The Actual and Potential Aroma of Winemaking Grapes
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biomolecules Review The Actual and Potential Aroma of Winemaking Grapes Vicente Ferreira * and Ricardo Lopez Laboratory for Aroma Analysis and Enology (LAAE), Department of Analytical Chemistry, Universidad de Zaragoza, Instituto Agroalimentario de Aragón (IA2) (UNIZAR-CITA), c/Pedro Cerbuna 12, 50009 Zaragoza, Spain; [email protected] * Correspondence: [email protected]; Tel.: +34-976-762-067 Received: 8 November 2019; Accepted: 30 November 2019; Published: 3 December 2019 Abstract: This review intends to rationalize the knowledge related to the aroma of grapes and to the aroma of wine with specific origin in molecules formed in grapes. The actual flavor of grapes is formed by the few free aroma molecules already found in the pulp and in the skin, plus by those aroma molecules quickly formed by enzymatic/catalytic reactions. The review covers key aroma components of aromatic grapes, raisins and raisinized grapes, and the aroma components responsible from green and vegetal notes. This knowledge is used to explain the flavor properties of neutral grapes. The aroma potential of grape is the consequence of five different systems/pools of specific aroma precursors that during fermentation and/or aging, release wine varietal aroma. In total, 27 relevant wine aroma compounds can be considered that proceed from grape specific precursors. Some of them are immediately formed during fermentation, while some others require long aging time to accumulate. Precursors are glycosides, glutathionyl and cysteinyl conjugates, and other non-volatile molecules. Keywords: wine aging; glycosides; glutathione; mercaptans; terpenols; norisoprenoids; volatile phenols; vanillin 1. Introduction Winemaking grapes are quite unique fruits because they are grown not to be immediately consumed, but to make wine with them. From this point of view, the study of grape aroma cannot be limited to the pool of molecules directly responsible for the odors and flavors of grape and grape juice but has also to include those other chemical structures that, more or less directly, are specific precursors of relevant wine aroma molecules. This task began more than 40 years ago when French and Australian researchers reported the existence of glycosides and other precursors of linalool [1,2]. The task, however, has proved to be extremely difficult due to many factors, such as the chemical and biochemical complexity of the precursor systems, the long times required to see aging effects in wine, or the analytical challenges associated to obtaining reliable representations of wine sensory properties from analytical data [3,4]. The truth is that nowadays, in spite of many significant advances, there are not accurate criteria or accepted methods able to provide a reliable assessment of the grape aroma potential, except perhaps for aromatic varietals such as Muscat or Gewürztraminer. This is a bit of a paradox; the grape genome was untangled more than 10 years ago [5], but yet, we do not have a clear understanding of all the grape metabolites which will ultimately contribute to the aromatic sensory properties of wine. The reasons for this rather sluggish progress in linking grape molecular systems and wine odorants can be better understood with the help of the schema in Figure1. The schema shows that grape contains at least seven different systems or pools of aroma precursors. Two out of the seven have Biomolecules 2019, 9, 818; doi:10.3390/biom9120818 www.mdpi.com/journal/biomolecules BiomoleculesBiomolecules2019 2019, 9, 818, 9, 818 2 of2 35of 36 have relevance in grape but are not particularly important in wine aroma (the Strecker amino acid relevancesystem inand grape the butfatty are acid/peroxygenase not particularly important system), inwh wineile the aroma other (the five Strecker play essential amino acid roles system in the anddevelopment the fatty acid of/peroxygenase wine varietal system),aroma during while thewine other aging, five and/or play essential in the development roles in the development of wine flavor of winenotes. varietal If at the aroma light duringof our present wine aging, understanding, and/or in the the development different analytical of wine flavorstrategies notes. and If concepts at the lightapplied of our along present the understanding, years for the study the diofff grapeerent analyticalaroma precursors strategies are and revisited, concepts it will applied become along evident the yearsthat for they the provide study of information grape aroma covering precursors a rather are revisited, limited fraction it will become of wine evident varietal that aroma. they In provide fact, the informationgeneral strategy covering followed a rather to limitedanalyze fraction grape glycosidic of wine