*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*

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,

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

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 '.

was shown that it is yeast strain-dependent [86, 87].

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respectively.

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 other herbaceous and fruity aromas [93].

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, and temperature/duration of contact with the skins.

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 TDN precursors [102, 106].
