8 Management of Vinification andStabilization to Preserve the AromaCharacteristic of Dehydrated Grape
Luigi Moio and Paola PiombinoDepartment of Food Science, University of Naples Federico II, Portici, Italy
8.1 INTRODUCTION
Wine aroma genesis largely depends on numerous and complex biochemical transformationsdeveloping during winemaking: oxidative and hydrolytic reactions arising during the pre-fermentative stages of grape pressing and maceration; primary and secondary metabolismsof micro-organisms conducting the alcoholic and malolactic fermentations; oxidation andtransfer processes occurring during wine maturation in wood; and chemical and enzymaticpost-fermentative reactions taking place during wine storage and bottle ageing. Alongsidethese sources of volatiles linked to the transformation and ageing processes, grape charac-teristics play a primary role in determining the quality and the evolution of wine aroma. Thisis true even though many wine aroma characteristics are not sensorially detectable in thegrapes. Among the volatile compounds that contribute to wine aroma, grape odour activemolecules play a determining role as they are responsible for the so-called ‘varietal aroma’,which represents an added value to wine quality. There is a substantial difference in theway varietal aroma forms in wines from ‘aromatic grapes’ or ‘neutral grapes’. Most of thevolatiles responsible for varietal notes in wines from aromatic cultivar are already presentat high levels as free odour active molecules in the must, which is characterized by odoursreminiscent of the grape variety. Musts from neutral grapes, however, are not recognizable,but are essentially dominated by herbaceous odours due to aldehydes and alcohols C6derived from fatty acids. Their varietal aroma will be expressed only after alcoholicfermentation and/or ageing, as secondary metabolites of biochemical transformations ordue to the release of volatiles from non-volatile aroma precursors. As a consequence ofthis knowledge, all factors affecting grape quality can potentially influence wine aromacharacteristics.
On-plant and postharvest dehydration importantly affect grape quality because the stressdue to berry water loss induces marked and variable effects on fruit physical structure,metabolism, chemical and biochemical composition. All these changes have both direct andindirect influences on the aroma of the final wine.
Wines from dehydrated grapes differ not only because of the grape-dehydration systems,which often reflect unique climatic and environmental conditions, but also due to the degreeof grape dehydration, the employment of different grape varieties (aromatic and/or neutral;white or red) and the vinification method, which is often the consequence of traditionalpractices.
All this variability (from field to winemaking) in the production of wines from with-ered/dehydrated grapes, gives rise to different wine styles corresponding to specializedproducts characterized by very distinctive sensory characteristics, which have little in com-mon other than high sugar concentration in the corresponding musts. This common featureprovides unusual conditions for yeast growth with a consequent impact on flavour pro-duction during fermentation. For this reason, in wines obtained from dehydrated grapes,the equilibrium between fermentative and grape aroma shifts towards the latter, and grapevarietal aroma, together with volatiles produced during dehydration, becomes even moreimportant than in table wines. As a consequence, all winemaking practices, from field tobottle, encouraging the release and the preservation of these aromas from dehydrated grapesto wine, are suitable to make products characterized by a complex and specific aroma. Thisflavour complexity is needed not only because wine quality is directly correlated with itsaroma richness, but also because this complexity must counter-balance the olfactory impactof acetaldehyde, acetic acid and ethyl acetate. These compounds may be present at high levelsin wines obtained from dehydrated grapes, and originate from both anaerobic metabolism,to which grapes shift during water loss, and unfavourable fermentative conditions.
This chapter provides an overview of the current knowledge available to optimize themanagement of vinification and stabilization to preserve the aroma characteristic of dehy-drated grapes. In particular, the chapter describes how the main stages of winemaking canbe enhanced in order to maximize the extraction and preservation of free and bound aromasfrom dehydrated grapes.
8.2 VOLATILE COMPOSITION OF WINES FROMDEHYDRATED GRAPES
Although over the years, several authors have investigated the volatile fraction of differentstyles of dessert wines, to the best of our knowledge, the study of the volatile fraction ofsound dehydrated winegrapes under different conditions, and of their musts, started less than10 years ago (Franco et al., 2004; Bellincontro et al., 2004; Costantini et al., 2006; Chkaibanet al., 2007; Moreno et al., 2008; Ruiz et al., 2010; Santonico et al., 2010; Piombino et al.,2011; Lopez de Lerma et al., 2012). The dehydration process markedly affects the primaryaroma compounds, both free and glycosylated, that are present at different concentrationsand ratios in the skin and in the pulp as described in Chapter 5. From the vinification pointof view, it is important to stress that all free volatiles are more concentrated in the skin, withthe exception of anaerobic metabolism derivatives that are mainly present in the pulp.
