*Wine Stabilisation: An Overview of Defects and Treatments DOI: http://dx.doi.org/10.5772/intechopen.95245*

*Chemistry and Biochemistry of Winemaking, Wine Stabilization and Aging*

dehydrogenase. Moreover, acetaldehyde can be formed by non-enzymatic oxidation throughout the storage and ageing of wine [52]. During wine oxidation, iron (II) reduces oxygen to the hydroperoxyl radical, which converts wine *ortho*-diphenols phenols into quinones and H2O2. Ferrous ion associated with H2O2 generates hydroxyl radical that can react with ethanol to yield acetaldehyde [53]. The sensory threshold for acetaldehyde in red wines is typically in the range of 40–100 mg/L [54]. If present at low levels gives a pleasant fruity aroma, but at high levels, it possesses a pungent irritating odour [55]. Indeed, excess acetaldehyde produces a 'green,' 'grassy,' 'nutty,' 'sherry-like,' 'bruised apple,' or even 'vegetative' off-flavour [30, 56]. The level of acetaldehyde in wine can be reduced by appropriate yeast strain selection, as well as the prevention of oxidation during the winemaking process [57]. The reduction of acetaldehyde can also be done by wine lactic acid bacteria (LAB) of the genera *Lactobacillus* and *Oenococcus* which can degrade free and SO2-bound acetaldehyde [58]. Acetaldehyde also strongly binds to SO2, reducing the free acetaldehyde content, and thus the perception of its aroma in wines [2]. Diacetyl (2,3-butanedione, III in **Figure 1**), is usually found in low levels, as a result of yeast metabolism (<1 mg/L), but it is principally formed during malolactic fermentation (MLF), by the metabolism of citric acid, which is usually naturally present in wines at levels between 0.1–0.7 g/L [59]. If present in an excessive

content sufficient to affect wine's flavour, is usually considered as a fault, generating a buttery, nutty or toasty, lactic off-odour. The detection threshold for diacetyl in a 10% aqueous ethanol is 0.1 mg/L [11]. However, the diacetyl detection threshold is dependent on the wine matrix. It has been reported as 0.2 mg/L in white wine (Chardonnay) and from 0.9 mg/L (Pinot Noir) to 2.8 mg/L (Cabernet Sauvignon) in red wines [60]. Also, diacetyl quickly bounds SO2, and the free and bound forms of diacetyl are in chemical equilibrium, depending on the pH, the level of SO2, and the presence of other SO2 binding components, such as acetaldehyde, α-ketoglutaric acid, and pyruvic acid are important [61]. It is assumed that only the unbound form of diacetyl is sensorially active. According to Nielsen and Richelieu [61] the addition of 80 mg SO2, which is within the range used in the wine industry, reduced the free

All wines contain a few tens of mg/L of ethyl acetate (30–60 mg/L, IV in **Figure 1**) produced by yeast, higher levels indicate AAB activity, formed by esterification between acetic acid and ethanol. This compound at low levels in wine (<50 mg/L) may not be unpleasant, contributing to 'fruity' aroma properties and add complexity to the wine, but at levels >150 mg/L ethyl acetate can confer an unpleasant 'fingernail polish' aroma [62]. Ethyl acetate has a perception threshold in the wine of around 160–180 mg/L, which is much lower than that of acetic acid (750 mg/L) [22]. The deleterious effect of ethyl acetate can be in part reduced by ageing [63] but, after 6 months of bottle ageing, the ethyl acetate levels (140–180 mg/L) affect the wine flavour, giving wines a hot flavour which reinforces the impression of bitterness on the aftertaste [22]. It is usually more perceived in white wine than red wines. Factors that can influence ethyl acetate formation include the yeast strain used during the AF as well as the temperature of fermentation and SO2 levels. Ethyl acetate is also produced by AAB and is related

Vinylphenols and ethylphenols are collectively known as volatile phenols (VPs). Vinylphenols (4-vinylphenol and 4-vinylguaiacol, V, and VI in **Figure 1**, respectively) are produced by the yeast *S. cerevisiae*, LAB such as *L. plantarum*, and *Dekkera/Brettanomyces* yeasts [65]. Their impact on wine quality is almost exclusively observed in white wines, as these wines can contain significant quantities of vinylphenols which, beyond a certain content (limit threshold = 725 μg/L of 4-vinylguaiacol+4-vinylphenol (1:1)), can be responsible for a depreciating

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diacetyl content (20 mg/L) by 75%.

to dissolved oxygen levels in the wine [64].

