Reductive Reasoning

ReductiveReasoning

Gettingtothebottomof'reduction'problemsinscrewcapwinesByJamieGoode

HIGHLIGHTS

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Withthewidespreaduseofscrewcaps,sometechnicalissueshaveemergedsurroundingpost-bottlingsulfurchemistry,knownas\inthetrade.

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Morethan2%ofwinesclosedwithtin-Saranscrewtopsmayshowmercaptanodors,whileSaranexscrewtopsarenotimplicated.

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Screwcap-sealedwinesaffectedbymercaptansshouldbeamajorconcernforwinemakersbecausethewineisemphaticallynotreachingtheconsumer\waythewinemakerintended.\

Thelastdecadehasseenarevolutioninthedull-soundingbutvitallyimportantfieldofwinebottleclosures.Tenyearsago,corkwasstillprettymuchtheuniversalclosure,althoughthemurmursofdissatisfactionaboutitspoorperformance,withunacceptablyhightaintrates,weregettinglouder.Themostprogressivewinemakerswerebeginningtotryoutalternatives,andbackin1997,mostpeoplewerebackingplasticcorksasthetaint-freealternativetosucceedcork.

Screwcapsweren'tyetontheradar:they'dbeentrialedinAustraliainthe1970sandhadbeenabandonedbecauseofpoorconsumeracceptance.Itwasn'tuntil1999thatOrlando'sPhilLafferreintroducedthemforahigh-endRieslinginAustralia,offeringconsumersachoicebetweenthesamewineinscrewcapandcork:Theformersoldoutlongbeforethelatter.Thefollowingyear,agroupof14vigneronsfromtheClareValleybandedtogetherandreleasedtheirRieslingsunderscrewcap.ThispromptedamassmigrationtothisclosureinbothAustraliaand,evenmorestrongly,inNewZealand.Itbecameclearthatinsomemarkets,notablyAustralia,NewZealandandtheUK,therewaslittleconsumeroppositiontothisnovelwayofsealingwinebottles.

Nowcork'smajoropponentwasnolongerplasticcorks,butscrewcaps.Thebattlelinesweredrawnbetweenthosewhostillchampionedcork,andthosewhoinsistedthatallwinesshouldbesealedwithscrewcap.

Screwcapshadestablishedthemselvesasthekeyalternativetocorkphilosophically,ifnotyetinvolumeterms--plasticcorkscurrentlystillsellsignificantlymoreworldwidethanscrewcap,buttheylackthesamesortofadvocacythatscrewcapsenjoy.AMatterofBalance

Butwiththewidespreaduseofscrewcap,sometechnicalissueshaveemerged,surroundingpost-bottlingsulfurchemistry,knownmorecommonlyas\inthetrade.It'shardtodiscussthesedispassionately,becausesuchisthevolumeofthewarofwordsbetweenadvocatesofscrewcapandcorkthatthesediscussionsrapidlygetfannedintoflames.Addtothisthatthesubjectmatteritselfishorridlytechnical,andthefactthatwedon'thaveallthedatawe'dlike,andthere'saneedforcalm,balancedtreatmentoftheseissues,whichiswhatI'mattemptingwiththisarticle.

Theissueunderdiscussion,screwcapreduction,firstcametolightintheclosuresstudybytheAustralianWineResearchInstitute(AWRI),whichcommencedin1999.\canbearguedthatclosingthebottleremainsoneofthegreatesttechnicalissuesfacingthewineindustry,\suggestedleadauthorsPeterGoddenandLeighFrancisinthereport'sintroduction.\winemakercancontrolmanyaspectsofwineproductiontocreateawinesuitableforthemarketplace,andyettherecanbeanunpredictableincidenceofproblemsoncethewineisbottled,dueinlargeparttothepropertiesoftheclosuresused.\

Thesamewine,arespectableClareValleySémillon,wasbottledusing14differentclosures,andfollowedwithregularchemicalandsensoryanalysis.Theresultsafter21monthsinbottlewerepublishedin2001,andshowedthatwhilethescrewcappedwinekeptfruitfreshnessandretainedfreesulfurdioxidethebestofalltheclosures,italsosufferedfromasensorydefectdescribedbytheexperttastersas\flint.\Thiswasasurprisingfinding,andcausedagreatdealofheadscratchingandanxietywithinthetrade.

