkingand

replenishingthechlorinesupplyatleastonceanhouris

oftenrecommended.Butarootcropintroducesmuchmore

soilintothesystem,sothechlorinemustbereplenished

morefrequently.Generally,chlorineshouldbemaintained

at100to150partspermillion(ppm)activechlorinewith

pHmaintainedbetween6.5and7.5.Thus,chlorinetestkits

thatmeasureactiveoravailablechlorineshouldbeused.

Youmayneedtodilutethesourceproductindistilledwater

forproperreadings.Commercialsystemsthatautomatically

24

Desired ppm of Free Chlorine Pints of 5.25% NaOCl Solution per 100 Gal. of Water

Pints of 12.75% NaOCl Solution per 100 Gal. of Water

Ounces of 65% Ca (OCl)2 per 100 Gal. of Water

50 0.8 0.3 1.0

75 1.1 0.5 1.5

100 1.5 0.6 2.1

125 1.9 0.8 2.6

150 2.3 0.9 3.1

175 2.7 1.1 3.6

200 3.0 1.3 4.1

Table 6.Guidelinesformixingchlorinesolutions*

rinse clean rinse sanitize

*From “Chlorine Use In Produce Packing Lines” Document hS-761 Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, and used with the permission of the authors.

monitorandadjustpHandchlorinearelikelytobemore

effectiveinmaintainingeffectivesanitizingconditions.

Alwaysfollowthelabeldirectionsforanyproductused.

Watertemperatureandwaterqualitycanalsoaffect

theefficacyofchlorine.Forexample,chlorinesolutionsare

moreeffectiveathighertemperatures,butthechlorineis

lostfaster.Registeredformulationsofchlorinatorsforuseon

producecontaineither5.25percentor12.75percentsodium

hypochlorite(NaOCl),or65percentcalciumhypochlorite

[Ca(OCl)2].Toestimatetheamountoftheseformulations

touseinmakingchlorinesolutionswithvariousconcentra-

tionsoffreechlorine,useTable6.Currentresearchdoes

notsuggestthatchlorinecanbeusedfordiseasecontrol.It

shouldonlybeconsideredasanitizerofwaterandequip-

ment.Also,checkcurrentguidelines,assomeproductsmay

notbeacceptablebycertainmarkets.

Oneadditionalproblemwiththesechlorineproducts

isthattheyproducetrihalomethaneswhentheyreactwith

organicmatter.Thesecompoundsarecarcinogensandpose

apotentialhealthrisk.Chlorinedioxidegascircumvents

thisproblembecauseitdoesnotreactwithorganicmatter

toformtrihalomethanes.Chlorinedioxideisalsoactiveover

awiderrangeofconditionsthanotherformsofchlorine

andisaneffectivesanitizer.However,itsuseisnotwithout

problems,asthegasmustbegeneratedonsite.Whenused

indoors,adequateventilationisessentialbecausesomeof

thegascancomeoutofsolution.

Asindicatedearlier,therehasbeenlittleresearchon

sanitizingsweetpotatopackinglines.However,several

classesofchemicalsanitizingagentshavebeenconsidered

forotherperishableproduce.Therearemanyfactorsto

considerwhenchoosingasanitizer,inadditiontowhether

itwilleffectivelyreducethenumberofpathogens.Many

sweetpotatopackershaveexperiencewiththecorrosive

natureofchlorine,andseveralhavehadtorebuildtheir

packinglinesbecauseofthisproblem.Table7comparesthe

propertiesofsomeoftheclassesofchemicalsanitizers.

Product Safety and Certification Programs

At the time of publication, no cases of foodborne illness

associated with consumption of unprocessed sweetpotatoes

have been reported in the U.S. In our rapidly changing

environment, with increasing concerns for food safety, stan-

dards in addition to those listed may be required by different

certifying organizations or markets.

Foodsafetyhasalwaysbeenaconcernandanimpor-

tantissuerelatedtobothdomesticandinternationalfood

supplies.Inthe1990s,throughtheU.S.ProduceSafety

Initiative,thefocusincreasedtoensurethesafetyofthe

nation’sfoodsupply,whethergrowndomesticallyorinterna-

tionally.Asaresult,theU.S.FoodandDrugAdministration

(FDA)developedguidelinesfortheproduceindustrywith

anoverallgoalofminimizingmicrobialfoodsafetyhazards.

Thoseguidelines,knownasGoodAgriculturalPractices

(GAP)andGoodManufacturingPractices(GMP),have

becomeincreasinglyimportantintheproduceindustry.

GrowersuseGAPandGMPextensivelytoimproveand

ensurethesafetyoftheirproduce.Growersalsoarereduc-

ingriskstofreshfruitsandvegetableswithcomprehensive

educationandtraningprogramsonfarms.GAPguidelines

areprimarilyassociatedwithproductionfieldpractices,

whereasGMPguidelinesareaimedatstorageandpacking

facilities.

Asafirststepinthecertificationprocess,allsweetpo-

tatoproducersshouldregisterwiththeFDA,afterwhichthey

willbeissuedafederalidentificationnumber.Producerscan

accesstheFDA‘sGAPandGMPguidelinesonlineathttp://

www.cfsan.fda.gov/~dms/prodguid.html.

ProducerswhousetheFDAguidelinesarenotonly

protectingthehealthofconsumers,butarealsoreducing

theirfinancialrisk.Participatingproducersareaudited,

25

Generalrecommendationsforpackinghousesanitation:Freshsanitizingsolutionsshouldbepreparedanduseddaily.Forsanitizingwashwater,itisrecommendedthatactive(nottotal)chlorinelevelsbekeptbetween100to150ppm,andthewaterpHshouldbekeptbetween6.5to7.5.Thewashwatershouldbereplacedasoftenaspossibleduringtheday,oratleastwhenitbecomesobviouslydirty.Thefrequencythatwashwaterisreplaceddependsonthesoilload,packinglineconfiguration,andregulationsregardingdisposal,anditisspecifictoeachpackingline.Storagebinsandpackinglinecomponentsshouldbecleanedandsanitizedaftereachuse,andsweetpotatorootsanddebrisshouldberemovedfromthepackinghousefloordaily.

inspected,andvalidatedbyanindependentthirdparty.

Thesecompaniesprovideachecklisttogrowersthatthey

willusetocompletetheauditingandinspectionprocesses

associatedwithcertification.

Producers,packers,andshippersmustcomplywith

70percentoftheguidelinestopasscertification.Certain

conditionsaremandatoryandcanresultincertification

failureiftheyaredetected.Examplesincludethedetermina-

tionofanimmediatefoodsafetyrisk,aviolationoftheU.S.

EnvironmentalProtectionAgency(EPA)orstatepesticide

regulations,andfailuretotestortreatwatersources.Two

criticalcomponentsofGAPandGMPinvolveeducating

employeesandmaintainingfacilities.Producersshould

stresspersonalhygieneandensurethatallemployeesare

providedacopyofsafetyguidelines.Employeeswork-

inginapackingfacilityshouldnotwearjewelryorbody

adornments,andtheyshouldalsowearhairnets.Itisalso

extremelyimportantthatsanitaryfacilitiesbeprovided

toemployeestopromotepersonalhygieneandsanitary

activity,suchasfrequenthandwashing.Communication

andcooperationbetweenaproducerandhisorherproduc-

tionandpackingemployeesisessentialforGAPandGMP

guidelinecompliance.

26

Chlorine Iodophors Quaternary Ammonium Compounds

Acid Anionic Fatty Acid Peroxyacetic Acid

Corrosiveness yes slightly no slightly slightly slightly

Irritating to skin yes no no slightly slightly no

Effective at neutral pH yes depends on type in most cases no no yes

Effective at acid pH yes, but unstable yes in some cases yes, below 3.0 to 3.5

yes, below 3.5 to 4.0

yes

Effective at alkaline pHyes, but less than at neutral ph

no in most cases no no less effective

Affected by organic material yes moderately moderately moderately partially partially

Affected by water hardness no slightly yes slightly slightly slightly

Residual antimicrobial activity none moderate yes yes yes none

Cost low high moderate moderate moderate moderate

Incompatibilitiesacid solutions, phenols, amines

highly alkaline detergents

anionic wetting agents, soaps, and acids

cationic surfac-tants and alkaline detergents

cationic surfac-tants and alkaline detergents

reducing agents, metal ions, strong alkalies

Stability of use solution dissipates rapidly dissipates slowly stable stable stable dissipates slowly

Maximum level permitted by FDA without rinse 200 ppm 25 ppm 200 ppm varied varied 100 to 200 ppm

Water temperature sensitivity none high moderate moderate moderate none

Foam level none low moderate low to moderate low none

Phosphate none high none high moderate none

Soil load tolerance none low high low low low

*Comparisons made at approved ”no rinse” use levels. Adapted from B.R. Cords and G.R. Dychdala (1993) and R.H. Schmidt (2003), and used with the permission of the authors.

Table 7.Comparisonofthechemicalandphysicalpropertiesincommonlyusedsanitizers.*

27

Thereisincreasinginterestinexportingsweetpotatoes

producedintheU.S.toothercountries,especiallytoEurope

andtheUnitedKingdom.Atthesametime,therehavebeen

rapidchangesinlaws,regulations,andattitudesofcon-

sumersandalllevelsoftheproduce-marketingindustries

regardingproducequalityandpesticideresidues.GlobalGAP

isathird-partycertificationprogramwithguidelinesand

standardsthatareusedwidelybyfruitandvegetableproduc-

ersandshippersinEuropeandaroundtheworldforthe

voluntarycertificationofgoodagriculturalpractices.Specific

informationcanbeobtainedfromtheGlobalGapWebsite

(http://www.globalgap.org/).

Packaging and Shipping for Quality

Packaging for QualiTy: Properpackagingisanimportantstepinthejourney

frompackertoconsumer.Packingandpackagingmaterials

constituteasignificantcosttothesweetpotatoindustry;

therefore,itisimportantthatpackersandshippershave

aclearunderstandingofthedifferentpackagingoptions

andtheirlimitations.Asignificantpercentageofproduce

buyerandconsumercomplaintsmaybetracedtoinferior

containerdesignorinappropriatecontainerselectionand

use.Aproperlydesignedsweetpotatocontainerwillcontain,

protect,andidentifythesweetpotatoes,satisfyingeveryone

fromgrowertoconsumer.

Thecontainermustenclosethesweetpotatoesincon-

venientunitsforhandlinganddistribution.Sweetpotatoes

arenowmarketedinavarietyofpackages,dependingonthe

requirementsofthemarket(Table8,Figures35AthroughF).

Althoughthemostcommonshippingsizeisthe40-pound

carton,inrecentyears,thetrendhasbeentowardsmaller

packagesdesignedtobecarriedhomebytheconsumer.

Allpackagingcontainersmustprotectthesweetpota-

toesfrommechanicaldamageandenvironmentalcondi-

tionsduringhandlinganddistribution.Torn,dented,or

collapsedpackagesusuallyindicatelackofcareinhandling.

Sweetpotatocontainersmustbesturdytoresistdamage

duringpackaging,storage,andtransportationtomarket.

Becausealmostallsweetpotatopackagesarepalletized,the

containermusthavesufficientstackingstrengthtoresist

crushinginahigh-humidityenvironment.Althoughthecost

ofpackagingmaterialshasescalatedinrecentyears,inferior

quality,lightweightcontainersthatareeasilydamagedby

handlingormoisturearenolongertoleratedbypackersor

buyers.

Thepackagemustalsoidentifyandprovideuseful

informationaboutthecontents.Itiscustomary(andmay

berequiredinsomecases)toprovideinformationsuchas

producename,brand,size,grade,cultivar,netweight,count,

fungicidetreatment,grower,shipper,andcountryoforigin.

Itisalsobecomingmorecommontoincludenutritional

information,recipes,andotherusefulinformationdirected

specificallyattheconsumer.

Thefederalgovernmentnowrequiresthatrecordsbe

keptonallsweetpotatoespacked.Currentlya“onestep

forward,onestepback”approachisbeingenforced.This

meansthatpackersmustkeeptrackofthesourceofthe

rootsaswellastheshippingdestination.Traceabilityis

theabilitytofollowapieceofproducefromthegrower

throughconsumption.Traceabilityofindividualcontainers

andpalletsofproduceingeneralisasystemundergoing

improvements.Currentguidelinesontheimplementationof

produceidentificationandtraceabilitycanbefoundonthe

ProduceMarketingAssociation’sWebsite(www.pma.com).

shiPPing for QualiTy: Itisestimatedthatasmuchas5percentofpacked

sweetpotatoesarelostannuallyduringtransportationto

market.Muchofthelossisadirectresultofmishandling

duringshipment.Toreducelosses,shippers,truckers,

andreceiversshouldbewellacquaintedwiththespecific

handlingrequirementsofsweetpotatoes.