The aroma descriptors generally cited to describe a dessert wine are apricot, peach,coconut, exotic fruit, orange zest, candied fruit, dried fruit (apricot, fig, prune, walnut),citrus blossom, floral, marmalade, honey, caramel. The volatile compounds identified indessert wines of different styles (from non-botrytized or partially infected grapes) belong toseveral chemical classes and those that have the greatest impact seem to be: ethyl acetate,acetoin, phenylacetaldehyde, benzaldehyde, 1-octen-3-ol, -butyrolactone, -hexalactone, -nonalactone, -decalactone, -decalactone, linalool, geraniol, nerol, 4-terpineol, �-ionone,
Preserving the Aroma Characteristic of Dehydrated Grape 133
apple (ethyl acetate)
A
B
butter (diacetyl)kiwi (ethyl butanoate)
red fruit (ethyl 2-methylbutanoate)exotic fruit (ethyl 3-methylbutanoate)
Figure 8.1 Aromagrams of sweet Fiano wine (A) and base Fiano wine (B) obtained by GC/O analysis;NI, not identified (from Genovese et al., Copyright 2007, with permission from Elsevier).
�-damascenone, furfural, 5-methylfurfural, furaneol, homofuraneol, sotolon (Moreno et al.,2008; Genovese et al., 2007; Franco et al., 2004; Castellano et al., 2001). Most of thesemolecules are detectable also in wines obtained from fresh sound grapes, but at lower levelsand consequently with less olfactory impact. Figure 8.1 shows the comparison between thearomagrams of two Fiano wines: the sweet wine (A) and the base wine (B) (Genovese et al.,2007). The profiles significantly differ both in the number and in the intensity of the odour
134 Sweet, Reinforced and Fortified Wines
Figure 8.2 Quantitative descriptive odour profiles of sweet Fiano wine (A) and base Fiano wine(B) (adapted from Genovese et al., 2007).
zones, with the sweet wine presenting a greater richness. Nineteen odour compounds detectedduring gas-chromatography/olfactometry (GC/O) analysis of the sweet Fiano differed fromthe base wine, and the most important were: ethyl 3-methylbutanoate (exotic fruits), 1-butanol (grass), 1-octen-3-ol (mushrooms), vitispirane (camphor), linalool, nerol and geran-iol (orange flowers), -nonalactone (coconut), and -decalactone (apricot), eugenol andisoeugenol (clove) and phenyl acetic acid (honey). The other 11 odour peaks of sweet Fianowine were not identified. They showed floral, apricot, plum, citrus jam, liquorice, almond,pepper and medicinal odours. Over-ripeness and dehydration significantly modify the flavourcharacteristics of the corresponding wine, as shown in Figure 8.2 where the sensory profilesof sweet (A) and base (B) Fiano wines were reported. The main influence of Fiano grapeover-ripeness and dehydration on the corresponding wine appears to be the enrichment ofdried fruits (apricot, fig, prune), citrus jam, honey and coconut odours. If further investigationseems necessary, these and other results by authors cited earlier show that among the com-pounds that derive from grapes, terpenes and C-13 norisoprenoids notably contribute to thearoma of most dessert wines. It is well known that terpenes concentration increases duringripening and after maturity fall off (Wilson et al., 1984). The higher levels usually detectedin wines from dehydrated grapes could be affected also by a better extraction as a conse-quence of the structural degradation of the skin, where most of the terpenes are located. C-13norisoprenoids have an important role, because they are molecules characterized by very lowodour thresholds. These compounds come from carotenoids, unstable compounds whosedegradation can occur in the presence of oxygen, high temperatures and exposure to the sun(Rapp and Marais, 1993). Both terpenes and C-13 norisoprenoids are present in grapes alsoas glycoconjugated compounds, and their high levels in dessert wines may be due to majorconcentration in over-ripe and dehydrated grapes and to higher contents of their glycosidicprecursors. The extraction and preservation of these compounds in their free and boundforms should represent one of the main targets during winemaking of dehydrated grapes.