Shortlyafterpublication,Goddenhadthistosayaboutit:\areveryconfidentthatthe'rubber-like'characterisnotataint,butisanunwelcomemodificationduetochemicallyreducedsulfur,asaresultoflackofoxygen.However,itiscertainlyanimportantcharacterinscrewcap-closedwine,andwehavehighlighteditsexistencetoavoidmass-bottlingofwineunderextremelyanaerobicconditionswhichmightthendevelopasimilarcharactersomewhereinthefuture.\Thisreductionwasstillevidentfiveyearspost-bottlingwiththeSémillonusedinthetrial.

WhatCausesReduction?

TheAWRIresultsraisedanumberofquestions.Whatistheexplanationforthisreduction?Wasthisproblemspecifictothiswine?Howmuchofanissueisitwithreal-worldwinesbeingdrunkbyconsumers?Andwhatdoesitsuggestabouttheroleoftheclosureinwinedevelopment?

Theprevailingviewatthetimewasthattheidealclosurewouldbeonethatsealshermetically,allowingno

oxygentransmissionatall.ButafurthertrialfromAWRIseemedtoindicatethatananaerobicclosureisentirelyunsuitableforwine.ThistrialinvolvedcomparingaChardonnaywinesealedthreedifferentways:withcork,withscrewcapandthenhermeticallysealedinaglassampule.TheChardonnaywineunderwentsome

developmentinallthreecases,butwiththescrewcaptherewasabitofreductionandwiththeampulealot.Anotherstudy,thistimelookingataPenfoldsBin389redwinesealedwithsyntheticcork,naturalcorkandscrewcap,alsoencounteredsomereductioninthescrewcappedbottle.

Weneedtopausehereforatechnicalaside.Thescrewcapitselfisjustameansforholdingalinerinappositiontotherimofthebottle.Itisthepropertiesofthislinerthatdeterminetheoxygentransmissionlevelsoftheclosure.Currently,twodifferentlinersareavailableforwine.Thefirst,andmostwidelyused,iscalled\ThisconsistsofaSaranbacking,withametallayer(usuallytin,butitcanbealuminium)thatiscoveredbyathinlayerofPVDC,whichisincontactwiththewine.Thismetallinerallowsverylittleoxygentransmissionatall.ItisthelinerusedalmostuniversallyinAustraliaandNewZealand.IntheU.S.andEurope,itisstillthemost

widelyusedliner,butyouarealsolikelytoencounterthealternativeliner,knownas\only,\andconsistssolelyofSaranex.Thisallowsmoreoxygentransmission,andisnotimplicatedinthescrewcapreductionstory.Backtotheissueofreduction.Whatishappeninghere?Itallhastodowiththeissueofsulfur-compoundchemistry.Duringfermentation,yeastscanproducesulfides.Thisoccurswhentheyarestressed:forexample,whenthereislowmustnitrogentheymayturntothesulfur-containingaminoacidcysteineasanitrogensource,resultinginsulfideproduction.Theycanalsoproducesulfideswhentheyareforcedtooperateanaerobicallyoraretemperature-stressed,whichiswhygoodwinemakingpracticeistomonitorjuicenitrogenstatus,giveplentyofaerationtofermentingmustsandavoiddramaticshiftsintemperature.Intheworstcasescenarios,reduced

winessmellofhydrogensulfide,whichgivesthemaneggy,dirty-drainsaroma.Hydrogensulfidecanbeoxidizedtomercaptans(alsoknownasthiols,andsmellingofburned-match,rubber,earthorcookedcabbage),whichcanbeoxidizedtodislufides(describedvariouslyashavingaromasthatarevegetal,cabbagey,garlic-like,rubberyoronion-like).

Mercaptanscanalsoarisebyothermeans.Thepresenceofthesesulfurcompoundsinwineisknownas\butthistermisactuallyabitofamisnomer,andcanleadwinemakersintotroublewhenthey

respondtotheirpresencebygivingthewineoxygen:Thisisbecauseitispossibleforpart-oxidizedwinestohavethesearomas,andgivingevenmoreoxygentothemclearlywouldn'tbeagoodidea.

There'sanothercomplicationhere.Someofthesulfur-containingcompoundswe'retalkingaboutcanbebadinsomecontextsandgoodinothers.ThebestexampleofthisisinSauvignonBlanc,wherearangeofthiolshavebeenimplicatedasbeingpositivecontributorstothearomaofwinesmadefromthisvariety.Intherightcontext,alittlebitofthiolcanbepositive--acomplexingfactor.RedoxPotential

Sowhyisreductionaproblemthat'sassociatedwithscrewcapuse?It'stodowithaconceptknownasredoxpotential.Thisisameasureofhowoxidativeorreductiveasystem,suchasawineinbarrelorbottleis,anditismeasuredinmillivolts(mV)--thehigherthereading,thelessreductive.Typically,anaeratedredwinewillhavearedoxpotentialof400--450mV,whereasstorageintheabsenceofairforsometimewillreducethisto200--250mV.Iflevelsgetaslowas150mV,thenthereisadangerthatreductionproblemscanoccur.