'phenolic' or 'pharmaceutic' characteristic [66]. *S. cerevisiae* possesses a cinnamate carboxylase enzyme which can transform by non-oxidative decarboxylation, the phenolic acids *p*-coumaric and ferulic acids, into corresponding vinylphenols. This activity is only expressed during AF and with a variable intensity depending on the yeast strain. Although *Dekkera/Brettanomyces* yeasts can also produce vinylphenols they are more likely to reduce the available vinylphenols to ethyl derivatives. It has been shown that *Dekkera/Brettanomyces* is the only known microorganism that under winemaking conditions can produce significant amounts of VPs [67]. The ethylphenols are formed by these yeasts through decarboxylation of the corresponding hydroxycinnamic acids to vinylphenols, and subsequent reduction to ethylphenols, yielding 4-ethylphenol (VII in **Figure 1**) from *p*-coumaric acid and 4-ethylguaiacol (VIII in **Figure 1**) from ferulic acid [67, 68]. Some attributes, such as animal, stable, horse sweat was designated by the widespread term '*Brettcharacter*' in oenology [69]. The perception threshold of EPs (4-ethylphenols, designated as 4-ethylphenol and 4-ethylguaiacol) is influenced by the wine matrix. The values reported by Chatonnet et al. [68] 440 μg/L for 4-EP and 135 μg/L for 4-EG were found in a model solution. In red wines, the 4-ethylphenol presents a detection threshold of 230 μg/L [70] while the combination of 4-ethylphenol with 4-ethylguaiacol shows a threshold of 400 μg/L [68]. Nowadays, perhaps it is the most problematic sensory defect in red wine production around the world, with million litres being be contaminated each year [71]. In the last years, research has been performed to remove these negative VPs from contaminated red wines [67] and efficient treatments include activated carbons (ACs) and fungal chitosan to avoid the growth of contaminated yeast or to reduce the head space volatility of these negative VPs [3, 72]. New materials have been evaluated for their removal aiming to decrease the negative impact of the former treatments on wine quality. Of the new material that includes molecularly imprinted polymers [73], chitosan [3] and degassed and ethanol impregnated cork powder [74], that can remove about 70% of ethylphenols allowing a significant recovery of the wine's fruit and floral character [74]. This material is cheap and easily prepared from cork powder wastes, being natural with good biodegradability, and low environmental impact.

The formation of mousy off-flavours can occur during (MLF) either by the action of LAB (particularly heterofermentative strains) or *Dekkera/Brettanomyces* yeast. This off-flavour can be associate to three compounds, namely the N – heterocyclic volatile bases 2-acetyltetrahydropyridine (sensory threshold in water =1.6 μg/L, IX, **Figure 1**), 2-ethyltetrahydropyridine (odour threshold in wine = 150 μg/L, X, **Figure 1**, [75] and 2-acetylpyrroline (detection threshold in water = 0.1 μg/L, XI, **Figure 1**, [76], being the first one produced at the highest levels. *Dekkera/Brettanomyces* are capable of producing at least two of these compounds, whereas LAB are capable to produce all the three [77]. Although the biosynthetic pathway for the mousy off-flavour compounds formation in wine is unknown, the conditions necessary for its production have been established. l-lysine and l-ornithine are the precursors of the heterocycle ring of the three mousy compounds, and ethanol and acetaldehyde are responsible for the acetyl side chain. The presence or absence of certain metal ions and oxygen has a substantial effect on off-flavour production [77]. However, there is still not know efficient treatment to remove the mousy off-flavour from wines [78]. At present research studies are being performed on the use of molecular imprinting technology for developing materials with the capacity to selectively remove the mousy off-odour [79]. Therefore, at present, it is necessary to prevent the biosynthesis of the mousy off-odour-forming compounds, by the elimination or strict control of the yeast and bacteria responsible for their formation. This can be achieved by implementing effective microbial control strategies in the winery [77].

