fruit drying tonnels design and operation (jim valentine) short

8/2/2019 Fruit Drying Tonnels Design and Operation (Jim Valentine) Short

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FruitDryingTunnels

Design&Operation

ReportpreparedforUSAID/ADP

September27October13,2006

STTCJimValentine


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INDEX

INTRODUCTION............................................................................................................. 3DRYINGTUNNELDESIGNPARAMETERS.............................................................. 4FINDINGSANDRECCOMENDATIONS..................................................................... 7

1.Indirectheatingsystemusedbymostoftheprocessors. ............................................ 72.Directheatingandmoderngasdistributorsinstalledwillgenerategassavings.........74.Avoidfruitsbleedingduringdrying. ........................................................................... 9

6.Varietiesfordrying. .................................................................................................. 108.Gradingisimportant.................................................................................................. 109.Humiditymeasuringdevices. .................................................................................... 1110.Moistureanalyzerstomonitorthequalityofdriedproducts. .............................. 11

APPENDIXI.CALIFORNIADRIEDPLUMBOARD.............................................. 12APPENDIXII.NATURALGASCOMPOSITION..................................................... 13APPENDIXIII.CALIFORNIAPRIMARYPLUMVARIETIESFORDRYING..13


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INTRODUCTIONProductionofdriedfruitsandvegetablesinMoldovaisconsideredatraditionaltypeofproduct preservation.Moldova was oneof the main suppliers of driedproducts to theSovietUnion,withtotalproductionofabout10,000tonsannually.Productionofdried

products decreaseddramaticallyin the 90s duringthe economicdeclinefollowing theSovietUnionscollapse.Asaresult,thepost-harvestinfrastructure,processingfacilities,and general supply chain linkages have fallen into disrepair or have been abandoned.Meanwhile, the lack of working capital to maintain existing orchards and investmentcapitaltoplantneworchardsforcednewsmallfarmerstotransformmanyorchardsintofieldsusedforgrainproduction.The mostly widely used drying methodin Moldova is convective drying in tunnels.Convective tunnels are also common in other countries with traditions in drying,especiallyUSAandFrance,wheretunnelsareusedmainlyfordryingplums-becauseofhighloadingcapacity,goodresultsintermsofendproductsandlowercosts.

Inthelasttwoyears,signsofrevitalizationoftheMoldovanfruitsandvegetablesdryingsector can be seen. Due to growing demand for dried products and good margins,investmentsweremadetorehabilitatedryingfacilities&infrastructure,torecoverplumsorchards,andtoidentifynewexportmarkets.AsseeninTable1below,since2002Moldovahasconsistentlyincreaseditsexportsofdriedfruitsandvegetables.The2006exportvolumesareexpectedtobeevenhigherduetohigherplumyields. Table1:DriedFruits&Vegetables,Volume&Value

Year ExportValue,K$ ExportVolume,tons

2002 463 490

2003 800 717

2004 1,792 2,146

2005 3,761 2,844

Source:UnitedNationsCommodityTradeStatisticsDatabasePrunesaccountforapproximately80%ofMoldovandriedproducts.Twoprunevarietiesare predominantly grown in Moldova: Stanley and Anna Spath. Moldovan plums areharvestedby hand.Afterharvesting,fruitsaretakento dryingfacilitieswheretheyare

washed, placed on large trays, dried and later stocked for further processing andpackaging.Traditionallyprunesaredehydrateduntilmoisturecontenthasbeenreducedtoabout18%.For heat production in tunnels mainly liquid fuel/diesel was used. Recently, someMoldovandryingfacilitieshavebeenconnectedtoanaturalgaspipeline.Itisreasonabletoexpectthat,wherepossible,thedryingfacilitieswillmovefromdieseltonaturalgasinordertoreducetheenergycosts.


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ThepresentreportisfocusedonimprovingthedesignandoperationofdryingtunnelsusedinMoldova,reducingproductionanddryingcostsforprocessors..

DRYINGTUNNELDESIGNPARAMETERSListedbelowisagenericdescriptionofagoodtunneldesign.