8.3 VINIFICATION
Several styles of wines can be obtained from dehydrated grapes. They may be sweet, slightlysweet or dry, white, rose or red and several methods are used to produce them. The different
Preserving the Aroma Characteristic of Dehydrated Grape 135
techniques often reflect environmental conditions, traditional practices, or are the evolutionof fortuitous events, or are a modern development in response to consumer preferences. Forthis reason, there are different dessert wine styles corresponding to specialized products pos-sessing distinctive characteristics. Variations in style depend not only on grape variety anddehydration methods, but also on the winemaking process, that is, practices applied before,during and after fermentation. Of course, it is not fair, nor yet possible, to standardize thewinemaking process of this type of wine, but some physical-chemical and compositionalcharacteristics, common to juices obtained from dehydrated grapes, universally require spe-cial attention during the pre-fermentative, fermentative and maturation stages of dessertwines production. In the following sections we mainly refer to the production of a whitedessert wine.
8.3.1 Pre-fermentative phases
Managing pre-fermentative treatments is crucial for the preservation of the aromas resultingfrom dehydrated grapes and will indeed influence the flavour of the final wine. The crushing ofdehydrated grapes represents a critical point in the production of high-quality dessert wines.Managing this step must be oriented to increase must yield and grape aroma extraction, butat the same time, prevent and minimize enzymatic oxidation due to polyphenol oxydases andincreased amounts of suspended solids in the juice. As a rule, any rough manipulation (fromfield to cellar) of dehydrated grapes should be avoided in order to prevent the excessive releaseof suspended solids causing instability, difficult clarification and vegetal off-flavours. This isparticularly true if part of the grapes is botrytized, because of the diffusion of glucan in themust (Ribereau-Gayon et al., 2006). This polysaccaride is located on the skin/pulp interfaceand, thanks to its colloid protector properties, interferes with settling of must during the pre-fermentative phases and with clarification of the future wine. The first crushing results in theextraction of most of sugars that could be enhanced by increasing the contact time betweensolid and liquid parts of the berries during the pre-fermentative operations. The juice recoveryis favoured by the draining effect of the stems, which facilitate the passage of liquid throughthe pressed skins. In the specific case of dehydrated grapes, the risk of off-taste and off-odour,deriving from tannic and vegetal substances in the stems, is minimized by their lignificationarising during dehydration and by the necessity for a careful pressing. Dehydrated grapes needpowerful, but at the same time, slow and delicate treatments. A recent study on the modifica-tion of mechanical characteristics and phenolic composition in berry and seeds of Mondeusewine grapes during on-vine dehydration for the production of Ice wines (Rolle et al., 2009),reported that: berry skin thickness and hardness tended to increase progressively duringwithering, and its springiness also increased; seed hardness decreased during withering whilespringiness increased; at the end of the withering process, the percentage contribution of tan-nins extracted from seeds to total tannins was higher. These are among the dehydrated grapescharacteristics that drive the pressing modality. A mighty force is necessary to break the harderwithered skins; slower pressing allows a more complete extraction of flavours and antho-cyanins from harder skins (Rolle et al., 2008); a controlled pressing process, especially fordessert wines, minimizes tannins extraction from seeds, which significantly affect sensorialproperties (astringency and bitterness) of wines (Gambuti et al., 2006). Pneumatic pressesgive high quality musts, but due to the high density of juices from dehydrated grapes, versionswith greater than 3 bars of strength may be insufficient for a proper extraction of juices withthe highest sugar levels (above 22–23% volume of potential alcohol) (Ribereau-Gayon et al.,