Aroma properties evocative of rotten eggs, cabbage, garlic, putrefaction are termed 'reduction'. These aroma attributes are generally considered to contribute negatively to overall wine sensory quality and are considered to be related to different low molecular weight volatile sulphur compounds, such as H2S, (odour threshold in red wine 1.1 μg/L), methyl mercaptan (methanethiol, odour threshold in red wine 1.8 μg/L, XII, **Figure 1**), ethyl mercaptan (ethanethiol, odour threshold in red wine 1.1 μg/L, XIII, **Figure 1**), and dimethyl sulphide (odour threshold in red wine 25 μg/L, XIV, **Figure 1**) [80]. Yeast fermentation is frequently associated with the occurrence of reductive off-odours, mainly linked to the formation of H2S and mercaptan by the yeast as mentioned by Pereira et al. [81]. As nitrogen availability is considered one of the main factors for H2S production by yeast, a strategy that could be adopted is the addition of yeast assimilable nitrogen to supplement fermentation [80]. The production of H2S during the AF is normal and the quantity produced is dependent on multifactorial factors such, yeast DNA, grape juice turbidity, level of assimilable nitrogen in the grape juice, levels of methionine and cysteine, fermentation temperature, high levels of SO2, and sulphates. This type of aroma sometimes masks completely the positive varietal and fermentative aroma, however, H2S is very volatile and usually, simple wine aeration is enough to remove them or can be precipitated with copper sulphate or copper citrate. The excessive aeration of the wine in the presence of H2S could lead, by oxidation, to the production of heavy thiols that could be exceedingly difficult to remove from the wine. On the other hand, mercaptans and the other sulphides, are more intractable. Mercaptans impart off-odours reminiscent of rotten onions and disulphides are formed under similar reductive conditions and generate cooked-cabbage odours. Related compounds, such as 2-mercaptoethanol (XV, **Figure 1**) and 4-(methyl thiol) butanol (XVI, **Figure 1**), produce intense barnyard and chive–garlic odours, respectively.

Light-struck refers to a reduced-sulphur odour that can develop in wine during exposure to light [62]. This defect is associated with the formation of volatile sulphur compounds with unpleasant aroma notes, formed by the methionine degradation catalysed by the photochemically activated riboflavin. Methanethiol (XII, **Figure 1**) and dimethyl disulphide (XVII, **Figure 1**) are the main compounds responsible for the light-struck taste in white wine termed as 'cooked cabbage' [82, 83]. Exposure of wine to light at wavelengths close to 370 or 442 nm is particularly effective in inducing the light-struck taste [84], manly when clear glass bottles are used [85]. The preventive strategies are the most efficient as this defect generally develops after wine bottling, and these are mainly related to the reduction of the riboflavin levels in grape juice and wine. There are classic and authorised fining agents, such as bentonite and AC (activated carbon) that can be used to remove with relative efficiency riboflavin from white wine [86]. After application, if bentonite the average residual riboflavin was 60% [86, 87]. Also during the AF, the selection of low riboflavin-producing yeasts can be used as it was shown that it is yeast strain-dependent [86, 87].

Several herbaceous off-odours may be detected in wines. The presence of excessive sensations of herbaceous off-odour results in a decrease in the fruit notes, normally not appreciated by consumers. The source of this off-odour can generally be due to the presence of alkylmethoxypyrazines or aldehydes and alcohols with C6. The main alkylmethoxypyrazines found in grapes, musts, and wines are 3-ethyl-2-methoxypyrazine (ETMP, XVIII, **Figure 1**); 3- sec-butyl-2-methoxypyrazine (SBMP, XIX, **Figure 1**); 3-isopropyl-2-methoxypyrazine (IPMP, XX, **Figure 1**); and 3- isobutyl-2-methoxypyrazine (IBMP, XXI, **Figure 1**), conferring aromatic notes described as 'green pepper', or 'tomato leaf '.