Exposuretooxygenthroughwinemakingpracticessuchasracking,toppingupbarrelsandfiltering,increasesthelevelofdissolvedoxygeninthewineandincreasestheredoxpotential,whichwillthenreturnto200--300mV.Inwhitewines,thisredoxlevelwillchangemuchmorerapidlythaninredwines,becauseredwineshaveahigherconcentrationofphenoliccompoundssuchastanninsthatareabletointeractwithoxygen,andactasbuffers.Anothervariablehereistheleveloffreesulfurdioxideinthewine,whichwillactprotectivelybyreactingwiththeproductsofoxidation.

SauvignonRepublic'sinternationalSauvignonBlancsallarepackagedwithscrewcaps.

Yeastleesalsoscavengeoxygenandprotectthewineinasimilarfashion,helpingtolowertheredoxpotentialandcreateamorereductiveenvironment.Inmodernwinemaking,reductiveconditionsareencouraged:Theprotectionofwinesfromoxygenbytheuseofstainlesssteeltanksandinertgaseshelpstopreservefreshfruitcharacters.Thesereductiveconditions--thoseinwhichoxygenismoreorlessexcluded--canalsofavorthedevelopmentofsmellyformsofsulfurcompounds.

Post-bottling,theredoxstateofthewinewillbeinfluencedbyanumberoffactors,includingthestateofthewineatbottling,thefreesulfurdioxidelevels,theoxygenpick-upduringthebottlingprocedure,headspaceextentandcomposition(airorinertgas),andtheoxygentransmissionbytheclosure.Aswe'veseen,reductionseemstobeaprobleminthesesortsofanalyticalstudiesinvolvingmetal-linedscrewcaps,andtheobviousexplanationisthatthelowredoxenvironmentofthescrewcap-sealedwineiscausingsomeunwantedsulfurchemistrytooccur,withsulfurcompoundsshiftingfromalesssmelly(andthusunnoticed)formtoamoresmelly(andthusnoticeable),morereducedform.Thisisassumingthewineisbottledclean,ofcourse.

AMinorTechnicalProblem?

Whatarewetomakeofscrewcapreduction?Isitarealworldproblemonaparwithcorktaint,orisitjustaminortechnicalproblem--ateethingissuethatjustneedsabitoftweaking?Thelatterpositionhasbeentheoneconsistentlyadoptedbyproponentsofscrewcaps.

SincethepublicationofthefirstAWRIreportin2001,therehasbeenjustatrickleofdataonthesubjectof

screwcapreduction.Butlittlebylittle,aclearerpicturehasemerged,andthecurrentweightofevidencesuggeststhattheissueofmercaptansinscrewcappedwinesisproblematicenoughthatsomecautionshouldbeexercisedintheiruse.Winemaker(Stonecroft,HawkesBay)andPh.D.chemistAlanLimmerhasbeenabitofathorninthesideofthescrewcaplobby.Hehaswrittenwidelyonthesubject,bringinghisknowledgeofwinechemistrytobear.Inparticular,Limmerhaspointedoutthatscrewcapreductionisnotaproblemthatcanbecompletelyeliminatedbybetterwinemaking,asmanyhaveclaimed.

\essencewearetalkingaboutthiolaccumulation,post-bottling,fromcomplexsulfidesthatdonotrespondtopre-bottlingcoppertreatment,\Limmerclaims,inresponsetotheassertionthatfiningwithcopperremoves

reductiondefects.\reactionoccurstoallwinescontainingtheappropriateprecursors,irrespectiveofclosuretype.Butthevaryinglevelsofoxygeningressbetweenclosuresleadstosignificantlydifferentoutcomesfromasensorypointofview.\

Limmer'sexplanationforscrewcapreductionisthatsulfidespresentinthewineatbottlingnecessitateaverysmalllevelofoxygeningressthroughtheclosure,otherwisetheycanbecomereducedtothiols.Becausesulfidesarelesssmelly,itispossibleforawinethatiscleanatbottlingtotastereducedafterbottlingiftheclosuredoesn'tpermitenoughoxygeningress.Sotheuseofaclosure,suchascork,whichdoesallowalittleoxygeningress(butnottoomuch)isanecessaryconcessiontothevagariesofsulfurchemistry.