varietal precursors aroma. deals In with fact, precursors the general belonging strategy to followedjust one to or analyze two out grape of the glycosidic five pools. precursors This is dealsnot to with blame precursors previous belonging research, tomost just of one which or two was outbrilliantly of the five carried pools. out This by is notpioneers, to blame but previous to acknowledge research, the most difficulties of which of was the brilliantly study, which carried with out the bylimited pioneers, analytical but to acknowledge tools available the in ditheffi 1980s,culties 1990s, of the and study, even which the 2000s, with the hardly limited could analytical have been tools done availableany better. in the 1980s, 1990s, and even the 2000s, hardly could have been done any better. Glutathionyl and cysteinyl DMS precursors Glutathionyl DMS precursors buccal and cysteinyl precursors time enzymes precursors Glycosides Wine Fatty acids + of aroma enzymatic/cat Grape Free varietal molecules alytic systems Aroma aroma time Glycosides of buccal time Strecker precursors of enzymes amino acids aroma Glycosides Precursors Other molecules of aroma of aroma precursors molecules molecules time ©V.Ferreira Glycosides of precursors of aroma molecules FigureFigure 1. Scheme1. Scheme showing showing the the main main systems systems/pools/pools in grapein grap ofe specificof specific precursors precursors of aromaof aroma molecules molecules andand their their involvement involvement in thein the development developmen oft wineof wine varietal varietal aroma aroma and and flavor. flavor. TheThe two two first first systematic systematic approaches approaches developed developed to studyto study grape grape aroma aroma precursors, precursors, which which are are yet yet thethe basis basis of of the the methods methods in in use use at present,at present, were were developed developed by by Patrick Patrick Williams Williams and and coworkers coworkers in in AustraliaAustralia [6] and[6] and by Ziya by Ziya Gunata Gunata and coworkers and coworkers in Montpellier in Montpellier [7]. In these [7]. approaches,In these approaches, grape glycosil grape aromaglycosil precursors aroma precursors are extracted are from extracted grape from must grape or macerated must or macerated grape solids grape with solids C18 orwith with C18 XAD-2 or with polymericXAD-2 polymeric sorbents, respectively. sorbents, respectively. Much later, Much the use later, of more the use advanced of more polymeric advanced sorbents polymeric providing sorbents a widerproviding extraction a wider of precursorsextraction wasof precursors proposed was [8], althoughproposed as [8], noted although by Hampel as noted et al.,by noHampel sorbent et wasal., no effectivesorbent for was all glycosideseffective for [9 ].all The glycosides glycosidic [9]. fractions The glycosidic are further fractions hydrolyzed are further well by hydrolyzed acid hydrolysis well by andacid enzymatic hydrolysis treatment and enzymatic [6], and welltreatment exclusively [6], an byd well enzymatic exclusively treatment by enzymatic [7]. treatment [7]. TheThe advantage advantage of of enzymatic enzymatic treatment treatment is that,is that, in in comparison comparison to to acid acid hydrolysis, hydrolysis, it providesit provides a a relativelyrelatively unbiased unbiased composition composition of of the the aglycones aglycones present present in in the the extract, extract, as as far far as as the the correct correct type type of of enzymeenzyme is usedis used [9]. [9]. Under Under this this approach approach the the aroma arom ofa of grape grape is dividedis divided into into the the free free and and the the bound bound fractionsfractions [10 [10,11].,11]. Its Its major major disadvantage disadvantage is that,is that, in manyin many cases, cases, the the aglycone aglycone is notis not an an odorant odorant relevant relevant forfor wine wine aroma, aroma, but but an an aroma-worthless aroma-worthless volatile volatile compound compound such such as as benzyl benzyl alcohol alcohol or or an an odorless odorless precursorprecursor that that only only after after a seriesa series of of reactions, reactions, which which can can take take a longa long time, time, will will form form the the odorant. odorant. AttendingAttending to theto the scheme scheme shown shown in Figurein Figure1, enzymatic 1, enzymati hydrolysisc hydrolysis provides provides a useful a useful estimate estimate of wineof wine aromaaroma molecules molecules derived derived from from the the pool pool of “glycosidesof “glycosides of aromaof aroma molecules”, molecules”, but but not not of thoseof those derived derived fromfrom the the pool pool of “glycosidesof “glycosides of of precursors precursors of of aroma arom molecules”a molecules” or or from from the the other other pools pools of