Packagedandpalletizedsweetpotatoesawaitingship-

mentshouldberefrigeratedat55°F(13°C)and85percent

relativehumidityimmediatelyafterpacking.Thisstorage

areashouldbeseparatefromtheareawhereunwashedroots

arestoredand,ideally,neartheloadingdock.

Exercisecarewhenstoringorshippingsweetpotatoes

withothercommodities.Packedsweetpotatoesusually

areshippedtodomesticandCanadianmarketsintrac-

tor-trailers,whichmaybeloadedwithothercommodities.

Thetwomainconcernswhencross-loadingarecompat-

ibilitybetweenethylenesensitivitiesandrequiredstorage

temperatures.Ethyleneisanaturallyoccurring,odorless,

colorlessgasproducedbymanyfruitsandvegetables,butit

canalsobeproducedbyfaultyheatingunitsandcombus-

tionengines.Insweetpotato,ethylenedamageisdifficultto

diagnosebutcancauseinternaldarkeningandpithyareas.

Sweetpotatoesarelowemittersofethylenewhenstoredand

handledproperly,buttheymayproducehigherlevelsof

ethylenewhenwounded,infected,orsubjectedtochilling

injury.Sweetpotatoesareconsideredtobesensitivetothe

ethyleneproducedbyothercommoditiesandmachinery,

soavoidshippinginmixedloadswithethylene-producing

commodities.Mostethylene-producingcommoditiesare

shippedatmuchlowertemperaturesthansweetpotatoes;

however,avoidmixingloadswithbananas,mangoes,papa-

2�

Table 8.Typesofpackagingforsweetpotatoes

Corrugated Fiberboard the most common container material. Available in many different styles and weights. Relatively low in cost and easy to print with customized labels. most 40-pound boxes are one-piece, regular slotted containers (RSC) and two-piece, full telescoping containers (FtC). the RSC is the most popular because it is simple and economical. however, the RSC has relatively low stacking strength and therefore is most often used with produce (for example, sweetpotatoes) that can carry part of the stacking load. the FtC is used when greater stacking strength and resistance to bulging are required. Smaller size cartons (14 lb) are also used, especially for overseas markets. Almost all corrugated fiberboard containers are shipped to the packer flat and require hand or machine assembly onsite. (Figure 35A)

Plastic Bags A newer, low-cost material for consumer-sized packaging. Film bags are clear, allowing for easy inspection of the contents. they readily accept high-quality graphics and are available in a wide range of thicknesses, grades, and gas permeability. Decay is a risk if a low-oxygen and high-moisture environment develops within the bags, which is more likely to occur with extended storage time or if the bags are stacked. Specialized bagging equipment is required. (Figure 35B)

Shrink Wrap One of the newer trends in packaging is shrink-wrapping of individual roots, which can reduce moisture loss, reduce mechanical damage during shipping, and provide a good surface for stick-on labels. Roots can be shrink-wrapped in a foam tray of two or three. Some consumers prefer individually wrapped roots for microwave cooking. Only healthy roots should be shrink-wrapped, and film that allows for root respiration must be used. Otherwise, disease problems, particularly Fusarium root rots, can develop. (Figures 35C-D)

Net Bags Net bags bundle roots into convenient consumer-sized packages. Although they cannot offer the modified- atmosphere possibilities of plastic, they are preferred by many consumers, and good air movement limits the chance of disease development. Specialized bagging equipment is required. (Figure 35E)

Bulk Bins Large double- or triple-wall corrugated pallet bins are used as one-way pallet bins to ship in bulk form to processors and retailers. Container cost per pound of produce is as little as one-fourth that of 40-pound containers, and some bulk containers may be collapsed and reused. (Figure 35F)

Figure 35. Examples of packaging: corrugated fiberboard (A) (PhOtO By B. EDmUNDS); plastic bags (B) (PhOtO By G. hOLmES); tray shrink wrapping (C) (PhOtO By G. hOLmES); individual root shrink wrapping (D) (PhOtO By G. hOLmES); net bags (E) (PhOtO By G. hOLmES); bulk bins (F) (PhOtO By t. SmIth)

a b c

d e f

2929

yas,andothertropicalfruitsthatarealsoheldat55°F(13°C).

Alsomakecertainthattherefrigerationunitisfunctioning

properlyandthatnoengineexhaustisenteringthetrailer.

Compatibleshippingtemperatureisthemainconcern

withsweetpotatoes.Manycommoditiesareshippedat

temperaturesnearfreezing,whichcanseverelydamage

sweetpotatoes.Besuretoinformshippersofthetemperature

requirements(Figure36).Rootsmaybechilledifthetrailer

thermostatissettoolowtoaccommodateothercommodi-

tiesorduringlongtripsincoldweather.

Export-marketshippinginvolvesonsitepackingof

marinecontainerswithpalletsorbulkbins(Figure37).

ShippingtimebyboattoEuropeanmarketscanrange

from8to14days,plusthedistributiontimeoneither

endofthetrip.Marinecontainersforsweetpotatoesare

usuallynotmodified-atmosphereenvironments,butmost

containersdohavetemperaturecontrolalongwithventsto

provideairexchange.Researchonoptimalconditionsfor

overseasshippingofsweetpotatoesisminimal.Thebest

sourceofgeneralinformationisapublicationfromthe

UniversityofCalifornia,“MarineContainerTransportof

ChilledPerishableProduce”(Thompsonetal.,2000).This

publicationstatesthatsweetpotatoesareconsideredto

havelowrespirationduringshipment,sotherecommended

air-exchangerateis15feet3perminute(25meter3perhour)

fora40-footmarinecontainertomaintainpropercarbon

dioxidelevels.Containertemperatureshouldbemaintained

at55°F(13°C).Allcontainershaveventsthatareleftopen

ataspecifiedsettingduringtransporttoensurefreshair

exchange.However,thismakescontrollingrelativehumidity

difficult.Ventsettingsvaryamongcontainermanufacturers,

soitisimportanttosetventsbyairflowratesinfeet3per

minuteormeter3perhour,because“percentopening”rec-

ommendationsmaynotbereproduciblebetweencontainer

manufacturers.Packersshouldinstalltemperature-recording

devicestoverifythatpropertemperaturesweremaintained

duringshipping.

markeT life of Packed sweeTPoTaToes: Marketlifebeginswhenrootsareremovedfrombulk

storagebins.Marketlifeincludeswashing,packing,and

distributiontomarket,anditconcludesatthepointof

consumerpurchase.Temperatureandrelativehumidity

oftenvarygreatlyatdifferentstepsastherootsmovefrom

storagetoconsumer.Marketlifeismaximizedwhenroots

arehandledgentlyandtemperatureandrelativehumidity

aremaintainedat55°F(13°C)and85percent,respectively.

Undertheseconditions,marketlifeistypicallytwotothree

weeks.(Marketlifeshouldnotbeconfusedwithstoragelife,

whichcanbeupto13monthsunderproperconditions.)

Manyfactorsinfluencethemarketlifeofsweetpota-

toes.Thecultivar,pre-harvestgrowingconditions,curing

conditions(orlackthereof),storagetemperature,relative

humidity,atmosphericoxygen/carbondioxidecomposition,

amountofmechanicalinjuryduringpacking,washwater

sanitation,typeofpackaging,andairflowandtemperature

duringtransportanddistributiontomarketareamongthe

mostimportantfactorsinfluencingmarketlife.

Weightlossduetomechanicalinjuriesreceivedduring

packingisamajorcauseofqualityreductionintheretail

market.Weightlossresultsinshriveledorpithyroots.Itcan

behigh,particularlyduringtransitorinsupermarketdisplay

areaswhererelativehumidityisgenerallylow.Forexample,

recentresearchwithcultivarBeauregardshowedweight

lossofwashedrootsintherangeof5to10percentaftera

four-weekperiodundersimulatedretailmarketconditions

(about70°Fand70percentrelativehumidity).Consumers

prefersweetpotatoesthathavelostlessthan5percent

duringtransitandretailmarketing,andthatrequiresproper

temperatureandhumiditycontrol.

Itisnotpossibletomakeanall-inclusivestatementthat

definesthemarketlifeofanycommodity.Marketlifecan

varygreatlyfromonelotofproducttoanotherfornumerous

diversecauses.Ultimately,themarketsubjectivelydeter-

minesthepointatwhichsweetpotatoqualitybecomesunfit

forconsumption.

Figure 36. temperature and air exchange controls for shipping container. (PhOtO By G. hOLmES)

30

Key References

Chemicalcontrol

Louisiana Insect Pest Management Guide.Availableonline:

www.lsuagcenter.com/en/communications/publications/Publications+Catalog/Crops+and+Livestock/Insect+and+

Disease+Control/management_guides/Insect+Pest+Management+Guide.htm

Louisiana Plant Disease Management Guide.Availableonline:

http://www.lsuagcenter.com/en/crops_livestock/crops/Integrated_Pest_Management/Publications/Plant+Disease+

Control+Guide.htm

North Carolina Agricultural Chemicals Manual.Availableonline:

http://ipm.ncsu.edu/agchem/agchem.html

Vegetable Crop Handbook for the Southeastern U.S.Availableonline:

http://www.sripmc.org/

Diseases,abioticinjuryandinsectdamage

Ahn,J.K.,W.W.Collins,andD.M.Pharr.1980.Influenceofpreharvesttemperatureandfloodingonsweetpotatorootsin

storage.HortScience15:261–263.

Chang,L.A.andS.J.Kays.1981.Effectoflowoxygenstorageonsweetpotatoroots.Journal of the American Society of

Horticultural Science106(4):481–483.

Clark,C.AandJ.W.Moyer.1988.Compendium of Sweet Potato Diseases.St.Paul:APSPress.

Corey,K.A.,W.W.Collins,andD.M.Pharr.1982.Effectofdurationofsoilsaturationonethanolconcentrationandstorageloss

ofsweetpotatoroots.Journal of the American Society of Horticultural Science107(2):195–198.

Kays,S.J.,A.S.Bhagsari,andD.H.Picha.1992.“Physiologyandchemistry.”InFifty Years of Cooperative Sweetpotato Research,

1939–1989,ed.A.JonesandJ.C.Bouwkamp.So.Coop.Ser.,Bull.No.369.

Kushman,L.J.andD.T.Pope.1972.Causes of Pithiness in Sweetpotatoes.TechBulletinNo.207.Raleigh,NC:NorthCarolina

AgriculturalExperimentStationandUSDA-ARS.

Picha,D.H.1987.Chillinginjury,respiration,andsugarchangesinsweetpotatoesstoredatlowtemperatures.Journal of the

American Society of Horticultural Science112(3):497-502.

Sanitation

Rangarajan,A.,E.A.Bihnr,R.B.Gravani,D.L.Scott,andM.P.Pritts.2000.Food safety begins on the farm: A grower’s guide.

Availableonline:www.gaps.cornell.edu/FSBFEng.html

Ritenour,M.A.,S.A.Sargent,andJ.A.Bartz.2002.Chlorine use in produce packing lines.DocumentHS-761,Horticultural

SciencesDepartment,FloridaCooperativeExtensionService,InstituteofFoodandAgriculturalSciences,University

ofFlorida.Availableonline:http://edis.ifas.ufl.edu/CH160

31

Schmidt,R.H.2003.Basic elements of equipment cleaning and sanitizing in food processing and handling operations.

DocumentFS14,FoodScienceandHumanNutritionDepartment,FloridaCooperativeExtensionService,Instituteof

FoodandAgriculturalSciences,UniversityofFlorida.Availableonline:edis.ifas.ufl.edu/FS077

Spotts,R.A.,andL.A.Cervantes.1994.Contaminationofharvestbinswithpeardecayfungiandevaluationofdisinfestantson

plasticandwoodbinmaterial.Acta Hort.367:419-425.

Suslow,T.1997.Postharvest Chlorination: Basic Properties and Key Points for Disinfection.UniversityofCalifornia,Divisionof

NaturalResources,Publication8003.Availableonline:anrcatalog.ucdavis.edu/pdf/8003.pdf

U.S.FoodandDrugAdministration.1998.Guide to minimize microbial food safety hazards for fruits and vegetables.Available

online:http://www.foodsafety.gov/~acrobat/prodguid.pdf

PackagingandTraceability

Anonymous.Fresh produce traceability: a guide to implementation.JointpublicationoftheProduceMarketingAssociationand

theCanadianProduceMarketingAssociation.October2006.Availableonline:www.pma.com

Shipping

Thompson,J.F.,Brecht,P.E.,Hinsch,T.andKader,A.A.2000.Marine container transport of chilled perishable produce.