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2006). Vertical presses are recommended in order to favour juice extraction, and mainly inthe final step, hydraulic presses can improve the yield. For these reasons, in recent years pro-ducers of dessert wines have gone back to using vertical presses, and particularly those withlow capacity (5–15 hL), as the pressing of several small aliquots, ensures a more efficientextraction of the juice. Pressing must be slow, long and constant, avoiding the shredding ofdehydrated grape berries and minimizing the pressing cycles in order to control suspendedsolids and entrance of oxygen. The presence of oxygen during pre-fermentative operationscauses the enzymatic oxidation of phenolic compounds due to endogenous tyrosinase, anda consequent browning of the juice, the extent of which also depends on the grape variety.Dehydrated grapes could be particularly susceptible to this phenomenon due to the senes-cence and de-structuring of berry tissue as a consequence of water loss. Even more browningshould be expected in musts from botrytized grapes because of the co-presence of laccaseproduced by the mould, but this is not frequently observed. Often, for a better extraction ofnoble rot juice, two to three pressing cycles and the manual breaking up of the press cakebefore each pressing start could be useful. Nevertheless, this practice could be hazardous inthe case of dehydrated grapes only partially infected by Botrytis cinerea. In totally botrytizedjuice, the risk of oxidation during pre-fermentative operations is lower because polyphenolswere jet oxidized by laccase directly into the berry during the mould growth. However, insound dehydrated grapes only partially infected by noble rot, the substrate for laccase activityis available, so that in this kind of juice the entrance of oxygen increases the risk of enzymaticoxidation. Laccase is a more general and active enzyme than tyrosinase and other phenoloxidases. This enzyme is particularly dangerous because it is active and stable at must pHvalues; it is not inhibited by SO2 at the levels used in winemaking, is able to consume dis-solved oxygen, and is reactive towards the quinone-glutathione browning protective complex(Ugliano, 2009). The most effective way to control its activity is by decreasing juice tem-perature and aeration (Boulton et al., 1999). According to the results reported by Selli et al.(2011), pressing uniformly increased the levels of aromatic constituents, but this treatmentlowered the grape juice quality for winemaking by increasing the total phenolic compounds,browning index, and C6-alcohol levels (green-herbaceous odour). Moreover, even if resultsreported by Moio et al. (2004b) were on wines from sound fresh grapes, these authorsdemonstrated that the protection of the must against oxidation gives wines with higher levelsof volatiles responsible for fruity aromas. On the contrary, hyperoxygenation provides wineswith increased colour stability (Schneider, 1998), but it seems to have a negative effect on thevarietal character of wine (Dubourdieu and Lavigne, 1990) becoming undesirable in winesthat emphasize the contribution of the grape (Boulton et al., 1999), as is the case in wines fromdehydrated grapes.
In order to aid the production of high-quality musts from dehydrated grapes, minimizesome risks linked to the pre-fermentative steps, maximize juice yield, and favour free andbound aromas extraction, it could be very useful to add enzymes in the production process ofdessert wines. Enzymes are able to help the grape must yield during pressing as they favourthe settling of musts, and improve clarification and filtration. Different types of enzymesare used in winemaking (pectinase, cellulase, hemicellulase, oxidoreductase, protease,�-glycosidase) and their principal and secondary activities significantly affect the qualityof the final wine (Guerin et al., 2009). For this reason, a careful selection of commercialenzymes is necessary and knowledge of their specific effect on different grape varieties,which may vary according to the specific condition of transformation. A recent paper (Espejoand Armada, 2010) evaluated the effect of pectolytic enzymes addition in the making ofPedro Ximenez sweet wines, using dynamic pre-fermentative maceration (room temperature,
Preserving the Aroma Characteristic of Dehydrated Grape 137
3 hours). Treated wines varied significantly with respect to the control sample in total solublesolids (� ◦Brix), total juice (� yield) and final sensory characteristics (� flavour intensity,flavour quality, aroma intensity, equilibrium; � astringency; = herbaceous); on the contrary,no effects on the content of total polyphenols and other chemical characteristics of the mustswere observed. The employment of selected pectolytic enzymes represents an effectivemeans of improving aroma and taste quality of dessert wines. The enzyme solution mustbe added at the beginning of the pre-fermentative operations, when the skins and pulps ofcrushed dehydrated grapes are in contact with their own juice prior to pressing: 1–2 g hL−1
directly added during the grape must and homogenized in a rotating macerator to achieve thebest distribution.