Figure1.Dryingtunneldesign(crossflow)The single most importantideato remember when designinga dryingtunnel forfruitsandvegetablesisthatairwillalwaysfollowthepathofleastresistance.Tunnelsmustbedesignedinsuchawayas toforcethehotairovertheproductbeingdried.Thetunnelcanbedesignedforeithercrossfloworlinearflowalongthelengthofthetunnel.Ineithercase,thesamedesignprincipleapplies.

Toforce the air flowoverthefruit, the trolley designshouldallow the traystonestoneachothertoformasolidsidewall.Thesolidsidewallspreventairfromflowingoutthesidesof the tray. The conceptof wooden tray stacked toforma solidwallonly workswhenthetraysarestackedonadolly.Whenatrolleyisused,thesidesofthetrolleymust be blocked off with rubber baffles and the tray separation maintained in the 5centimeterrange.


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Wooden trays are normallystacked up to 25high, but the height of the stack may belimitedbythetunnelheight.Whenstackedtoformasolidwallontwosidesofthetrays,theopeningfortheairflowoverthefruitshouldbeabout5centimeters.A5centimetergap between the sidewall of the tray stack and the side wall of the tunnel should bemaintained.Rubberbafflesshouldbeinstalledonthesidewalls,ceilingandfloortolimit

theamountofairflowingaroundthestackedtraysoffruit.Thetraysshouldbestackedhighenoughtolimitthegapbetweentheceilingandthetoptraytoabout5centimeters.Withthistypeofconfiguration,theairwillbeforcedovereachofthetraysataboutthesamevelocitybecauseeachofthegapsbetweenthetraysoffersthesameresistanceandtheairwillentertheopenspacesatthesamerate.Theairhasnootherplacetogoexceptoverthefruit.Notethatthetraymustbeflatinthedirectionoftheairflow.Nolipshouldbeaddedtocontainthefruit.Itwillnotgoanywhere,andthelipwouldinterferewiththeairflowandcausedeadspotsnearthelipandunevendryingacrossthelengthofthetray.

Thedryingtunneldesignisusuallytwostoriesforthelineartunnelandasinglestoryforthecrossflowdryer.Thesecondstoryofthelineartunneliswheretheincomingairisheated and forced over the fruit on the first story. Thedrawings previouslyprovidedillustratebasictunneldesign.Thesecondstoryofthetunnelcontainsaheatsourceandafan.Theairentersthetunneljustbehindtheheatsource.Thefandrawstheairovertheheat source whereitis heated tothe desired temperature and then forcesthe air downthroughawideopeningtothefirststorywhereitflowsoverthefruit.Ithasnoplaceelseto go because the doors that are shown as opened in the drawing are closed duringoperation.Theyareopenedonlywhenadollyfulloftraysisplacedinthetunnel.Theopeningthroughwhichtheairpassesfromthesecondstorytothefirst storyiswideinorder that the transfer of air to the first story does not create any unnecessary backpressureonthefan.Theonlybackpressureshouldcomefromtheairbeingforcedoverthefruitthroughthesmallopeningsbetweeneachtray.Anybackpressureotherthanthatonlyleadstoinefficientairflow.Astheairexitsthelaststackoftrays,itisexhaustedintotheatmosphere.Again,thereshouldbenounnecessarybackpressurecreatedonthefanbecauseofrestrictionsoftheairflowattheexhaustend.Ifthetunnelsarelocatedindoors,theexhaustairshouldbedischarged outside the building through chimney-likevents. Notethat thereis also anopening between the two floors of the tunnel just behind and below the heater. Thisopening allows for some of the hot air to be re-circulated through the system, thusconservingonenergy.Caremustbetakenthatthemoistairbeingre-circulateddoesnotraisethehumidityoftheincomingairtoanunacceptablelevel.Temperature controls are installed at the hot end of the unit. Humidity controls areinstalledatthecoolendofthetunneltomeasurethemoistureintheairbeingdischarged.Humidityiscontrolledthroughairflow.Thedoorsonthesecondstoryattheintakeendofthetunnel(wheretheburnerislocated)areusuallyslidingdoorssothattheopeningcanbeeasilyadjusted.Ifthehumidityistoohigh,thedoorsareopenedwiderinordertoallowmorefresh air into the systemandreducingthehumidity. Ifthehumidity istoo