UniversityofCalifornia,AgricultureandNaturalResources.Publication21595.

Figure 37. marine containers are utilized to transport bulk bins and pallets to overseas export markets. (PhOtO By G. hOLmES)

Appendix 1.

Guide to Common Postharvest Diseases, Abiotic Damage, and Insects

diseases ThaT develoP in sTorage Bacterialrootrot,causedbyDickeya didantii

(Pectobacterium (Erwinia) chrysanthemi),canoccasionally

causeheavylossesinstorageorafterpacking.Infection

resultsinmostlyinternal,verywet,softrotthatusu-

allyoccursduringwarmhumidconditions(Figure38).

Occasionally,bacterialsoftrothasdevelopedaftersweet-

potatoescomingfromthefieldwerewettedtohelpsetthe

skin,orwhenpalletswerewrappedtooheavilyinplastic

forstabilityduringshipping,deprivingthemofadequate

gasexchange.Bacterialsoftrotcanbeinternalandwithout

symptominroots,developingonlyunderthepropercondi-

tions.Thediseaseismanagedbyusingplantingstocksthat

arefreeofdisease,avoidingwoundingduringharvestand

handling,usingcleanwateronpackinglines,andpreventing

highcarbondioxide/lowoxygenenvironmentswithhigh

temperatures.

BlackrotinfectionscausedbythefungusCeratocystis

fimbriataoccurinthefield.However,thediseasecanbe

spreadduringharvestfromdiseasedtohealthyroots.The

presenceofevensmallblackrotlesionsinthecropduring

harvestmayleadtolargelossesinstorage(Figure39Aand

B).Ifasmallamountofblackrotispresentatharvest,the

lesionswillexpandinstorage.Blackrotwasoncethemost

importantpostharvestdiseaseofsweetpotato,butusing

disease-freeseedroots,cuttingslipsabovethesoilline,and

rotatingcropshavedramaticallyreduceditsoccurrence.

FusariumsurfacerotandFusariumrootrotdecay

causedbyFusariumiscommonlyseeninstorage.Thereare

twospeciesofFusariumthatcausetwodistinctsymptoms

onroots.Fusariumrootrot,causedbyFusarium solani,is

adecaythatextendsintothefleshoftheroot.Externally,

lesionsaredarktanandoftenhaveadistinctringpatternof

lightanddarkbrownbands(Figure40).Therotisfirm,dry,

anddarkbrown,withinternalcavitiesoftenfilledwithwhite

myceliumofthefungus(Figure41).Fusariumsurfacerot,

causedbyFusarium oxysporum,hassurfacelesionsthatdo

notextendintotheflesh.Theyaregenerallycircular,light

todarkbrown,firm,anddry.Bothdiseasescanbemanaged

byminimizinginjuriesduringharvestingandhandling,

harvestingwhensoilmoistureisoptimal,curingpromptly

afterharvest,andusingcultivarsresistanttoFusariumroot

rot.Fusariumsurfaceandrootrotsdevelopslowlyand,

therefore,arenotconsideredapost-packingdiseasebecause

thereisinsufficienttimeforthediseasestodevelopbetween

packingandconsumption.

Javablackrot,causedbythefungusLasiodiplodia

theobromae,isoccasionallyfoundduringstorage.Thisdis-

easeiseasilyidentifiedbyitsdistinctsymptoms.Symptoms

usuallybeginattherootendasafirm,moistdecaythat

turnscolorfrompaleyellow,tobrown,toblack(Figure42).

32

Figure 38. Bacterial root rot causes a soft, dark rot. (PhOtO By C. AvERRE)

Figure 39. Black rot caused by Ceratocystis fimbriata. (A and B) (PhOtOS By G. hOLmES)

a b

Asthediseaseprogresses,hardblackmassescalled“stroma”

breakthroughthesurfaceoftheroot(Figure43).Withtime,

thesemasseswillbreakdowntoreleasepowderyspores.The

fungusentersthroughawoundedareaandcanbecontrolled

withgoodpackinghousesanitationandwithpromptcuring

atthepropertemperatureandhumiditytohealwounds.Java

blackrotismorecommonintropicalareasoftheworld.

Rhizopussoftrot,causedbythefungusRhizopus

stolonifer,isthemostcommondiseaseofstoredandpacked

sweetpotatoes.Afterthefungusentersthetissuesthrough

wounds,itcancauseasoft,wetdecayoftheentireroot

withinthreedays.Rhizopussoftrotischaracterizedby

white,whiskeryfungalgrowthandprolificdustyblackspores

onthesurfaceofinfectedroots(Figure44).Wetsoilandlow

temperatureatharvestcausesweetpotatoestobeespecially

susceptible,andsymptomsappearsoonafterstorage.

Rhizopussoftrotismostlikelytooccurafterpacking,

becauseinfectionsoccurthroughwounds.

33

Figure 41. Fusarium root rot shows internal decay with characteristic cavities that may contain white fungal growth. (PhOtO By G. hOLmES)

Figure 40. Fusarium surface rot is characterized by sunken, scalloped-edge rings on the root surface. (PhOtO By G. hOLmES)

Figure 42. Internal symptoms of Java black rot include a clearly demarcated, firm, dark rot. (PhOtO By B. EDmUNDS)

Figure 43. External symptoms of Java black rot are hard black “stroma” protruding out of the sweetpotato skin. (PhOtO By G. hOLmES)

Figure 44. Rhizopus soft rot is characterized by a wet, soft rot and whiskery fungal growth covered with powdery black spores.(PhOtO By G. hOLmES)

Rhizopussoftrotcanbemanagedbyproperlycuring

rootstohealwoundsincurredduringharvestandbyavoid-

ingdamageonpackinglines.Mostpackersapplyfungicide

onthepackingline.Lesssusceptiblecultivarsareavailable;

however,noneshowcompleteresistance.Forspecific

fungiciderecommendations,refertothelatesteditionofthe

North Carolina Agricultural Chemicals Manual, Louisiana

Plant Disease Management Guide,orcontactyourlocal

Extensioncenterforguidelinesinyourstate.

diseases ThaT occur in The field and are also seen in sTorage and Packing Circularspotiscausedbythesoilbornefungus

Sclerotium rolfsii,whichalsocausessclerotialblightinplant

beds.Thecircular,brown,shallowspots(Figure45)develop

onrootsjustbeforeharvestbutdonotdevelopfurtherafter

harvestunlessthereisfreemoistureontherootsduring

curing.Usually,therootshealduringstorage,andthespots

begintopeelawayfromtheroot.

Geotrichumsourrotbeginsinthefieldwhenflooding

occursandcontinuestodevelopinstorage.Awet,softrot

developsthathasadistinctivefruity-alcoholodor.Tuftsof

whitemyceliadevelopontheoutsideoftheroot(Figure46).

Sourrotcanalsooccurpostharvestwhenrootsareexposed

tohightemperaturesandlowoxygenenvironments.

Mottlenecrosisiscausedbyatleasttwospeciesofthe

soilbornefungusPythium.Althoughitoccasionallycauses

significantlosses,itisnotconsideredacommondisease.

Externalsymptomsconsistofslightlysunken,brownspots

withirregularmargins.Whenaffectedrootsaresliced,a

marbleddecayisrevealed(Figure47).Avoidcool,wetcondi-

tionsnearharvest,andpracticecroprotation.

Soilrotorpoxiscausedbyasoilbornefilamentous

bacterium(Streptomyces ipomoea).Soilrotcausesdarkcorky

lesionsthatareusuallyindented(Figures48and49).The

typeofsymptomproduceddependslargelyonthetimingof

infectionwithrespecttorootdevelopment.Infectionsoccur-

ringpriortorootenlargementaremorelikelytoconstrict

34

Figure 45. Circular spot is caused by the fungus Sclerotium rolfsii. Lesions are unusually circular. and the center of the spot typically cracks. (PhOtO By G. hOLmES)

Figure 46. Geotrichum sour rot can occur in the field or in storage. It is favored by conditions of low oxygen and high temperature. the disease has a distinctly sour smell. (PhOtO By G. hOLmES)

Figure 47. mottle necrosis is caused by Pythium spp. and produces a distinctive marbled necrosis of the root pith. (PhOtO By G. hOLmES)

Figure 48. Constrictions can be caused by early season infection with soil rot/pox. (PhOtO By G. hOLmES)

anddisfiguretheroot,butdonotdevelopfurtherinstorage.

Growingresistantvarietiescangreatlyreducetheincidence

ofsoilrotorpox.LoweringsoilpHthroughapplicationsof

sulfurcanreducediseaseproblems.

Russetcrackisadiseasecausedbyastrainofthesweet

potatofeatherymottlevirus(SPFMV).Inthefield,brown

bandsdevelopontheskinandextendlaterallyaroundparts

oftheroot.Withinthesebrownbandsthereareusually

shallowcracksthatrunperpendiculartothebands(Figure

50).Thesymptomsdonotchangeinstorageandareoften

misseduntiltherootsarewashed.Russetcrackcanbe

managedbyagoodseedprogramthatutilizesvirus-tested,

Certified,orFoundationseed.

Scurf,causedbythefungusMonilochaetes infuscans,is

acommonsweetpotatodiseasethatistransmittedprimarily

byinfectedplantingstock,butitalsomaypersistinsoilfor1

to2years.Scurfisadiseaseoftheskin(Figure51).However,

rootsthatareheavilyinfectedwithscurfmayshrinkmore

rapidlythannormalroots.Researchhasshownthatscurf

doesnotspreadtootherrootsduringstorage.Small,

insignificant,infectedareasmayenlarge,especiallyinthe

presenceofhumidityhigherthan90percent.

Scurfcanbemanagedbyusingscurf-freeplanting

stock,treatingseedwithfungicide,avoidingproblemfields,

rotatinglandusedforplantbedseachyearand,mostimpor-

tantly,cuttingplantsabovethesoillineinsteadofpullingto

reducethespreadfromseedplants(Figure4).Scurf-infected

rootsshouldnotbeusedforseedordisposedofinafield

wheresweetpotatoeswillbeplantedlater.USDAStandards

(Appendix2)listscurfcoveringmorethan15percentofthe

rootsurfaceasadefect.

Slimemoldsoccasionallydeveloponthesurfaceof

sweetpotatoes.Littleisknownabouttheconditionsthat

favorslimemolddevelopmentonstorageroots.Although

theylooksuperficial(Figure52),itisdifficulttowashslime

moldoffsweetpotatoeswithoutpermanentlydamagingthe

skinoftheroot.

35

Figure 49. Soil rot symptoms also include circular, sunken, dark scabby lesions. (PhOtO By G. hOLmES)

Figure 50. Russet crack is caused by a virus; slight constrictions and parallel cracking are characteristic symptoms of the disease. (PhOtO By G. hOLmES)

Figure 51. Scurf only affects the root skin; it does not penetrate into the root flesh. USDA standards allow for 15 percent of the root surface to be covered with scurf. (PhOtO By G. hOLmES)

Figure 52. Slime mold damage to root skin. (PhOtO By C. CLARk)

abioTic damage ChillingInjuryisrare,butitcanoccurifrootsarekept

incommonstorageareasduringthewintermonths(or

duringtransitiftemperaturesaretoolow).Storagebelow

55°F(13°C)canresultinchillinginjurythatmaynotbe

evidentforseveralweeks.Therootsmayappearnormal,but

thefleshwillbespongyandwatery.Whenchilledrootsare

cooked,thecentralareaoftherootmayremainhard.Chilled

rootsareoftencolonizedsecondarilybyPenicilliumspecies

(Figures9through11).

GrowthCracksaregenerallycausedbyunevengrowing

conditions,usuallywhendroughtisfollowedbyheavyrain

(Figure53).Thesecracksoftenserveasinfectioncourtsfor

secondarypathogens.Crackingmayalsobecausedbyroot-

knotnematode(Figure54).Crackscausedbyrootgrowth

andthosecausedbynematodescanbedistinguishedby

thepresenceofnematodesnearthesurfaceoftheroot

(Figure55).

Mutations/Chimerasinfleshcolorarecommonin

sweetpotatoes(Figures56AandB).Sproutsemergingfrom

themutatedfleshcolorwillproducerootsofthesameflesh

color.AgoodseedprogramthatusesFoundationorCertified

seedthathasbeenselectedforfreedomfrommutationscan

minimizethisproblem.