Together with pectolytic enzymes, in the winemaking of some dessert wines, the pre-fermentative maceration also constitutes a valid means to obtain high-quality products fromdehydrated grapes. On the one hand, the pre-fermentative maceration enhances the extrac-tion of desirable compounds from grapes (sugars, free and bound aromas), and on the otherhand, in the absence of ethanol, there is a relatively low extraction of tannin compounds andflavan-3-ols (Salinas et al., 2003; Pinelo et al., 2006). Pre-fermentative maceration may becarried out at room temperature, but in modern enology, the pre-fermentative maceration atlow (∼5◦C) or very low (≤0◦C) temperatures is here to stay, and Amarone is a prestigiousexample of dessert wine that is produced using cryomaceration (Paronetto and Dellaglio,2011). Regarding another renowned Italian wine produced from sun-dried grapes, Piombinoet al. (2010) reported that as an effect of cold-maceration (6◦C, 12 hours), both total freeand glycoconjugated aromas of Malvasia delle Lipari wines increased by 45% and 36%,respectively. Among free volatiles, alcohols increased by 79% (mainly due to 3-methyl-1-butanol and 2-phenylethanol) while esters were 14% lower, probably due to grape solids inthe must, which limit esters and enhance alcohols production during alcoholic fermentation(Moio et al., 2004a). The cold-macerated Malvasia was richer in the following free ter-penols: �-citronellol (96%), farnesol (69%), and 3,7-dimethyl-1,5-octadien-3,7-diol (21%).The latter compound is considered to be one of the key aromas of Malvasia delle Liparipassito wine (Guarrera et al., 2005). With regard to the increased amount of glycoconjugatedvolatiles, bound terpenoids were the most affected by cold skin contact increasing by 44%and becoming 78% of the total bound compounds detected in the cryomacerated sample.Four terpenols significantly increased: linalool (67%), �-terpineol (37%), geraniol (33%),nerol (21%); only epoxylinalool decreases by 52%. The potential varietal aroma of the Mal-vasia delle Lipari was increased by cryomaceration, which augmented the concentration ofbound terpenoids. Sensory analysis showed that cryomaceration of Malvasia grapes affectedthe aroma of the corresponding wines and perceptible odour modifications occurred. Theseresults show that cryomaceration is an interesting means of managing the aroma profile ofsweet wines, offering the advantage of preserving grapes’ varietal compounds.
At this stage of winemaking, a light juice sulfiting is recommended. A concentrationranging from 30 to 50 mg L−1, depending on grape health, generally represents a goodequilibrium between enological requirements and legal total SO2 limits. Sulfiting juice fromdehydrated grapes leads to high levels of bound SO2 in the final wine, due to interactionwith higher presence of acetaldehyde (from both grape and unfavourable fermentation con-ditions). In grapes with noble rot, some compounds produced by the infection ( - and-gluconolactone produced by gluconic acid, combined with the bacterial byproducts 5-oxofructone and dihydroxyacetone), act as the principal SO2-binding compounds in mustand wine (Barbe et al., 2002). Moreover, thiamine deficiency due to Botrytis infectioncauses accumulation of pyruvic acid, which is responsible for much of the non-acetaldehyde
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sulphite binding pool (Boulton et al., 1999). Fortunately, only a portion of SO2 added to juiceis found in the wine as bound form, because generally at least 50–60% is in the free form.Juice sulfiting has more crucial effects, some of which become even more important duringvinification of dehydrated grapes: protection against excessive oxidation of grape phenolicsubstrates; inhibition of the proliferation of spoilage micro-organisms, all forming byprod-ucts responsible for wine off-flavours and quality depletion, as well as of the development ofacetic and lactic bacteria; blockage of fermentation for several hours, enhancing clarificationby natural sedimentation.
The spontaneous settling of musts from dehydrated grapes is not easy. The level of sus-pended solids is high as a consequence of berry tissues degradation due to over-ripeningand water loss, and also by the presence of glucan in the case of Botrytis-infected grapes.Moreover, the high sugars concentration increases must viscosity. Therefore, natural set-tling is difficult as the suspended solids and high density must present comparable specificweights. The addition of selected pectolytic enzymes at the beginning of pre-fermentativeoperations when skins and pulps are crushed, accelerates must clarification. This, togetherwith controlled temperature (8–12◦C) as an accelerator of phases separation, leads to therecovery of spontaneous clarified musts within 24–72 hours. Must turbidity will not be aslow as in dry white wine production (80–120 NTU), but a value between 500–600 NTU isabsolutely appropriate. The improvement of clarification efficiency by adding fining agentsis not necessary, and on the contrary, it could be detrimental. Indeed, from the sensory pointof view, dessert wines are less subject to development of reduction and vegetal off-odoursderiving from insufficient clarification, due to the masking effect of the aroma richness.Moreover, Moio et al. (2004a) evaluated the impact of several clarification treatments on thearoma of Falanghina must and wine. Results showed that glycosylated precursors of linalool,geraniol, benzyl alcohol, 2-phenylethanol and eugenol were significantly decreased by mustclarification with fining agents, which also affected flavour production during fermentation.The authors concluded that pre-fermentative clarification might influence both expressionof varietal character and ageing potential of the treated wine. Moreover, as mentioned bythe same authors, clarification by fining agents could significantly affect the composition ofmust, reducing the concentration of some constituents important for yeast growth, such aslong-chain fatty acids and sterols, and reducing the level of amino nitrogen. It could alsoaffect nutrient depletion. For all these reasons, the best way to preserve grape aroma is toclarify musts from dehydrated grapes by the spontaneous settling that could be favoured byadding pectolytic enzymes at the beginning of the pre-fermentative stage.