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TDN precursors [102, 106].

*Wine Stabilisation: An Overview of Defects and Treatments*

Alkylmethoxypyrazines represent a narrow, delineated group of extremely powerful odorants characterised by extremely low sensory perception thresholds (1–2 ng/L in distilled water [88]; being present in green plant tissues, including grapes [89]. The content of methoxypyrazine in the wine depends primarily on grape composition [90], being observed a complex relationship between viticultural practices and varietal aroma, being difficult to predict the final wine aroma because of the multiple compounds and pathways involved. This vegetative character is most commonly, although not exclusively, associated with Sauvignon Blanc, Cabernet Sauvignon, and other Bordeaux varietals [91]. IPMP may also be present in certain grapes and thus found in the derived wine as a varietal character. The excessive green bell pepper aroma found in red wines containing IBMP is generally considered unfavourable to wine quality. However, the presence of this compound at low levels is often noted to augment the quality of certain wines obtained from red varieties (Cabernet Franc, Cabernet Sauvignon, Carménère, Merlot) or white varieties (Sauvignon Blanc, Sémillon) by adding to the intrinsic flavour complexity of these varietals [92]. The presence of IBMP can be a positive quality factor when it is not dominant but is in balance and complemented by

Aldehydes and alcohols with 6 carbon atoms are volatile, odorous molecules that can contribute to the herbaceous aroma in the wine. Their cut-grass-like aroma is the characteristic odour of freshly damaged green leaves; therefore, these compounds are often referred to as green leaf volatiles [94] and may also impart a bitter flavour [95]. The C6 alcohols frequently found in grapes include hexanol (XXII, **Figure 1**), (Z)-3-hexenol (XXIII, **Figure 1**), and (E)-2-hexenol (XXIV, **Figure 1**). (E)-2-hexenol, (E)-3-hexenol may also be found in wine at levels of μg/L [96]. The C6 aldehydes commonly identified in grapes are hexanal (XXV, **Figure 1**) and (E)-2-hexenal (XXVI, **Figure 1**); also C7 aldehydes have been found, but at lower content concerning C6 aldehydes [97]. At low levels (< 0.5 mg/L threshold), these C6 volatiles compounds contribute positively to the overall aroma of the wine. These C6 compounds may be present in a free volatile form or in bound form, as glycosides [98]. They are mainly generated through the enzymatic breakdown of C18 polyunsaturated fatty acids contained in plant membranes. The C6 aldehydes and alcohols derive from the oxidation of grape polyunsaturated fatty acids such as oleic acid, linoleic acid, and linolenic acid initiated by the lipoxygenase pathway when the berries are crushed [99]. Their levels in must can be in the order of several hundreds of μg/L [100] or even more than 13,000 μg/L [101], with very variable odour thresholds (400–8000 μg/L) [11]. Their levels depend on several factors, including the grape variety and ripeness, treatments before fermentation,

1,1,6- trimethyl-1,2-dihydronaphthalene (TDN, XXVII, **Figure 1**) exhibits kerosene- and petrol-like off-flavour when present at high levels. Precursors of TDN are carotenoid derived compounds originating from the grapes [102]. These precursors are slowly converted to TDN in the wine acidic medium. Kerosene/petrol aroma usually becomes perceivable after several years of wine storage. TDN is an ambiguous aroma compound, defining the varietal character of Riesling wine but also constituting a repelling taint [103] Comparing wines made of various grape varieties, a perceivable amount of TDN is found mostly in Riesling wines. The recognition threshold of TDN has been reported by Simpson [104] to be in the range of 20 μg/L, while Sacks et al. [105] determined a detection threshold of 2 μg/L. Exposing the grapes to more sunlight by defoliation increases both TDN levels [106]. Low pH and bottle ageing will increase their content likewise due to hydrolytic cleavage of the

*DOI: http://dx.doi.org/10.5772/intechopen.95245*

other herbaceous and fruity aromas [93].

and temperature/duration of contact with the skins.