36

Figure 54. Cracking and galling caused by root knot nematode infection on cultivar Beauregard. (PhOtO By G. hOLmES)

Figure 53. Cracking and galling caused by rapid expansion of roots during a period of high soil moisture on cultivar Beauregard. (PhOtO By G. hOLmES)

Pithinessusuallyisaresultofpoorstorageconditions

(suchasinadequatetemperatureorhumiditycontrol)andis

characterizedbywhitespongyfleshthatmaycontainspaces

orcavitiesandbyrootsthatweighmuchlessthannormal

(Figure10).

Skinningiscommonduringroughhandlingatharvest

andpacking(Figure5).Skinninginjuriescanbehealed

throughthecuringprocess,althoughtheoriginalappear-

ancewillneverberegained.

insecTs Inadditiontodiseases,potentialharmfrominsects

instoredsweetpotatoesisofsignificanteconomiccon-

cern.Amongtheinsectsfoundinsweetpotatostorage

facilitiesarefruitflies,soldierflies,cornearworm,beet

armyworm,fallarmyworm,southernarmyworm,andthe

sweetpotatoweevil.Specificrecommendationsforcontrol

ofinsectsusingchemicalsarefoundintheNorth Carolina

Agricultural Chemicals Manual andthe Louisiana Insect Pest

37

Figure 55. Nematode damage in sweetpotato root. Small dark spots with light centers reveal the locations of female nematode. (PhOtO By G. hOLmES)

Figure 56. Chimera in orange flesh cultivar (A) (PhOtO By C. CLARk) and purple flesh cultivar. (B) (PhOtO By G. hOLmES)

a b

Management Guide,oryoucancontactyourlocalExtension

centerforrecommendations.

Fruitflies(Drosophilaspp.)(Figure57),alsoknownas

vinegarflies,pomaceflies,sourflies,bananaflies,ordrunk-

ards,areverycommonandareusuallyconsiderednuisance

pests.Theyquicklybreedintotremendousnumbers,but

theydonotdirectlyattackhealthyroots.Theylaytheireggs

andreproduceindiseased,soured,ormechanicallydam-

agedsweetpotatoes.Theirpresenceingreatnumbersisa

sureindicationofasignificantdecayproblem.Conditions

thatfavorfruitfliesarehightemperatures,coupledwith

largequantitiesofsour,diseased,andmechanicallybruised

sweetpotatoes.Storingonlythebestquality,undamaged

rootsandmaintainingthatqualityaretheprimarymeans

ofavoidingfruitflies.Aninsecticidespraymaybejustified

whenextremelyhighpopulationsarepresent.

Soldierflies(Hermetia illucens)(Figure58)aremost

oftenseenintheformofscavenging,thick-skinned,

flattenedlarvae.Muchlikefruitflies,theyfavorwarm,wet

storageconditionswithdecayingroots.Otherinsectssuch

asearwormsandarmyworms,whichfeedonthefoliageand

exposedsweetpotatoesinthefield,areattimescarriedinto

storagefacilitiesduringharvestingoperations.There,they

continuetofeedandcauselargeholesinroots.Managing

3�

Figure 59. External damage caused by sweetpotato weevil feeding includes small puncture wounds. (PhOtO By J. mURRAy)

Figure 58. Soldier flies are an occasional pest of stored sweetpotatoes. (PhOtO By B. DREES)

Figure 57. Fruit flies are a common nuisance pest in storage. (PhOtO By m. wEStBy)

theseinsectsinthefieldpriortoharvestwillhelpavoid

introducingwormsintostorageareas.

Sweetpotatoweevil(Cylas formicarius,Fab.)(Figures

59through61)canbeaseriouspestofstoredsweetpotatoes

onceitisestablished.Althoughtheyarenotconsidereda

probleminthemajorityofsweetpotatocommercialproduc-

tionareasintheU.S.,weevilscanbecomeaseriouspestof

storedsweetpotatoesintheSouthifnotproperlymonitored

andmanaged.Regulatorymandatesdiffer,andgrowers

shouldbecomefamiliarwiththeirlocalregulations.

Preventionisthekeytosuccessfulweevilmanagement.

Avoidbringinginsweetpotatoesorcontainersfromareas

whereweevilsareknowntobeaseriousproblem.Always

insistonweevil-freerootswhenpurchasingseedorrootsfor

storage,andinspectshipmentsuponarrivaltoverifyplace

oforiginandpropertagging.Ifathreatofweevilsexists,sex

pheromonetraps(Figure62)thatarespecifictosweetpotato

weevilscanbeusedtomonitorplantbeds,storageareas,

andproductionfields.Ifsweetpotatoweevilsarediscovered

outsidethequarantinearea,promptlyalertregulatory

officials.Certaininsecticidesareavailableandlabeledfor

useinthefieldandinstoragefacilities.Contactyourlocal

Extensionagentformoreinformationonthepotentialrisk

ofandmanagementoptionsavailableforthesweetpotato

weevilinyourarea.

39

Figure 60. Internal damage caused by sweetpotato weevil feeding. (PhOtO By A. hAmmOND)

Figure 61. Adult sweetpotato weevil. (PhOtO By DIvISION OF PLANt INDUStRy ARChIvE, FLORIDA

DEPARtmENt OF AGRICULtURE AND CONSUmER SERvICES, BUGwOOD.ORG)

Figure 62. Pheromone traps for sweetpotato weevil are required in infested areas. (PhOtO By G. hOLmES)

Appendix 2.

U.S. Standards for Grades of Sweetpotatoes

SummarizedfromtheU.S. standards for grades of sweetpo-

tatoes.PublishedbytheUSDAAgricultureMarketingService

andeffectiveApril21,2005.Forcompleteinformationon

gradestandards,seehttp://www.ams.usda.gov/

gradesU.S.ExtraNo.1.

U.S.No.1.

U.S.No.1Petite.

U.S.Commercial.

U.S.No.2.

TolerancesU.S.ExtraNo.1,U.S.No.1andU.S.No.1Petitegrades.10

percentofthesweetpotatoesinanylotmayfailtomeetthe

requirementsofthesegrades,butnotmorethanone-halfof

thisamount,or5percent,shallbeallowedforsweetpotatoes

whichareseriouslydamaged,includingthereinnotmore

than2percentforsweetpotatoesaffectedbysoftrotorwet

breakdown.

U.S.Commercial.25percentofthesweetpotatoesinanylot

mayfailtomeetthe

requirementsofthisgrade,butnotmorethanone-fifthof

thisamount,or5percent,shallbeallowedforsweetpotatoes

whichareseriouslydamaged,includingthereinnotmore

than2percentforsweetpotatoesaffectedbysoftrotorwet

breakdown.

U.S.No.2.10percentofthesweetpotatoesinanylotmayfail

tomeettherequirementsofthisgrade,includingthereinnot

morethan2percentforsweetpotatoesaffectedbysoftrotor

wetbreakdown.

Off-size.10percentofthesweetpotatoesinanylotmayfail

tomeetanyspecifiedsize,butnotmorethanone-halfof

thisamount,or5percent,shallbeallowedforsweetpotatoes

whicharebelowtheminimumdiameterandminimum

lengthspecified.

definiTionsSimilarvarietalcharacteristics:sweetpotatoeshavethe

samecharacteroffleshandpracticallythesameskincolor.

Forexample,drytypeshallnotbemixedwithsemi-moistor

moisttype.

Firm:notmorethanslightlyflabbyorshriveled.

Smooth:thesweetpotatoisfreefromveiningorother

defectscausingroughnesswhichmorethanslightlydetract

fromtheappearanceoftheindividualsweetpotatoorthe

generalappearanceofthelot.

Fairlyclean:theindividualsweetpotatoisnotcakedwith

dirtandthatdirtorotherforeignmatterdoesnotmaterially

detractfromthegeneralappearanceofthelot.

Fairlywellshaped:thesweetpotatoesarenotsocurved,

crooked,constrictedorotherwisemisshapenastomaterially

detractfromtheappearanceoftheindividualsweetpotatoor

thegeneralappearanceofthelot.

Damage:anydefectwhichmateriallydetractsfromthe

appearance,ortheedibleorshippingqualityofthesweetpo-

tatoorwhichcannotberemovedwithoutalossofmorethan

5percentofthetotalweightofthesweetpotatoincluding

peelcoveringthedefectivearea.Specificdefects:

(a)Sproutsoverthree-fourthsinchinlength

(b)Growthcrackswhichdetractmateriallyfromappearance

(c)Scurfcoveringmorethan15percentofthesurface

(d)Pox(SoilRot)whendetractingfromappearance

(e)Wireworm,grassrootorsimilarinjurywhenanyholeina

sweetpotatoranginginsizefrom6to8ouncesismorethan

three-fourthsinchlongorwhentheaggregatelengthofall

holesismorethan1¼inches,orcorrespondinglyshorteror

longerholesinsmallerorlargersweetpotatoes.

Length:thedimensionofthesweetpotato,measuredina

straightlinebetweenpointsatorneareachendofthesweet-

potatowhereitisatleastthree-eighthsinchindiameter.

Diameter:thegreatestdimensionofthesweetpotato,

measuredatrightanglestothelongitudinalaxis.

Onetype:sweetpotatoeshavethesamecharacterofflesh,

anddonotshowanextremerangeinskincolor.Forexample,

drytypeshallnotbemixedwithsemi-moist,ormoisttype,

anddeepredorpurpleskincolorshallnotbemixedwith

yelloworreddishcopperskincolor.

Fairlysmooth:thesweetpotatoisfreefromveiningorother

defectscausingroughnesswhichdetractfromappearance.

Seriousdamage:anydefectwhichseriouslydetractsfrom

theappearanceoredibleorshippingqualityorwhich

cannotberemovedwithoutalossofmorethan10percentof

thetotalweightofthesweetpotatoincludingpeelcovering

thedefectivearea.Thefollowingspecificdefectsshallbe

consideredasseriousdamage:

(a)Dirtorotherforeignmatterwhentheindividualsweet-

potatoisbadlycakedwithdirt,orwhenseriouslydetracting

fromtheappearanceofthelot;

(b)Growthcrackswhenunhealedorwhenseriouslydetract-

ingfromtheappearanceoftheindividualsweetpotatoor

generalappearanceofthelot;

(c)Pox(SoilRot)whenseriouslydetractingfromtheappear-

anceoftheindividualsweetpotato;and,

(d)Wireworm,grassrootorsimilarinjurywhenanyholeina

40

sweetpotatoranginginsizefrom6to8ounces,ismorethan

1¼incheslong,orwhentheaggregatelengthofallholesis

morethan2inches,orcorrespondinglyshorterorlonger

holesinsmallerorlargersweetpotatoes.

Agradingboard(Figure63)canbeusedbygrowers,graders

andpackerstodeterminesizeclassification.Agradingboard

canbemadefromplywood,metal,Masonite,orplastic.

Unlessotherwisespecified,sweetpotatoesthatgothrough

thelargehole(left)butnotthroughthesmallhole(right)

andarebetweenthreeandnineinchesinlengthmeetthe

requirementsforU.S.No.1size.Additionalholescanbe

madeforsizingothergrades.

41

Figure 63. A grading board can be used in the field and on the packing line to manually grade roots. (ILLUStRAtION By B. EDmUNDS)

Table 9.SizerequirementsforU.S.sweetpotatogrades

Grade Length (in.) Diameter (in.) Weight (oz.)

min. max. min. max. max.

U.S. Extra No.1 3 9 13/4 31/4 18

U.S. No. 1 & US Commercial 3 9 13/4 31/2 20

U.S. No. 1 Petite 3 7 11/2 21/4 -

U.S. No. 2 11/2 - - - 36

Table 10. AppearancerequirementsforU.S.sweetpotatogrades

U.S. Extra No. 1 US No.1U.S. No. 1 Petite U.S. Commercial U.S. No. 2

Firm Firm Firm Firm

Smooth Fairly smooth Fairly smooth No serious damage

Fairly clean Fairly clean Fairly clean Not dirty

Fairly well shaped Fairly well shaped Fairly well shaped No shape requirement

Similar varietal characteristics One type One type One type

Free of damage* Free of damage* Free of serious damage* Free of serious damage*

Free of chilling injury, internal breakdown and dry rot

Free of chilling injury, internal breakdown and dry rot

Free of chilling injury, internal breakdown and dry rot

Free of chilling injury, internal breakdown and dry rot

<2% soft rot <2% soft rot <2% soft rot <2% soft rot

*See definitions above for description of damage and serious damage

Appendix 3.

Construction Guidelines for Negative Horizontal Ventilation (NHV) Curing and Storage Facilities

Revisedfrom The Postharvest Handling of Sweetpotatoes:

with construction guidelines for negative horizontal

ventilation curing and storage facilities.M.D.Boyette,E.A.