8.3.2 Fermentation
After decantation (24–72 hours at 8–12◦C), juice obtained from dehydrated grapes (500–600 NTU) is on its way to becoming wine through the alcoholic fermentation. The fermen-tation of dehydrated grape juices is generally slower than fermentation of musts from soundgrapes vinified at their technological maturity. This is due to the sluggish rate of yeast growthas a consequence of multiple inhibitions of the substrate. The high sugar content is theprincipal limiting factor, but the nutrient deficit, especially in the case of botrytized grapes,also contributes to slowing alcoholic fermentation. At this stage of dessert wine production,the main target in order to preserve the aroma quality should be the contraction of the yeastsgrowth latency. In dessert wines the so-called ‘fermentation aroma’ plays second fiddle toaromas originating from dehydrated grapes, because their production is not supported by
Preserving the Aroma Characteristic of Dehydrated Grape 139
the physical-chemical and chemical characteristics of the must. For this reason, it is veryimportant to manage the alcoholic fermentation in order to control the spread of off-flavours,and prevent too great a rise in acetaldehyde, acetic acid and ethyl acetate. This is possible byrefining fermentation conditions by inoculating with selected yeast strains, adding nutrientsand controlling temperature.
As long ago as 1935, Hopkins and Roberts (1935a, b), showed that at the sugar concentra-tion normally found in late-harvest musts (250–300 g L−1), the fermentation rate decreases atleast by 20% only because of sugar levels. It has been largely reported that increasing sugarlevels in fermentative media reduces yeast cell size, cell growth, viable cell concentration,and fermentation activities (Charoenchai et al., 1998). During yeast growth in dehydratedgrape musts, they are exposed to a strong osmotic stress. Erasmus et al. (2003) reported thatthe transcription of 589 genes in wine yeast was at least twofold lower in must containing40% (w/v) sugar (water activity ∼ 0.939) when compared to 22% (w/v) sugar (wateractivity ∼ 0.981). As a consequence of osmotic stress due to high sugar levels, the structuralgenes concerned with the synthesis of acetic acid from acetaldehyde and of glycerolfrom dihydroxyacetone phosphate is up-regulated. Yeast cells produce more glycerol as aresponse to osmotic stress, as it limits water loss from the cytoplasm preventing dehydrationof the yeast. The production of acetic acid has been correlated by different authors (Remizeet al., 1999) to the over-production of glycerol. The shift in redox balance (NADH/NAD+)caused by the rise of glycerol is compensated by using acetic acid to transform NAD+ backto NADH. The acetic acid is produced by yeasts through the oxidation of acetaldehyde toacetate. A recent study on Ice wines (Erasmus et al., 2004) has shown that different yeaststrains in the same condition of osmotic pressure, respond differently to stress, producingdifferent concentrations of glycerol and acetic acid. The same paper also shows that, thehigher the sugar concentration is (high osmotic stress), the more acetic acid and glycerol areproduced. They also found a positive correlation between acetic acid and ethanol formation,indicating that as more sugar is consumed, the yeast could potentially form more aceticacid. Therefore, the choice of yeast strain may strongly affect the overall sensory quality ofdessert wines, mainly as a function of the initial juice sugar content. This is true for differentreasons: first, because an overproduction of acetic acid is obviously detrimental for a dessertwine; second, because too high a level of ethanol reduces the headspace concentration ofsome volatile aroma compounds, likely suppressing their perception during wine tasting(Robinson et al., 2009). In descending order, the importance of each wine matrix-volatileinteraction is ethanol � glucose � glycerol � catechin. For 20 volatiles, also includingsome esters (isoamyl acetate, ethyl esters), terpenes (linalool, nerol) and norisoprenoids (�-damascenone, �- and �-ionone), increasing ethanol in the matrix was negatively correlatedwith the analyte peak area and was linear over the range 10–18% v/v. In the light of this,from an aromatic point of view, it is very important that the fermentation of high sugar mustsfrom dehydrated grapes should be stopped as soon as sufficient ethanol (13.5–14.5 % v/v)is produced.