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low,theopeningisreduced,thuslimitingtheamountoffreshaircomingintothesystemand increasing the humidity. Notethat the amount ofre-circulatedair comingintothesystem isalsocontrolledby the door openings. Ifthedooropening isreduced,theairpressureinfrontofthefanisreducedandtheamountofre-circulatedairincreases,andviceversawhenthedooropeningisincreased.Inpractice,thedooropeningonceset

seldomneedsadjustment.Afanoutputof2,000to2,500cubicmetersperminuteshouldbeadequateforoneortworowsoftrays.Theheatingelementshouldhaveanoutputofapproximately250,000kilo-calories. In a cross flow system the fan and heater on one side of the system areequivalent tothesecond storyin the linear tunnelsystemand the large emptyspace infrontofthefanallowsforasimulationofthelineartunnelwithacoolendandahotendAir velocity will be the same throughout the tunnel, although the humidity andtemperature of the air will probably vary. The significanceof the variations in watercontentofthefinalproductcannotbedeterminedwithoutempiricaldatafromthetunnel

itself. It may not be significant if sweating equalizes the moisture content to anacceptablelevel.Inanycase,thedryingtimecanbeprolongedsothatthefruitisatanacceptablelevelofdrynessaftersweating.Asummaryoftheserecommendationsfollows:1. Useapropellertypefanwithaminimumoutputof2,000cubicmetersperminute.2. Purchaseaheatsourcewithatleast250,000kilo-calorieoutput.Theheatsourcecan

beeithergasorelectricityoperated.Gasisabetteroptionbecauseitisimmediatelyresponsivetochangesinoutputtemperatures.Withelectricity,thereis residualheatthatmaytaketimetodissipate.Operatingcostsshoulddriveyourchoice.

3. Thesecondstoryshouldtotallyenclosethefan,theheatsourceandtheopeningfortheairtoenterthehotendofthetunnel.Theenclosureshouldhaveslidingdoorsinfrontofthefantoadjusttheintakeairflow.Anopeningabout2/3sthewidthofthetunnelandabout30centimeterswideshouldbebuiltinthefloorbetweentheheatsourceandthefantoallowairfromthecoolendofthetunneltore-circulateinthesystem,conservingenergy.

4. Theopeningfortheairtoenterthehotendofthetunnelshouldbethewidthofthetunnelandabout50to70centimetersfromtoptobottom.

5. Theopening attheexhaustendshouldbe,ataminimum,thesamesize astheinletopening. The air should be exhausted to the outside to prevent buildup oftemperatureandhumidityinsidethebuilding.

6. Thesidesofthetrolleysfacingthetunnelwallsshouldbeclosedtopreventairfromflowing out the sides. The closed sides of the trolleys should be within fivecentimetersofeachsidewallofthetunnel.

7. Purchasehumiditysensorstoinstallattheexhaustend(coolend)ofthetunnel.Forallproducts,therelativehumidityattheexhaustendshouldbemaintainedat60%.

8. Temperaturesensorsmustbeinstalledatthehotendofthetunnel.Ingeneral,thetemperatureatthehotendismaintainedat650Cdegrees.


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FINDINGSANDRECCOMENDATIONSTen different processors were visited by Jim Valentine during his stay in Moldova(September 27 October 13, 2006). There were several areas of concern that werecommonamongmostoftheprocessors:

1. Indirect heating systemused by most of the processors. Part of thelegacyoftheSovietUnionwasacumbersomeandinefficientsystemforheatingtheair.Originally, it was designed for diesel fueled driers where direct heating would haveresulted inundesirableresidues onthe fruit. Theindirect heating systemconsistsof alarge diameter pipe (40 cm or more) with another iron pipe inside and the heatingchamberforburningthefuelinsidetheinnerpipe(Figure1).Afanpullsorpushestheairbetweentheouterandinnerpipesandthechamberinwhichthedieselfuelisburningheatstheair.Suchasystemisinefficientbecausetheopeningbetweenthepipesrestrictsairflow and heating all the iron piping and maintaining temperature during the entiredwelltimewastesenergy.Withtheincreasingpopularityofnaturalgas,thetechnologydidnotchangetotakeadvantageoftheefficienciesachievablewithgasburners.