Estes,A.R.Rubin,andK.A.Sorenson.1997.NorthCarolina

ExtensionServicepublicationAG-413-10-B

Inthe1970s,theSoutheastsweetpotatoindustrybegan

toevolveasareliablesourceofgood-qualitysweetpota-

toes,notonlyinthefallandearlywinter,butessentially

year-round.Agrowingconcernaboutthequalityofstored

sweetpotatoes,combinedwithamovetowardlargerand

moremechanizedoperations,eventuallyprompteda

re-examinationofthen-currentcuringandstoragefacility

design.InEurope,adesignknownasthe“letter-box,”or

negativehorizontalventilation(NHV)system,hadlongbeen

employedforventilationof(Irish)potatostoragefacili-

ties.Duringthe1980s,avariationofthisdesignproduced

excellentresultsinCaliforniaandotherregionsforforced-air

coolingoffruitandvegetablesinpalletbins.

IntheNHVsystem,theventilationairisgenerally

pulledhorizontallypastthepalletbinsbyaslightnegative

pressure.Thenegativepressureiscreatedbyfansmounted

internallyalongthetopofaplenumwallononeendofthe

room(Figure12).TheNHVsystemmixestheair,which

resultsinlittleinternalvariationintemperatureorhumidity

throughouttheroom.Further,becausetheairisinmotion

andpassingthroughthemassofsweetpotatoes(norootis

morethanone-halfthedepthofapalletbinfromamoving

streamofair),thereisopportunityforheattransfer.Good

heattransferisimportantforwarmingthesweetpotatoesat

thebeginningofthecuringcycle,orcoolingattheend,and

fortheremovalofheatgeneratedbyrespirationthroughout

thestorageperiod.

Asweetpotatocuringandstoragefacilityisaninvest-

mentinqualitymaintenance.Ingeneral,thelongerthe

storagetime,themoresophisticatedthefacilityandhence,

themoreexpensive.Postharvestfacilities,unlikemany

othertypesoflargecapitalexpenditures(suchastractors

andharvesters),arefrequentlysubjecttoalterationsand

additions.Itisamajormistaketoassumethatanybuilding

designis“final.”Economicconditions,marketingopportuni-

ties,andmanyotherforcestendtoaltertherequirementsof

thesestructures.Veryfewofthesebuildingsremainasthey

werebuiltformorethanafewyears.Afacilitywithexcessive

andunusedcapacitymakeslittleeconomicsense.However,

planningforfutureexpansionoralterationshouldbegin

withsiteselection.Afterafacilityisbuilt,allfutureexpan-

sionsmustnecessarily“workaround”thatfacility.Agreat

manyproblemscanbeavoidedwithproperplanning.

Thefollowingimportantpointsshouldbeseriously

consideredduringtheplanningprocess:

Function.Thefirststepinplanningistodetermine

theprimaryfunctionofthefacility.Willthefacilitybeused

onlyforcuringandstoringsweetpotatoes?Doimmediate

orlong-termplansincludeprovisionsforapackinglineand

possiblecoldstorage?Willthefacilitybeusedforothertypes

ofproduceorforotherpurposes(forexample,equipment

storageorstorageofothercropsorsupplies)duringthe

off-season?Willactivitiesatthefacilitybemostlyseasonal,

orwilltherebeworkersatthefacilityyear-round?Auseful

andefficientfacilitycanbebuiltonlywhenalltheneedsand

functionshavebeenthoroughlyexplored.

Location.Thechainfromsweetpotatofieldtothe

consumer’stableisessentiallyanexerciseinmaterial

handling.Visualizingthemovementofsweetpotatoesto

thefacility,throughthefacility,andawayfromthefacility

asaflowofmaterialcansignificantlysimplifytheplanning

process.Landisexpensive.Situatingan8,000-square-foot

sweetpotatofacilityinthemiddleofa10-acrefieldmay

seemawasteofvaluableland.However,therearehundreds

ofpostharvestfacilitiesonsitesthateffectivelyeliminate

anyfutureexpansion,eventhoughthefacilitywasoriginally

convenientlylocated.Therearenogoodguidelinestofollow

inprojectingwhatfuturerequirementsmightbe,although

itisoftenrecommendedthatasiteshouldhaveroomfora

facilitytobedoubledinsizetwice.Theoutlinesofagener-

ousexpansionprogramshouldbeincludedontheoriginal

siteplan.

Asyoubeginplanning,itisagoodideatobecome

familiarwithanyzoningregulations,localordinances,or

land-useplansthatareapplicabletothesite.Youshould

alsobecomefamiliarwithanylawsorregulationspertaining

toconstruction,electricalsystems,workerhealthorsafety,

theproperuseandstorageofpesticides,andtheproper

handlingandstorageoffoodproducts.Althoughbuilding

contractorsshouldbefamiliarwiththelawsgoverning

construction,donotassumetheyarefamiliarwithlaws

governingtheuseofthefacility.

Beforeanyconstructionactivity,theownersoropera-

torsofasweetpotatofacilityareurgedtocompleteacom-

prehensivesiteevaluationtoensurethatwetlandsorother

sensitiveareaswillnotbeaffectedbytheproposedactivity.

Mostcountyplanningdepartmentsmaintainacomprehen-

sivelistofrulesandregulationsthatmayimpactsmallbusi-

42

nessesandindustries.Earlycontactwithregulatoryagencies

isrecommendedtoensurethattheconstructionanduseof

thefacilityareincompliancewithallapplicablerulesand

regulationsandthatallnecessarypermitsareacquired.

Alevel,elevatedsitewithgoodnaturaldrainageisideal.

Standingwaterisnotonlyanuisance,butitcontributesto

theearlyfailureoffloors,foundationwalls,andcolumnfoot-

ings.Further,pumpingwaterfromloadingdocks—especially

pumpingwastewaterfrompackinglines—canbeexpensive.

Avoidsitestoonearpropertylines,creeks(anddesignated

wetlands),andrailroad,highway,orpowerrights-of-way.It

isalsoprudenttoavoidresidentialareaswheneverpos-

sible.Althoughhighwayaccessibilityisessentialbothfor

themovementofsweetpotatoesandforthoseworkingin

thefacility,trytomaintainareasonablesetback.Allowfor

adequateparkingandtruckturnaroundspace,provideroom

forfutureexpansion,andavoidpotentialproblemsshould

thehighwaybealteredorwidened.

Power.Allexceptthesmallestsweetpotatocuringand

storagefacilitiesshouldhaveaccesstothree-phasepower.

Accessmaybeproblematicinsomeruralareas,butthe

amountofpowerrequiredforfans,pumps,andrefrigeration

systemsmakesaccesstothree-phaseessentialinmostcases.

Althoughthecostofthree-phasepowermaybeconsider-

ablylessthansinglephase,connectionfeesmaybequite

high.However,thecostofrunningthree-phaseequipment

maybeconsiderablylessthansinglephase.Forexample,a

five-horsepower,three-phaseelectricmotormaycostabout

thesameasaone-horsepower,single-phaseelectricmotor.

Someelectricalequipment,suchasrefrigerationsystems

withacapacitygreaterthan10tons,mayrequirethree-

phasepower.Powerstand-bychargesorminimummonthly

chargesmaybeleviedifpowerisnotdisconnectedinthe

off-season,socheckdetailswithyourutilitycompany.

Water.Somewaterisrequiredforallsweetpotatocuring

andstoragefacilities.Forfacilitiesintendedforcuringand

storageonly,8to12gallonsperminutemaybeadequate

forhumidifiers,drinking,bathrooms,andmiscellaneous

cleaning.However,forfacilitieswithpackinglines,thewater

requirementsmayrangefrom1,000gallonsperhourfora

smalllinetomorethan3,000gallonsperhourforalargeline.

Ifwellwateristobethesource,followlocalpermitrequire-

ments.Back-flowprotectionshouldbeincorporatedintoany

well’sconstructiondesign.

Wastedisposal.Foracuringandstoragefacilitythat

mayincludeapackingline,youshouldcloselyexamine

properwastedisposaloptions.Allsuchfacilitiesmusthave

meansforpropermanagementanddisposalofbothdomes-

ticandindustrialprocesswasteandmayalsorequireoneor

moreregulatorypermits,dependingonstateregulations.

Potentialsitesthatareotherwisesuitablemaybe

rejectedbecausetheydonotprovidefortheproperdisposal

ofwastewater.Landapplicationofthewastewater,through

sprayirrigationorothermeans,shouldbeexploredfirst.

Theguidelinesforthisoptionmayrequireanassessment

byaqualifiedirrigationengineer,dependingonthestate.

Thisisdefinitelythebestoptionformostoperations.A

secondalternativeistodischargetoamunicipalwastewater

treatmentsystem,whichmayhaveindustrialpretreatment

requirementsofitsown.

Afinaloptionistodischargewastewaterintothe

surfacewaters(stream,canal,drainageditch,roadditch,

andsoon).Ifyouareclassifiedasanindustrialoperation,

(youpackanysweetpotatoesotherthanyourown),youmay

alsoberequiredtoobtainawastewaterdischargepermit.

Youshouldtrytoavoidthisstepifatallpossible.These

permitsareissuedonlyafteralloptionsforlandapplication

orreuseareevaluated.Permitstodischargetosurfacewaters

areissuedonlyafterthesurfacewaterandflowregimehas

beenmodeledanddischargelimitsareestablished.Thisisa

longandoftencostlyprocess,resultinginonlyatemporary

permitthatissubjecttoreview.

Inadditiontopermitsforwastewatermanagement,

additionalpermitsmayberequiredformanagingthesolid

waste.Thesolidwastegeneratedinasweetpotatopacking

facility,suchasculls,mud,sand,andplasticandpaperpack-

agingmaterials,mustbedisposedofinapermittedlandfill

orthroughsomepermittedon-sitecompostingfacility.

Thequalityofthecompostedmaterialsmustbemonitored

andcanbedeterminedthroughtestingbystateregulatory

agenciesforasmallfee.Detailedinformationoncomposting

methodsandequipmentmaybeobtainedfromyourcounty

CooperativeExtensioncenter.Landapplicationorlivestock

feedingofcullpotatoesmayalsobeanoption.

Capacity.Manyofthedetailsofasweetpotatocur-

ingandstoragefacilityareultimatelydeterminedbythe

requiredcapacity.Thecapacityrequirementsarenotalways

sosimpleasthesumofyouranticipatedproduction,plus

theamountyoumightbewillingtostoreforothers.The

practicesofsegregatingtherootsfromdifferentgrowers,

fromdifferentfields,ofdifferentgradesordifferentvarieties,

andarrangingthemforeasyaccess,canallhaveasignificant

effectontheworkingcapacityofanindividualfacility.It

makesnoeconomicsensetobuildmorecapacitythancan

beefficientlyutilized.Ameasureofexcesscapacitymaybe

awiseinvestmentwhenitwillbeneededtoaccommodate

higherthannormalyieldsortoprovideadequatespaceto

storesweetpotatoesfromothersources.Rememberthat

thecostpersquarefootdecreasesandenergyefficiency

increaseswithincreasingfacilitysize.

43

Curing,unlikestorage,isaprocessthatinvolvesa

definiteperiodoftime.Curingfortooshortortoolonga

timecanhavenegativeeffectsonthequalityoftheroots.

Forthisreason,thesizeoftheindividualroomsinanNHV

facilityiscustomarilylimitedtonomorethantwodays’

harvest.Twodays’harvestforsomegrowersmaybeasfewas

5,000bushels,whileforothersitmaybeasmanyas30,000

bushels.Thetypicalcapacityofanindividualroominan

NHVfacilityisabout15,000bushels.Largerroomsmaymake

slightlymoreefficientuseofspacethansmallerrooms,but

theycanresultinundercuredorovercuredrootsifnotfilled

onatimelybasis.Smallerrooms,however,aremorequickly

emptiedandtakenoutofservice.

Althoughtherehavebeensomenotableeffortsinthe

handlingandstorageofsweetpotatoesinbulk,almost100

percentofthedomesticsweetpotatocropisstillharvested

into,cured,andstoredinpalletbins.Althoughthereare

publishedstandardsforpalletbins(ASAE337.1,American

PlywoodAssociationandtheNationalWoodenPalletand

ContainerAssociation),theconstructionanddimensions

ofthebinshaveevolvedthroughvoluntaryagreements

betweenthemanufacturersandthesweetpotatoindustry.