The winemaking of most wines produced from dehydrated grapes is usually strictly linkedto traditional procedures that in practice bring a faint control of fermentation parameters,such as microbial population and temperature. In spite of this, inoculation of the musts withselected yeasts is strongly recommended. The principal intent of this operation is a bettercontrol of the fermentative process in order to control the volatile acidity and optimize thewines’ quality level. The criteria followed to choose a good selected yeast strain for theproduction of a dessert wine are: high tolerance to sugar and ethanol; low production ofacetic acid in difficult fermentation conditions; low production of SO2-binding compounds
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such as acetaldehyde; low demand of nutrients (nitrogen, vitamins) mainly if grapes arepartially infected by B. cinerea; sensibility to SO2 at levels necessary to stop the alcoholicfermentation (∼100 mg L−1) in order to allow easy mutage.
Inoculation modalities also have a significant impact on the fermentation of highlysugar concentrated juice obtained from dehydrated grapes. Kontkanen et al. (2004) studiedthe effect of yeast inoculation rate, acclimatization, and nutrient addition on Ice-winefermentation. The authors showed that fermentation at the lower inoculation rate (0.2 g L−1)consumed less sugar. In these conditions, cells converted a higher proportion of sugar con-sumed to acetic acid and glycerol in comparison to cells at a higher (0.5 g L−1) inoculationrate. The main impact of acclimatization with respect to the direct inoculation of rehydratedyeast into the juice was higher biomass and viable cell concentration, which allowed for moresugar to be consumed in a shorter time and higher ethanol production. The addition of theyeast nutrient during yeast rehydration increased the rate of biomass accumulation, reducedthe fermentation time, reduced the ethanol concentration in the Ice wines, and reduced therate of acetic acid produced as a function of sugar consumed. Such nutrient supplementation(vitamins and minerals) at the yeast rehydration stages improved cell viability, reducingvolatile acidity and off-odours. When necessary, the addition of nitrogen to fermentationsalso reduces sulfur-like off-odour due to high hydrogen sulphide, production of which isassociated with sluggish or stuck fermentations caused by a lack of nitrogen in the must. Lowmust yeast assimilable nitrogen (YAN) leads to higher alcohols, and low production of estersand long-chain volatile fatty acids. High must YAN leads to a rise of ethyl acetate, aceticacid and volatile acidity due to increased biomass and higher fermentation temperature as aconsequence of the vigour. Too high YAN increases the concentration of proteins inducinghaze, urea, ethyl carbamate, biogenic amines, and the risk of microbial instability, potentialtaint from botrytized bunches and atypical ageing character (Bell and Henschke, 2005). Inter-mediate must YAN favours the best balance between desirable and undesirable chemical andsensory wine attributes, and in harmony with Bely et al. (2003), in high Brix musts a correctlevel of nitrogen could be 160–200 mg L−1. Another survival factor is oxygen. Ribereau-Gayon et al. (2006) suggest that in sweet wine production, a late aeration prevents increasesin volatile acidity, and for this reason oxygen should be introduced during the stationaryphase of the yeast growth, rather than during the growth phase. This is particularly truewhen sweet must is not fermented in wood, but when fermentation occurs in tanks in stricteranaerobic conditions.