Mostoftheprocessorsvisitedthathadconvertedtogaswerestillusingtheoldindirectheatingsystem.Gaswasusedbecauseitwascheaperthandieselfuel.Insomecases,theprocessorswerecontinuingtousedieselbecausenaturalgaswasnotyetavailable.

Figure2.IndirectheatingsystematoneoftheMoldovandryingfacility

2.Directheatingandmoderngasdistributorsinstalledwillgenerategas

savings.Presentations on the advantages of direct heating of the air were presented to all theprocessors.Directheatingmeansthatthegasdistributorisinstalleddirectlyintheupperlevel of the drying tunnel avoiding the chamber for burning. The ones who had gasavailable were encouraged to convert their systems during the off season so that theycouldstarttakingadvantageofthesavingsatthebeginningofnextseason.Thesavings


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estimates range from 20% to 40% depending on the characteristics of the tunnels, thedrying temperatures, air velocity and the effectiveness of management in controllingdryingparameters.

Figure3.Directheatingsystematoneof

theMoldovandryingfacilityFor those processors that did not yethave natural gas available, it wassuggestedthattheconversionbemadeassoonasgasbecameavailable.Itmustbekeptinmindthatnaturalgaspriceslastyearwere1.42leipercubicmeter.Thisyear, the price is 2.96 lei per cubicmeter. In January 2007, there will beanotherpriceincrease. It is imperative

that energy conservation be one of theprimary considerations of the processor in order to stay competitive and remain in

business,giventhat30%-40%oftheircostsarerelatedtoheating.BecausetheoldSovietgasdistributorswillnotoperateinthesemi-closedenvironmentoftheupperlevelofthedryingtunnel,amoremoderngasdistributorthatallowsfortheadjustmentof the gas air mixtureandoperateswellinsidetheupper levelofthetunnelwas recommended in case of direct heating. The more complete combustion plus thethermostaticcontroltomaintainaconstanttemperatureatthehotendofthetunnelcanresultinevenmoreenergysavings,inpart,becausetheshortironpipeusedasanopencombustion chamber willbeeliminated. The gas distribution system shouldbe placedabout 1 to 2 meters from the air intake entrance. It was recommended that one bepurchased toreplace the old Sovietstyle distributor that was originally builtfor dieselfuel.Thenewdistributorwillallowtheironpipethatcurrentlyhousestheflametobeomittedandfurtherreductionsinenergyusagerealized.Completecombustionisrealizedby adjusting the flame to burn blue instead of the orange that is characteristic ofincompletecombustion.

3.Puttunnelsoutside. When building new tunnels, it is always best to put themoutsidewitharoofoverheadtoprotecttheworkersandproductsfromtheweather.Ifnotoutside,thedischargedmoistairwillre-circulatethroughthetunnelagain,prolongingthe

drying cycle unreasonably. With

reconditioned tunnels, an alternative isto exhaust air outside via chimney-linevents.Figure 4. Drying tunnels built outsidewitharoof


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Figure5.Reconditionedtunnels-chimney-likeventstoexhaustairthroughtheroof(seecoloredingrey)

4.Avoidfruitsbleedingduringdrying. Withoneexception,theprocessorswerelosing alot ofthe sugar content oftheplumsdue toexcessive bleedingduring drying.Thislossmeanslessyield.Italsomeansthattheskinoftheplumrupturesduringdryingwhich affects the appearance of the product. Finally, excessive bleeding makes itdifficulttokeepthetraysandthefloorofthetunnelclean,creatingasanitationproblem.Thebleedingproblemissolvable.It is caused partly by toohigh a temperature andpartlybyinadequateairvelocity.Oneofthesolutionscanbetousetwotunnelstodrytheplums.Inthefirsttunnel,theplumsshouldbedriedattemperaturesnohigherthan60degreesuntilthemoisturecontentwasreducedbyabout50%.Theplumsthenshouldbetransferredtoasecondtunnelandfinishedatatemperatureof75-80degrees.Oneofthe Moldovan dryingcompanysolved the problem with bleedingusing this method. A

secondprocessorwitha largecabinettypedrier foundthathe couldavoid the problemthroughstageddryingandbyincreasingtheairvelocity.InCalifornia,stageddryingisnotpracticed,becausetheplumvarietiesdonotrequireit.