IntheSoutheast,mostpalletbinsbuiltsinceabout1985for

sweetpotatoeshavegenerallymeasured42incheswide,47

inchesdeep(inthedirectionoftheforkslots),and35inches

high.Thelevel,fullcapacityofthesebinsisslightlymore

than20bushels.Inpractice,however,topreventcrushing

andskinningtheroots,thebinsarenotfilledtocapacity,

butareassumedtohold18bushels.Thegrossweightofa

fullbinatharvestisapproximately1,250pounds.Inrecent

years,somegrowershavemovedtodouble-palletbinsfor

increasedefficiency.Thesedoublebins(84incheswide,47

deep,and35incheshigh)aredesignedtoexactlyreplacetwo

singlebinsbutrequireheavierforkliftsandspecializedbin

rotationequipment.

Palletbinsaredesignedtobestackedoneontopofthe

othertomakeefficientuseoffloorspace.Itiscustomaryto

stackbinsatleastsixhigh(171/2feet).Thisallowsonly21/2

feetofheadspacearoundthewallsinbuildingswith20-foot

eaveheight.ThisshouldbeconsideredaminimuminNHV

facilities.Ampleheadspaceisnecessaryabovethestack

forproperairmovement.Somerecentlybuiltfacilitieshave

22-,24-,oreven30-footeaveheights.A30-footeaveheight

allowspalletbinstobestackedeighthigh(23.3feet).

Allstackedpalletbinsshouldbeperfectlyalignedandin

goodcondition.Thecollapseofonelowerpalletbincan

causeastack—orevenanentirerow—tofallover,with

catastrophicresults.

Asmentioned,thetotalcapacityofasweetpotato

curingandstoragefacilitydependsonproductionplussome

prudentexcess,whereasthecapacityofindividualroomsis

usuallylimitedtotwodays’harvest.Mostsweetpotatocuring

andstoragefacilitiesconsistofseveralindividualrooms.

Usethefollowingformulasandguidelinestodeterminethe

actualdimensionsoftheserooms:

C=(1.2)(L)(W)(N)Where:

C=roomcapacityinbushels,normallylimitedtotwo

days’harvest

L=lengthoftheroominfeet

W=widthofroominfeet

N=numberofbinsinstack,(forexample,ifthebinsare

stackedsixhigh,N=6)

Thelengthoftheroom(dimensionparalleltothedirec-

tionofairflow)isnormallylimitedto100feetbecauseof

thedifficultyofthefanstoconsistentlymovetheairgreater

distances.Althoughtherearenosuchlimitsonthewidth

oftheroom,goodplanningsuggeststheselectionofroom

widthsthatutilizestandardbuildingcomponents(beams,

girders,andbarjoists)andlimitthelengthofunsupported

roofspans.Itisalsoagoodideatoselectwidthsthatmatch

wellwiththecombinedwidthofthepalletbins,plusabout

twofeettotakecareofgapsbetweenbinsandclearance

alongbothwalls.Supportcolumnsinthemiddleofaroom

areamajorinconvenience,apossiblesafetyhazard,and

shouldbeavoidedwhenpossible.

Example:Aroomeightbinswidewouldrequirea

roomwidthof:W=(8)(42)+24=360inches=30feet.

Assumingaroomcapacityof12,000bushelswithpallet

binsstackedsixhigh,theformulaabovewouldyielda

calculatedlengthof:12,000=(1.2)(L)(30)(6)L=55.6feet.

Whenthewidthoftheplenumisadded,thetotallength

oftheroomapproaches60feet.Therefore,thecapacityof

asingleroomforanNHVfacilitywithinsidedimensions

of30feetwideby60feetlong,withbinsstackedsixhigh,is

approximately12,000bushels.Table11providesestimated

capacitiesforroomsofvariousdimensions.

Ifthepalletbinsarecarefullystackednearthedoor,

almostallthefloorspaceinsidetheroommaybeused.Often

thepalletsinthelastroworpairofrowsmustbeturned

sideways(theforkliftslotsareperpendiculartotheflowof

airintheroom)toallowtheforkliftmaneuveringroom.

Thisarrangementwillpresentnoairmovementproblems,

providedafreespaceof6inchesto1footismaintained

aroundthesebins.

Rarelywillacuringandstorageroombebuiltwithout

sometypeofpaved(andoftencovered)stagingareafor

44

45

unloadingandloadingtrucks.Carefullyconsiderthisaspect

ofthefacilitydesign.Suchareasareseldomconsidered

ininitialdesignsbutoftenbecomeacentralalleywayof

anexpandedfacility.Thisareaisimportanttotheoverall

functioningofthefacilitybecauseconvenientaccesstoeach

roomfromacentrallocationisessentialforeaseofmaterial

handling.Accessaislesandalleywaysshouldbeatleast16

feetwidetoallowforsafeoperationofloadedforklifts.When

loadingdocksarepartofthedesign,theyshouldbeconve-

nientlylocatedtokeepthetripsshortfromtrucktostorage.

consTrucTion PoinTs and eQuiPmenT selecTion Buildingtype.Manytypesofbuildingconstruction

haveprovedsatisfactoryforsweetpotatocuringandstorage

facilities.Insulatedconcrete-block,curtain-wallbuildings

withbarjoistroofsupportswereoncepopularwithgrowers.

Becauseofthelaborcosts,blockconstructionhasbeen

almostreplacedbyengineeredpost-framebuildingsorall

steelbuildingsofvariousdesigns.Thesemodernbuildings

aredurable,functional,economical,andcanbecustom

designedandconstructedinafractionofthetimeittookto

buildtheoldermasonrybuildings.

Foundationandfloor.Althoughthecostofthefounda-

tionisrelativelysmall,thefoundationdirectlyorindirectly

affectstheperformanceofallotherpartsofthebuilding.

Thefoundationdistributestheweightofthebuildingand

itscontentsoveranareasufficienttopreventexcessiveand

unevensettling.Awell-drainedsolidgradeisessentialtoan

acceptablefoundation.

Allsweetpotatocuringandstoragefacilitiesare

constructedonaslabfloorofatleast4inchesofwire-mesh-

reinforcedconcreteoverawell-compactedgrade.Thefloor

shouldbecapableofsupportingatleastthreetimesthe

ratedcapacityoftheforklift.Fiveto8inchesofreinforced

concretemaybenecessarywhereloadsareunusuallyheavy,

suchasaroundloadingdocks.Thefloorshouldbeassmooth

andlevelaspossible.Evenaslightunevennessof1/4inch

betweenthetwofrontwheelsoftheforkliftmayresultina

3-inchsidemovementofthepalletbin16feetoffthefloor

(Figure64).Becausetherecommendedsweetpotatostorage

temperatureisnearthedeepsoiltemperatureinmostgrow-

ingareasoftheU.S.,under-floorinsulationisnotnecessary.

Itiswise,however,toincludeavaporbarrierundertheslab.

Insulation.Thermalenergyflowsnaturallyfrom

warmobjectstocoldones.Allmaterial,evengoodheat

conductorslikemetals,offerssomeresistancetotheflow

ofheat.Insulation,however,isanymaterialthatoffers

highresistancetotheflowofenergy.Hundredsofdifferent

Figure 64. Extended forklift. (PhOtO By t. SmIth)

46

materialshavebeenusedatonetimeoranotherforthermal

insulation.Thecharacteristicsoftheinsulationmaterials

differconsiderably.Suitabilityfortheparticularapplication,

aswellascost,shouldbethedecidingfactorsinchoosinga

material.Thermalresistance(R-factor)shouldbeconsidered

inadditiontodurabilityandthelaborrequiredtoinstall

thematerial.Somecommoninsulationmaterialssuchas

fiberglassbatsarenotrecommendedforsweetpotatocuring

andstoragefacilitiesbecausetheyperformpoorlyinahigh-

humidityenvironmentandoftenharborrodentsandbirds.

Ameasureofaninsulation’sresistancetothemovement

ofheatisitsR-value.TheR(forresistance)numberisalways

associatedwithathickness:thehighertheR-value,the

highertheresistanceandthebettertheinsulatingproper-

tiesofthematerial.TheR-valuecanbegivenintermsofa

1-inch-thicklayerorintermsofthetotalthicknessofthe

material.TheR-valuesofsomecommonbuildingmaterials

areshowninTable12.

Themostpopularinsulationmaterialnowusedin

newsweetpotatocuringandstoragefacilitiesissprayed-on

expandedpolyurethanefoam.Properlyapplied,thismaterial

hasanR-factorofapproximately7perinch,although

thisvaluemaydecreasesomewhatwithage.Itisdurable,

moistureresistant,andeffectivelysealscracksandsmall

openings.Itsmostimportantcharacteristic,however,is

thatitmaybeappliedveryrapidlywithcomparativelylittle

labor.Butthismaterialisextremelyflammableimmediately

afterapplicationandgivesoffatoxicsmokewhenburned.

Somelocalfirecodesorinsurancecompaniesmayrequirea

fire-resistantcoatingonthismaterial.Fire-protectivecoating

mayprovidesomemoistureresistancethatisnotpermanent

inurethane,causingthefoam’sR-factortodeclinewithage.

Rigidboardsoffoilorplastic-backedpolyisocyranuate

foamhavebeenoccasionallyusedforinsulatingsweetpotato

facilities.Thismaterialvariesinthicknesses(1/2inchto3

inches)andinsize(4by8feettoaslargeas8by30feet).

CertainformulationsofthismaterialmayhaveR-factors

ashighas8perinch.Inthepast,similarmaterialshave

delaminatedandotherwiseperformedpoorlyinthehigh

humidityofasweetpotatofacility.Thismaterialalsorequires

considerableinstallationlabor.Themainobjectiontothis

materialinanNHVfacilityinthepastwasthedifficultyin

sealingthejointsbetweenboardsandatroofandfloor

level.BecauseNHVfacilitiesoperateatslightlyreducedor

elevatedairpressures,failuretoadequatelysealallopenings

totheoutsidecansubstantiallyreducetheperformance

ofthesystem.Newermaterials,jointsealingmethods,and

installationprocedureshaveallbuteliminatedmanyof

theseobjections.

Bothsprayed-onexpandedpolyurethaneandpolyiso-

cyranuateboardmaterialsaremadeofclosedcellfoams.

Closedcellfoamsactasvaporbarriersbyretardingthe

movementofmoistureoutofthebuilding.Agoodvaporbar-

rierisdesirabletomaintainthehumidityinsidethefacility,

therebypreventingexcessiveevaporationandweightloss

ofthesweetpotatoes.Underhigh-humidityconditions,suf-

ficientinsulationwillhelppreventmoisturefromcondens-

ingonthewallsandroof.Whentherelativehumidityinside

thebuildingapproachessaturation,moisturewillbeginto

condenseonanysurfacethatisbelowthetemperatureof

theair.Condensationismorelikelytooccurinthetopofthe

building,ismostprevalentatnight,andindicatesinsuf-

ficientinsulationratherthanexcessivehumidity.

InthesweetpotatogrowingareasintheSoutheast,the

totalR-factorshouldbeaminimumof12forwallsanda

minimumof16forroofs.Roofsrequiregreaterinsulation

becausetheirsurfacesaremoredirectlyexposedtosunlight

duringthedayandradiationcoolingatnight.

Doors.Thedoorisanotherimportantpartofthesweet-

potatofacility.Thelargerthedoor,thebettertheaccessto

theroomandtheeasierpalletbinsmaybemovedinandout.

However,well-designeddoorscanbeexpensive.Theirprice

isroughlybasedontheirtotalarea,notwidth.Forexample,

a16-footwideby16-foottallslidingorgaragedoormay

costuptofourtimesasmuchasan8-footdoor.Although

smalldoorsarelessexpensive,fewsweetpotatofacilitiesare

builtwithdoorwayslessthan12feetwide.Doorwaysshould

exceedtheheightofaloadedforklift(about10to11feet).

Improperlydesignedormaintaineddoorscanwastelarge

quantitiesofenergy.Doorsshouldhaveasmuchinsulation

asthewallsandshouldalwaysprovideagoodairseal

whenclosed.

Plasticstripcurtainsareeffectiveinreducingtheenergy

losswhenlargedoorsmustremainopenforlongperiods.

Becausethedoorsofsweetpotatoroomsareusuallyopened

onlywhenfillingandunfilling,plasticstripcurtainsare

probablynotagoodinvestment.Smallaccessdoorsarea

goodinvestment,however,becausetheyallowforinspection

oftheroomanditscontentswithoutthebotherandenergy

lossofopeningthelargeaccessdoor(Figure65).

Heatingsystem.Thosefamiliarwiththeheating

requirementsoftheoldtrenchfloorsystemofcuringare

sometimessurprisedathowmuchlowertheheatingrequire-

mentisfortheNHVsystem.Ingeneral,theheatingsystems

selectedforthetrenchfloorfacilitieswerebasednotonthe

requiredheatcapacity,butonthefanorair-movingcapacity

oftheheatingunit.Itisnotunusualtoseea12,000-bushel,

trench-floorcuringroomwithaheatingsystemcapacityof

1millionBtuperhourormore.Theactualheatingrequire-

47

mentforanNHVroomofthesamecapacitywiththerecom-

mendedminimuminsulationisabout300,000Btuperhour.