Finally, temperature control is also a crucial factor in managing fermentation of dehy-drated grapes juice. As reported earlier, the composition of this kind of juice is characterizedby different inhibitory factors affecting alcoholic fermentation. As a consequence, the settingof a constant temperature (18–20◦C) represents a means to allow a good rate of alcoholicfermentation in musts from dehydrated grapes. Extreme temperatures during fermentationcan severely affect yeast growth and metabolism, also affecting sensory characteristics ofthe final wine. Too low a temperature prolongs yeast growth latency causing a rise in volatileacidity, ethyl acetate and acetaldehyde, and affecting yeast population dynamics, mainlyduring spontaneous fermentation. Too high a temperature during fermentation, influencesyeast ethanol resistance because the cell membrane fluidity increases and ethanol can enterthe cell more readily, adversely affecting metabolism and cell viability. Moreover, toleranceto both ethanol and temperature is also very strain-dependent (Bisson, 1999). At this point,as soon as sufficient ethanol (13.5–14.5% v/v) is produced, fermentation must be stopped(mutage) by adding SO2 (100–200 mg L−1, depending on pH). The strong addition of SO2
is necessary to block yeast activity as rapidly as possible, in order to limit the acetaldehyde
Preserving the Aroma Characteristic of Dehydrated Grape 141
production, which strongly combines with SO2. Yeasts are more sensitive to SO2 at highertemperature, but at the same time low temperatures negatively affect yeast activity. For thesereasons, after SO2 addition, a gradual diminution of the temperature until 4–6◦C is useful,but this operation should be carried out at least 2–3 hours after sulfiting.
8.3.3 Maturation
The overall sensory quality, and principally the aroma of dessert wines, improve with matura-tion. This is possible largely due to their richness in bound volatiles arising from dehydratedgrapes, which represent a real ‘aroma tank’ throughout maturation and bottle ageing ofdessert wines. The structure of the whole volatile fraction of this kind of wine benefits along maturation in wood barrels more than dry wines. Wines from dehydrated grapes aresufficiently rich in aromas, so that volatiles arising from the wood barrel (e.g. whisky lac-tone, eugenol, vanillin) do not create a disharmony, but rather, add complexity to the finalwine. The wood barrel adds woody, spicy and toasted notes to dried fruit, honey, citrus andorange blossom flavours resulting from dehydrated grapes. This union enhances the qualityof the wine, determining a significant increase in the overall aroma complexity. This is truethroughout the time because wines from dehydrated grapes are richest in flavourless precur-sors (Genovese et al., 2007) which could be transformed during maturation and ageing intoflavour active compounds. Table 8.1 shows the concentrations of bound aroma compounds
Table 8.1 Quantitative data of main glycosylated aroma compounds in sweet Fiano wine (A) andbase Fiano wine (B) (adapted from Genovese et al., 2007).
measured in the volatile fraction of a sweet (A) and base (B) Fiano wine obtained from thehomonym grapes, a neutral variety from southern Italy. Interestingly, the sweet wine showedthe highest levels (four times greater) of total bound compounds with respect to the baseFiano wine. The main increments have been observed for compounds that have an active rolein wine aroma: terpenes, �-damascenone, benzyl alcohol and 2-phenylethanol. Ultimately,correct barrel maturation has an important fining action on sweet wines. Wood tannins sta-bilize the proteinaceous colloids, thus minimizing the employment of fining agents such asbentonite, which unfortunately impoverishes the wine of some aromas (Lisanti et al., 2009),and small-sized barrels favour the spontaneous process of clarification, therefore they are tobe preferred in the case of dessert wines.
Finally, a microbiological stabilization of these wines is necessary before bottling as, dueto high sugar concentration, fermentation re-start is possible with consequent alteration ofthe wine quality. The free SO2 concentration should be monitored and the level correctedin order to obtain the right stability during ageing. For example, for a safe ageing of awine with 14% (v/v) of ethanol and a pH of 3.4, an approximate level of 1.5–2 mg L−1 ofmolecular SO2 corresponding to 40–50 mg L−1 of free SO2 is suitable before bottling. Itis important that bottling is carried out by sterile microfiltration and periodically monitoredthrough microbiological stability tests.
8.4 CONCLUSIONS
Modern enology is not based on a single approach, but on the adaptation of winemakingpractices to the compositional peculiarity of a grape variety and to the ‘wine design’ intendedby the winemaker. This chapter is an example of a proper interpretation of this concept ofenology, where the main goal is to produce a wine that best expresses the sensory qualitiesof a particular grape variety.
ACKNOWLEDGEMENTS
Review paper published in the framework of the project PRIN 2008, coordinator F. Mencar-elli, supported by the Italian Ministry of Education, University and Research. The authorswould like to thank Dr Alessandro Genovese for his kind collaboration.
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