\notthewinemaker'sfaultthesecompoundsexistinthewineatbottling.\--AlanLimmer,Ph.D.

Ofcourse,we'drathernothavethesulfidesinthewineatall,whichwouldthenavoidproblemswithreductiontomercaptansatalaterstage.But,asLimmerpointsout:\fermentstonotproducethecomplexsulfidesisbeyondourmeanscurrently.Thissulfidebehaviorofthefermentismorecontrolledbytheyeastgeneticsthanthewinemaker,\heexplains.

\isnotthewinemaker'sfaultthesecompoundsexistinthewineatbottling.Wecanminimizeittosomeextentbyprovidingoptimumnutrientconditionsfortheferment,andemployingsomespecificwinemakingregimes.But,theresearchtellsusthisonlyhasaslightimpactonthecomplexsulfidepatternproducedbytheyeast.\Limmerreinforceshispoint:\patternsarequitespecifictoeachyeasttype,almostirrespectiveofnutrientconditions.Everywinecontainsthesecomplexsulfides.\KeepitClean

Butothersthinkthatcaretakenduringwinemaking,andparticularatbottling,canreducetheriskofanyreductionsignificantly.Wisecounselwouldbethatwinemakersintendingtousetin-linedscrewcapsshould

ensurethattheyareincontrolofbottlingparameters.Itisimportanttogetthewinecleanbeforebottling,without

anytraceofmercaptanordisulfide.Ahealthyfermentshouldbethefirstpriority.

Whenitcomestobottling,freesulfurdioxidelevelsthatarealittlelowerthanthoseusedwithotherclosuresareadvisable,asisaknowledgeofthevariationinoxygenpick-uponthebottlinglineused.Itshouldbeemphasizedthatallowingmoreoxygenpick-uporleavingalargerheadspaceofairdoesnotcounterpost-bottlingreduction,butratherresultsinoxidation.

Copperfiningcanhelpinsomecircumstances:Thiscertainlygetsridofmercaptans,butitdoesn'teliminatedisulfideswhich,aswehaveseen,canrevertinalowredoxenvironmenttomercaptans.Besides,copperfiningwillalsoremovethedesirablesulfideswhichareimportantforvarietalcharacterinSauvignonBlancandothergrapes.

DoConsumersNotice?

Theextentofscrewcapreductioniscurrentlyunclear.Theonlylargesurveytolookforit,thefaultsclinicoftheUK-basedInternationalWineChallenge(IWC),reportedthat2.2%ofscrewcappedwinessufferedfrom

mercaptanproblemsin2006,andin2007provisionalresultswereslightlyup,at2.7%.Thissoundsalarming,butitshouldbeborneinmindthatcorktaintirredeemablyruinsbottlesitaffects,whileveryfewconsumerswillhaveaproblemwithlow-levelmercaptansintheirwines.

Idoubtthatmostofthewinetradewouldspotthisasaprobleminallbutthemostextremecases,soitisunfairtoequateitwiththeverywellrecognizedproblemofcorktaint.Havingsaidthis,though,screwcap-sealedwinesaffectedbymercaptansshouldbeamajorconcernforwinemakers,becausetheclosureismodifyingtheflavorofthewine,whichisemphaticallynotreachingtheconsumer\waythewinemakerintended.\

Itwouldbedangerouslycomplacentfortheindustrytotaketheviewthatiftheconsumerdoesn'tnoticeit,thenitdoesn'tmatter.Howdoyouspotlow-levelmercaptans?\impactfromanorganolepticperspectivetowardstheendofthepalate,\Limmersays,\a'mineral'orbitter/hard/astringentaspect.Thishasthe

appearanceofshorteningorclosingupthepalate,sothewinedoesnotdisplayafine,fresh,longfinish,butendsabruptly,andsomewhatharshly.\

Thepictureemergingisacomplexone,butasimplified\taintwine\messagewouldbedangerousifitcausedproducerstobackawayfromadoptingalternativeclosuresolutions,whichwouldthenhavethesecondaryeffectofremovinganyincentivefromthecorkindustrytoputitshouseinorderanddoallitcantoreducetaintlevels.However,complicationslikethisreductionissueshouldputpressureonwinemakerstobemorecuriousabouttheclosurestheyareusing,andpaymoreattentiontothepotentiallossofqualitythatcanoccurduringthebottlingprocess.