BecauseintheNHVsystem,thefansthatmovetheairabout

theroomareseparatefromtheheatingsystem,theheating

systemmayberightlybasedontheheatrequirementand

notthefancapacity.

Sweetpotatoesdeliveredtothecuringandstorage

facilityusuallyarriveatatemperaturenearthatofthe

outsideair.Earlyintheharvestseason,thismaybenear85°F

(30°C),therecommendedcuringtemperature.Oncolddays

lateintheharvestseason,however,thearrivingrootsmay

besignificantlycooler.Theamountofheatrequiredtowarm

thesweetpotatoestotherecommendedcuringtemperature

dependsontheirpulptemperatureatarrival,thequantity

andratedelivered,andtherateofwarmup.Generally,suf-

ficientheatshouldbeprovidedinthecuringroomtoraise

thetemperatureto85°F(30°C)within48 hours.Thisisan

importantpoint.Inthepast,heatingsystemsweredesigned

toraisethetemperatureoftherootsasmuchas40°Fin24

hours.Thisrateofheatinputisexcessivefortworeasons.

First,itoftencausesdryingandotherphysiologicalproblems

associatedwithhightemperaturesinlocalizedareasnearthe

heater,eveninfacilitiesequippedwithNHVsystems.The

fanssimplycannotmovetheheatedairawayfromtheheater

fastenoughtopreventlocalizedoverheating.Second,there

issomeevidencethatabruptchangesintemperature(from

cooltowarmorvice-versa)maynegativelyaffectsweetpo-

tatometabolism,decreasingqualityandshelflife.

Excessivelyhighcuringtemperaturescontributeto

excessiveweightloss,sprouting,andotherpossibledamage.

Although85°F(30°C)istherecommendedcuringtempera-

ture,itisprudenttostoporslowtheinputofheatwellbefore

thattemperatureisreachedtopreventovershootingthe

targettemperature.Excursionsto90°F(32°C)oreven100°F

(38°C)arecommonproblemsthatoccurbecauseofthe

productionofrespirationheatathightemperatures.Forthis

reason,itisrecommendedthatthecuringthermostatbeset

nohigherthan80°Ftoallowthepulptemperatureto“coast”

to85°F(30°C).

Heatingsystemloadcalculationsarebasedprimarily

ontwofactors.Themajorfactoristheheatrequiredtoraise

thetemperatureofthesweetpotatoesfromtheambient

Figure 65. Small access doors to storage rooms limit excessive temperature fluctuations that occur when checking root quality or room conditions. (PhOtO By G. hOLmES)

4�

temperatureto85°F(30°C)atthebeginningofthecuring

cycle.Inaproperlyinsulatedbuilding,thistypicallyrepre-

sents90to95percentofthetotalload.Therestoftheloadis

composedofheatlostbyconductionthroughthewallsand

heatlostwithventilationair.

Theroomfromthepreviousexample(30feetwideby

60feetlong,12,000bushelcapacity),insulatedtoR-12inthe

wallsandR-16intheroof,wouldrequireaheateroutputof

approximately300,000Btuperhour.About94percentofthis

total(282,000Btuperhour)isneededtoraisethesweetpo-

tatoestocuringtemperaturein48hours.Only6percentof

thatcapacity(18,000Btuperhour)isneededtooffsetthe

conductionandventilationlossesoncolddays.

Asthisexampleshows,inaproperlyinsulatedandoper-

atedcuringfacility,mostoftheheatloadisusedtoraisethe

temperatureofthesweetpotatoes.Onlyasmallpercentage

oftheheatloadisusedtooffsetinfiltrationlosses.Evenin

verycoldweather,heatersareseldomoperatedbecauseheat

fromrespirationofthesweetpotatoesgenerallymaintains

thecorrectstoragetemperature.Forthisreason,itispossible

toaccuratelyestimatetherequiredheatingsystemoutput

basedsolelyonthecapacityofthefacility,provideditis

adequatelyinsulatedandproperlyoperated.Operating

experiencewithwell-constructedNHVfacilitiesineastern

NorthCarolinahasshownthatheatingsystemssizedat25

Btu per hour per bushel of capacityareadequate.Curingand

storagefacilitieswithaslittleas10Btuperhourperbushel

havebeensuccessfullyoperated,butincoolweathermay

takefourdaysormoretoraisethecuringtemperatureto

85°F(30°C).

Manydifferenttypesofoilorgasheatershavebeen

usedsuccessfullyinsweetpotatocuringandstoragefacili-

ties.MostrecentlybuiltNHVfacilitieshaveusedunvented

LPornaturalgasheatersofthetypeusedinanimalconfine-

mentbuildingsorsomegreenhouses.Theseunventedgas

heatersaresimple,relativelyinexpensive,andeconomical

tooperate.Nomatterwhattypeofheaterisused,itis

importantthatthecombustionairfortheheaternotcome

frominsidetheroom.Unventedgasheatersinparticularcan

quicklydepletetheoxygeninsidetheroom,whichresultsin

poorcombustion,theproductionofcarbonmonoxide,anda

potentiallydeadlysituation.Ethyleneisaby-productofheat-

ersandcannegativelyaffectsweetpotatoquality.Heaters

shouldalwaysbemountedoutsidetheroominsome

convenientlocation.Aboveandtothesideoftheroll-up

orslidingdoorattheendoftheroomoppositetheplenum

wallhasprovensatisfactory.Aductcanbeputthroughthe

wallfromtheheatersothehotairenterstheroomabovethe

stackofpalletbins.Theheaterscanalsobelocatedatthe

oppositeendofthebuildingandductedintotheplenum.

Ideally,thethermostatusedtocontroltheheatershouldbe

locatedawayfromtheheaterinapositionthathasgoodair

movementandallowseasyaccess(butnotdirectlyonan

exteriorwallifpossible).

Refrigerationsystem.Therefrigerationsystem

removesexcessheatfromthesweetpotatofacility.Itis

possibletostoresweetpotatoesforextendedperiodsin

bothconventionalandNHVfacilitiesduringcoolweather

withoutrefrigeration.However,refrigerationallowsthe

timelyremovalofheatattheendofthecuringcycle,which

eliminatesexcessivesproutingandweightloss.Additionally,

refrigerationallowsformuchmoreprecisecontrolofthe

temperatureduringstorageinwarmweatherandisessential

forsuccessfulsweetpotatostorageduringlatespringand

summer.Inlate-seasonstoragesituations,thecostof

refrigerationgenerallypaysforitselfintwoyearsorlessby

reducedweightlossandincreasedqualitymaintenance.

Heatcanbethoughtofasaformofenergythatalways

flows“downhill”fromwarmobjectstocoolobjects.For

example,heatwillflowfromawarmsweetpotatotothe

coolsurroundingair.Arefrigerationsystemisfunctionally

likeapumpthatpumpsheat“uphill”fromcooltowarm

objects.Witharefrigerationsystem,itispossibletoremove

heatfromasweetpotatostorageroomat65°F(18°C)tothe

outsideair,whichmaybeat90°F(32°C).

Arefrigerationsystemconsistsofthreemajorparts:

amotor/refrigerantcompressorset,condensercoils,and

evaporator(cooling)coils.Thesepartsareconnectedby

pipingtoformaclosedloop.Sealedinsidethepipingisa

liquidorvaporrefrigerant.Themotor/compressoractsas

apumpthatcirculatestherefrigerantthroughthesystem.

Boththecompressorandcondensercoilsareheattransfer

devicesandhavefanstohelpmoveheatfromthecoilstoor

fromthesurroundingair.Theevaporatorcoilsarealways

locatedinsidetheroomandaredesignedtotransferheat

fromtheairtotherefrigerant.Thecondensercoilsarealways

operating experience with well-constructed nhv facilities in eastern north carolina has shown that heating systems sized at 25 btu per hour per bushel of capacity are adequate.

49

locatedoutsidetherefrigeratedroomandaredesignedto

transferheatfromtherefrigeranttothesurroundingair.The

refrigerationsystemiscontrolledbyacoolingthermostat.

Liketheheatingthermostat,thecoolingthermostatshould

beconvenientlylocated,butnotbedirectlyintheairstream

fromthecoolingcoilsoronanexteriorwall.

Thecoolingcoilsoftherefrigerationsystemmustbe

coolerthantheairinsidetheroomiftheairistobecooled.

Thelargerthetemperaturedifference,thegreatertherateof

heattransferandthesmallerandlessexpensivethecool-

ingcoilsforagivencapacity.However,thecolderthecoil

surface,themorewatervaporfromtheairwillcondenseon

thecoils.Aswatercondensesonthecoils,itgivesupheat

attherateofapproximately1,000Btuperpint.Thecooling

energyusedtoremovethislatentheatiswastedbecause

itcontributesnothingtocoolingtheroom.Inadditionto

wastingrefrigerationcapacityandelectricalpower,excessive

condensationonthecoolingcoilsshouldbeminimized

becauseitreducestherelativehumidityoftheroom.

Tominimizecondensationandmaintaintheoptimum

curingandstoragehumidityinsidethefacility,itisgood

practicetoselectacoolingcoilthatminimizesthetempera-

turedifferencebetweentheairandcoilsurfacesbyselecting

alargercoil.Thetemperaturedifferencebetweentheairand

thecoilsurfaces(knownintheindustryas“deltaT”)should

belimitedto12degreesorlesstohelpmaintainarelative

humidityof85percent.Unfortunately,notallrefrigeration

contractorsareawareofthespecialhumidityrequirements

ofsweetpotatostoragefacilities.Inselectingacoolingcoil,

besuretospecifya12-degreemaximumdeltaT.Thecooling

coilsofarefrigerationsystemarenormallyequippedwith

fansthatbloworpullairpasttheheattransfersurfaces.Itis

advantageoustopositionthecoolingcoilsinsidetheroomin

awaythatallowsthefanstomoveairinthesamedirection

astheNHVfansintheplenumwallasshowninFigure12.

Allowingthecoolingcoilsfanstomoveairacrosstheroom

perpendiculartothedirectionofairfromtheNHVsystem,or

evenworse,oppositeofthatfromtheNHVfans,willdisrupt

theairflowpatterns,andtheresultispoorperformanceof

theNHVsystem.

Similartotheheatingsystem,therefrigerationload

calculationsarebasedontwofactors.Themainfactoris

therefrigerationcapacityrequiredtoremovetheheatfrom

therootsattheendofthecuringcycle.Indeterminingthis

partoftheload,timelyremovalofheattoreduceweightloss

mustbebalancedwiththecostofthesystem.Thelongerthe

timeallowedtoreducethetemperaturefrom85°F(30°C)to

58°F(15°C),thesmallertherefrigerationloadandhence,the

smallerandlesscostlythesystem.Experiencehasshown

thatacool-downperiodof96hours(fourdays)strikesa

goodbalancebetweenqualitymaintenanceandcost.Ina

well-insulatedandoperatedNHVfacility,thiscool-down

generallyrepresentsapproximately90percentofthetotal

requiredrefrigerationload.Similartoheating,theother

factorrepresentingtheremaining10percentistherefrigera-

tioncapacityrequiredtoovercometheconductionlosses

throughthewallsandroofandtheventilationlosses.

Usingthepreviousexample—a30-by-60-foot,12,000-

bushelcapacityroom—thefacilitywouldrequirearefrigera-

tioncapacityofapproximately185,000Btuperhour(15.4

refrigerationtons).Morethan90percentofthisamountis

neededtolowerthetemperatureattheendofcuring.The

other10percentisneededtooffsettheheatgainedthrough

conductionandventilation.Aswiththeheatingsystemcal-

culations,itispossibletoaccuratelyestimatetherefrigera-

tionsystemcapacityrequiredbasedsolelyonthecapacityof

thefacility,provideditisadequatelyinsulatedandproperly

operated.Again,operatingexperiencewithwell-constructed

NHVfacilitieshasshownthatabout1.3tonsofrefrigeration

per1,000bushelsofcapacityisadequate.

Humidificationequipment.Forsuccessfulcuringand

storageofsweetpotatoes,maintainingtheproperhumidity

shouldbeconsideredalmostasimportantasmaintaining

thepropertemperature.InNHVfacilitiesthatarecooled

withoutsideaironly,lowhumidityandtheresultingweight

losscanbeaseriousproblemwithouthumiditycontrol.