5.Sanitation.Someoftheotherconcernsencounteredinthefieldwerepoorsanitationdueinparttothebleeding.Traysandtrolleyswerenotthoroughlycleanedaftereachcycle.Thetunnelfloorswerestickywithdriedplumjuice.Theenvironmentintheplumpreparation area and in the dried fruit storage area was not clean. This included

unprotected lighting, walls floor and someof the older equipment that did not havefood grade contact parts. The equipment

was painted which is always a bad choicesince the paint chips and harbors bacteria.The paint may also contain substances thatareharmfulwheningested.Figure6.Caramelizedplumjuiceduetoexcessive fruits bleeding during the dryingprocess


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6.Varietiesfordrying.Processorsthatplantoplanttheirownplumorchardwereencouragedtoselectthebestvarietiesfordryingtocorrespondtoglobalmarketdemandandtoavoidbleedingwhiledrying.(SeeAppendixIII).

7.Woodentrays.Withtwoexceptions,thetrayswhereplumsareplacedfordryingwere perforated aluminum. Although easy to clean and durable, aluminum is a poorsurfacefordryingbecauseitretainsheatandcanburnthefruitorcausecaramelizationofthesugars.Woodentraysareabetterchoice. Althoughahighmaintenanceitem,theywillcontributetoproducingabetterproduct.Woodentraysideallyhaveasolidwooden

bottom made of wide boards that areflushagainst each other. The trays built inMoldovaarethinslatsofwoodwithagapbetweentheslats.Itisalmostimpossibleto

clean that area of the tray. A smooth softwood bottom that will not impart flavortothefruitisthebestchoice.PineisthewoodofchoiceinCalifornia.Figure7.Traysfor dryingwith bottom andframefromwood

8.Grading is important. It was recommended that each of the processors grade

their plums before drying to optimize the drying process and to ensure the consistentqualityofthefinalproduct.

Grading fruits and vegetables is important notonly for the products designated for the freshmarket, but also for fruits, especially plums,usedfordrying.Moldovanprocessorsdonotgradeplumsbeforedrying. However in US and in Franceprocessors are grading plums to optimize theenergy costs and to make the pitting process

easier.

Figure8.Agraderusedtogradefreshfruits


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9.Humiditymeasuringdevices. It wassuggested toall clients that a humidity-measuringdevicewithremotereadoutbepurchasedforinstallationatthecoldendofthetunnel.BecausehumidityishighinMoldova,monitoringhumidityandadjustingtheair

intakeismoreimportantthanindrierclimates.

10.Moistureanalyzerstomonitorthequalityofdriedproducts. Itwasalsosuggestedtoallclientsthatamoistureanalyzerthatprovidesaninstantreadoutbepurchasedtomonitorthemoistureoffinalproducts.Laboratoryanalysistakestoolongtobeofanyvalueduringthedryingprocess.Additionally,over-dryingcostsmoneywithnoadditionalpremiumpaidforover-driedplums.


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APPENDIXI.CALIFORNIADRIEDPLUMBOARD

The CaliforniaDriedPlumBoardrepresentsthe growers,processors andpackers. The

Boardwasestablishedundertheauthorityof theCaliforniaSecretaryof Agricultureandisauthorizedtodirectandmanageactivitiesinthefollowingareas:

1. Advertising,publicrelationsandsalespromotion2. Marketresearch3. Production,processingandnutritionresearch4. Education

The Board is composed of 22 members. Seven members are dried plumhandlers/packers/processors.Fourteenmembersaredriedplumproducers/growers.Onememberisanon-industrypublicmember.

ThemissionoftheboardistoexpandtheworldwidedemandforCaliforniadriedplums.To this end, the Board will develop a domestic generic program for advertising, salespromotion,andpublicrelations,andaconsumereducationalprogramonthenutritionalvalueof the dried plum. The Boardwillalsocoordinate aninternational programthatmay include both generic and non-generic advertising sale promotion and publicrelations. To improve the quality of the dried plum, the Board conducts research onproductionandprocessing. Finally,theBoardmustdevelopgoodworkingrelationshipswithtradeandconsumerorganizationsaswellasgovernmentagencies.TheBoardwilllobby the government agencies to prevent adverse legislation from being passed andchangeanylegislationwhichadverselyaffectstheindustry.