Somewatervaporiscontinuallygivenupbysweetpotatoes,

eitherbyevaporationorbyrespiration.Undernormalcuring

andstorageconditions,however,thisisnotenoughto

maintainthehumidityat85percent.Tomaintainoptimum

humidity,someprovisionsmustbemadetoaddadditional

watertotheair.Traditionalrecommendationstowetthe

wallsorfloorsareineffective.Thewatermustbeevaporated

directlyintotheairstreamtoraisethehumidityquicklyand

uniformlythroughouttheroom.Awell-designedhumidifier

istheonlywaytoefficientlyaccomplishthis.

again, operating experience with well-constructed nhv facilities has shown that about 1.3 tons of refrigeration per 1,000 bushels of capacity is adequate.

50

Manygoodhumidifiersareavailablethataresuitable

forasweetpotatocuringandstoragefacility.Mostconsist

ofanelectricallydriven,high-speedfanandatomizerthat

producesafinemist.Ideally,thismistisquicklyevaporated,

pickedupbythecirculatingair,anddistributedthroughout

theroom.Properlyadjusted,humidifiersinasweetpotato

facilitywillnotmoistenthefloor,thesweetpotatoes,or

anyothersurface.Thepurposeofhumidificationisto

fillthesweetpotatofacilitywithwatervapor,notafogof

waterdroplets.Forthisreason,thosehumidifiersdesigned

primarilyforgreenhousesshouldbeavoided,becausethey

oftenproducerelativelylargedropletsthatsettleoutofthe

airstreambeforetheyevaporate.

Theselectedhumidifiershouldhavearatedcapacity

ofapproximatelyonegallonperhourper5,000bushelsof

facilitystoragecapacity.Theratedcapacityofthehumidi-

fierisindicativeofitsperformanceinlowhumidity.Asthe

humidityinthefacilityincreasestoideallevels,someof

thewaterdropletsmaynotcompletelyevaporateanddrip

ontothefloororothersurfaces.Additionally,watersources

oftencontainlimeorotherchemicalsthatformdepositson

nozzles,fanblades,andotherpartsofthehumidifier.The

depositscanadverselyaffecttheperformanceoftheunitand

shouldberemovedduringperiodicmaintenance.Forthis

reason,humidifiersshouldbepositionedconvenientlyfor

easeofservicing.Becausetheevaporationofmoisturecools

theair,itisadvisabletomountthehumidifierneartheheat

ducttohelpwarmtheaircooledbyevaporationandfurther

acceleratetheevaporationofwaterdroplets.

Whatcannotbemeasured,cannotbecontrolled.

Humiditycanbemeasuredbyhandusinganinexpensive

devicecalledaslingpsychrometer,usingwetbulbanda

drybulbairtemperaturewithaconstantairspeedpassing

overthethermometer(Figure66).Otherdevicesalsocan

beincorporatedintoautomatedsystems.Theymeasure

humidityelectronicallyorbytheshrinkingorswelling

ofspeciallytreatedmaterials.Ahumidistat,whichisan

adjustablehumiditysensorthatactivatesaswitch,should

alwaysbeusedtocontrolthehumidifier.Manualcontrolof

thehumidifierbyasimpleon/offswitchwillinvariablyresult

inpoorhumiditymaintenance.Thehumidistatshouldbe

properlypositionedforbestresults.Itshouldnotbelocated

toonearthehumidifiernordirectlyintheairstreamfrom

thecoolingcoilsorheater.

Ventilationequipment.Thefansmountedinthe

plenumwallaretheessentialelementsoftheNHVsystem.

Alongwiththedampersandlouvers,thefansregulatethe

movementofairinsidethefacility.Guidelinesfortheproper

selectionofthisequipmentarebasedonresearchand

practicalfieldexperienceinavarietyofNHVfacilities.The

horizontalmovementofairnotonlypromotesuniformcon-

ditionsoftemperatureandhumiditythroughoutthemassof

sweetpotatoes,butmoreimportant,itprovidesameansfor

heattransferbyforcedconvection.Thisforcedconvection

allowsthetemperatureofthesweetpotatoestoberaised

uniformlyatthebeginningofcuringandloweredattheend.

Italsoallowsforthetimelyremovalofrespirationheatand

themaintenanceofuniformconditionsoftemperatureand

humiditythroughouttheroom.

Experimentaldataforheattransferbetweenthe

sweetpotatoesandtheairformsthebasisonwhichthe

requiredfancapacityisdetermined.Ifeachofthepalletbins

wereexactlythesamesizeandfittedperfectlytogetherso

thatairleakagebetweenbinswaseliminated,approximately

onecubicfootperminute(cfm)perbushelwouldbemore

thanenoughairtoeffectgoodheattransfer.Experimental

evidencehasshown,however,thatabouthalftheair

passingthroughthefansleaksintotheplenumthrough

...the total fan capacity of the nhv facility must equal approximately 2 cfm per bushel based on a fan static pressure differential of 0.10 inches of water.

Figure 66. A sling psychrometer is used to manually measure humidity. (PhOtO By G. hOLmES)

51

gapsbetweenthepalletbins.Therefore,toyieldsatisfactory

results,thetotalfancapacityoftheNHVfacilitymustequal

approximately2cfmperbushelbasedonafanstaticpres-

suredifferentialof0.10inchesofwater.Fanmanufacturers

normallysupplychartsgivingvolumeflowrateasafunction

ofstaticpressure.

Correctfanselectionisessentialtotheproperoperation

ofanNHVfacility.Industrialventilationaxialflowfanssimi-

lartothoseshowninFigure13areofferedinmanydifferent

combinationsofbladediameter,motorhorsepower,and

fancapacity.Ingeneral,wheretwofansofdifferentblade

diameterandhorsepowerhavesimilarcapacity,theonewith

thelargerbladediameterismoreenergyefficient.Although

itispossiblethatonlyonefancouldprovidesufficient

capacity,experiencehasshownthatmultiplefansspaced

10to12feetapartalongtheplenumwallwilldistributeair

moreuniformly.Allthefansinaroommustbeoperated

simultaneously.Ifsomefansareallowedtooperatewhile

othersareswitchedofforotherwiseinoperable,airwillbe

drawnbackwardsintotheplenumthroughthenonoperating

fans.Thiswillsignificantlyreduceairmovementthroughthe

sweetpotatoes,wasteenergy,andcompromiseenvironmen-

talcontrol.Alwayspromptlyrepairinoperablefans.

Whenthefansareusedtosimplycirculatetheairinside

theroom,themotorizeddampersacrosstheplenumfrom

thefansremainclosed.Whenoutsideairisrequired,the

dampersareopened.Whencorrectlysized,thetotaldamper

areashouldequalapproximatelytwo-thirdsofthetotalfan

area.Thetotalareaofthegravityshuttersattheendofthe

roomoppositetheplenumshouldequalthetotalareaofthe

motorizeddampers.

Usingtheexamplefacilityofa30-by60-foot,12,000-

bushelcapacityroom,thetotalrequiredfancapacitywould

beapproximately2(12,000)=24,000cfm.Assumingthat

threefansspacedapproximately10feetapartalongthe

plenumwallwouldbesufficient,eachfanwouldneeda

capacityofabout8,000cfmat0.10inchesofstaticpressure.

Fromtypicalmanufacturers’specifications,three30-inch

Table 11.Estimatedcapacities(inbushels)forroomsofvariousdimensions,withpalletbinsstackedsixhigh.

Width in Feet (Number of Bins)

Length (ft) 30 (8) 34 (9) 37 (10) 40 (11) 44 (12) 48 (13) 52 (14) 56 (15) 60 (16)

40 7,776 8,820 9,576 10,332 11,376 12,420 13,464 14,508 15,560

45 8,856 10,044 10,908 11,772 12,960 14,148 15,336 16,524 17,720

50 9,936 11,268 12,240 13,212 14,544 15,876 17,208 18,540 19,880

55 11,016 12,492 13,572 14,652 16,128 17,604 19,080 20,556 22,040

60 12,096 13,716 14,904 16,092 17,712 19,332 20,952 22,572 24,200

65 13,176 14,940 16,236 17,532 19,296 21,060 22,824 24,588 26,360

70 14,256 16,164 17,568 18,972 20,880 22,788 24,696 26,604 28,520

75 15,336 17,388 18,900 20,412 22,464 24,516 26,568 28,620 30,680

80 16,416 18,612 20,232 21,852 24,048 26,244 28,440 30,636 32,840

85 17,496 19,836 21,564 23,292 25,632 27,972 30,312 32,652 35,000

90 18,576 21,060 22,896 24,732 27,216 29,700 32,184 34,668 37,160

95 19,656 22,284 24,228 26,172 28,800 31,428 34,056 36,684 39,320

100 20,736 23,508 25,560 27,612 30,384 33,156 35,928 38,700 41,480

Note: for rooms with bins stacked seven or eight high, multiply the numbers in the table by 1.17 or 1.33 respectively

52

diameter,one-halfhorsepowerfansshouldbesufficient.

Basedonthefansizes,two30-inchmotorizeddampersand

two30-inchgravityshutterswouldalsoberequired

perroom.

Controls.Propersweetpotatocuringandlong-term

storagerequiresprecisecontroltemperature,humidity,

andventilation.Thelevelofenvironmentalcontrolcannot

exceedtheprecisionofthecontrolsystem.Theelectrical

controlsoftheheater,refrigerationsystem,ventilation

system,andhumidifierformthebrainoftheNHVfacility.

Althoughtherearemanydifferentpossibledesignsforthis

equipment,eachdesignessentiallyconsistsofsensorsand

associatedswitchingequipment.Therehavebeentwobroad

categoriesofcontrolsystemsutilizedinNHVfacilities.

Electromechanicalcontrolsconsistingofthermostats,

humidistats,timers,andassociatedrelaysarewiredintoa

centralpanelbox.Allthenecessarypartsandwiringcanbe

obtainedfrommostelectricalsupplydealers,andthesystem

canbebuiltandservicedbymostlicensedelectricians

Generally,aseparatesystemisbuilttocontroleachroom.

Electromechanicalcontrolsystemscouldbedesignedto

controlseveralrooms,butthecostoflongrunsofcontrol

wiringmakesthisoptioneconomicallyunfeasible.These

controls,ifcorrectlyinstalled,aresimpleandreliable.With

theadditionofseveralremotethermostatsandindicator

lights,thecontrolpanelmaybemountedjustoutsidethe

roomtoprovidethestatusoftheroomandequipment

ataglance.

ProgrammableLogicControllers(PLCs),Figure67,are

industrialcomputersthathavealsobeenusedtosuccessful-

lycontrolNHVfacilities.Thesesophisticateddevicesoperate

thecontrolequipment(suchasheatersandhumidistats)

throughrelaysinthesamemannerastheelectromechanical

controls.However,unliketheelectromechanicalcontrols,

Table 12.InsulationR-valuesforcommonbuildingmaterials

Material Full Thickness R-value

fiberglass batt insulation, 3-1/2 inches thick 13

fiberglass batt insulation, 5-1/2 inches thick 19

loose fill cellulose, per inch 3.5

loose fill glass wool, per inch 3.0

vermiculite, per inch 2.2

expanded polystyrene (blue board), per inch 5.5

foil-backed polyisocyranuate, per inch 8.0

aged expanded sprayed in place polyurethane, per inch 6.3

sprayed in place urea formaldehyde, per inch 5.0

solid concrete, per inch 0.08

eight-inch concrete block 1.1

eight-inch concrete block with vermiculite fill 5.0

pine or fir lumber, per inch 1.2

metal siding <0.01

3/8 inch plywood 0.5

1/2 inch plywood 0.7

53

PLCscanbeprogrammedtomakevariousenergy-saving

decisionsabouttheoperationoffans,heaters,andrefrigera-

tionequipment.Theyalsomaybeprogrammedtocollect

data,providesecurity,andsoundalarmsoractivateauto-

matictelephonedialingdevicesinemergencysituations.

OnePLC-basedsystemcaneasilycontrolmanyroomsand,

therefore,maybetheleastcostlyalternativeformulti-room

facilities.FielddatasuggestthatPLCbasedsystemscan

providesignificantlymoreoverallenergyefficiencythan

electro-mechanicalsystems.

AlthoughmanypartsofaPLC-basedcontrolsystem

canbeinstalledandservicedbyalicensedelectrician,the

actualhardwareselectionandprogrammingmustbedone

byanexperiencedprofessional.Thesedeviceshavea

proventrackrecordofreliableserviceinmanydifferent

industries.Additionally,likemanytypesofcomputer

equipment,thepowerandversatilitycontinuetoincrease

asthepricedecreases.

Figure 67. Programmable logic controller (PLC) for an eight-room negative horizon-tal ventilation (Nhv) facility. (PhOtO By CCU INC.)

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