The advertising and sales promotion activities might include TV ads,newspaper/magazineads,in-storepromotions,contests,give-awaysetc.Someoftheseactivities, especially the ads, may be directed toward consumer education on thenutritionalvalueofplums.Publicrelationsmayincludetradeshows,relationshipswithfood magazine editors, ads in trade publications, giveaways at fitness events, ads inconjunction with cereal producers, and ads as a substitute for butter, sugar oil, etc inbakedgoods.TheBoardalsoworksinconjunctionwiththePruneMarketingCommitteeinenforcingCaliforniamarketingordersfordriedplums.Themarketingordersrestrictthesaleof

plumsthatdonotreachtheminimumsizestandardof1.83cminlength. About3%to6%ofCaliforniaplumsfailtomeetthisstandard.Theseplumsarethendivertedtopureeorjuiceproduction.Thereisalsoaweightstandardof.45kgper100plums.Plumsthatdonotmeetthisstandardaftersizegradingaredivertedtojuiceorpuree.ThePruneMarketingcommitteehastheauthoritytochangethestandards,buttheyneverexercisethatauthority over size andweight. Tocontroltheflowofdriedplums tothemarketandtomaintainthepriceandqualityoftheproductonthemarket,themarketing


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committeeusuallyusesthesurplussetaside.Thisistocontrolthesupplytothemarketbyputtingdriedplumsininventoryuntilabadcropyearorincreaseddemandallowthemtobemarketedwithoutanymarketdisruption.Itshouldbenotedthattherearemarketingorders for every fruit produced in California and that these marketing orders areenforceablebylegalactioninthecourtsthatcanresultinheavyfinesforviolations.

APPENDIXII.NATURALGASCOMPOSITIONThere was much discussion about the possible residues left by natural gas. The by-products of burning are water and carbon dioxide, both compounds that are found inabundanceintheairthatwebreathereveryday.Theworldsnaturalgassupplyconsistsof80%to95%methane.ThelowerpercentageistypicalofEasternEurope.Naturalgasalsocontains ethane, propane,butane, hexane, heptane,andoctane. All ofthesegases

contain only carbon and hydrogen which produce water and carbon dioxide uponburning.Therearealsotracesofhydrogensulfide,nitrogenandargoninthenaturalgas.Nitrogenandargonareinertgasesandarefoundintheairwebreathe.Thehydrogensulfide is only in trace quantities that are harmless. The hydrogen sulfide occurs insignificantquantitiesinwine,grapejuiceandotherproductsthatusethesulfurcontenttoinhibit fermentation. There is also an additive in the natural gas that gives it thecharacteristicodor.Thisisthesafetyfeatureforwhatisacolorless,odorlessgasinitsnatural state. Notethat it is the same gas that restaurant use in their ovensto bake apizza,bread,andcakesorthatthehousewifeusesinherovenandstovetopathome.Iftherewereharmfulresidues,wewouldallbeillordead.

APPENDIXIII.CALIFORNIAPRIMARYPLUMVARIETIESFORDRYING

The primary varietiesused inCaliforniafordrying arethe Frenchplum,the ImprovedFrenchPlum andtheSutter. Thesevarietiesarecharacterizedbytheirhigh Brix. TheSutterisinthe19degreeBrixrange.TheFrenchvarietiesareinthe17degreerange.ThehighBrixplumisthechoiceofdryersbecausethereisnofermentationaroundthepitafterdryingwhenthedriedplumsareinstorage.Itisestimatedthat97%ofthedried

plumsinCaliforniaarefromthesevarietiesoffreshplums.NotethattheFrenchplumwasintroducedfromFranceintotheUnitedStatesinthemid1800s. However,theFrenchPlumoriginated inWesternAsia. TheImproved FrenchPlumandwasdevelopedintheUS.TheSutterwasrecentlydevelopedbytheUniversityofCaliforniaatDavis.

fruit drying tonnels design and operation (jim valentine) short

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