**3.1 Wine off-odours**

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

and may contribute to the yellow hue of oxidised wines [44].

to displace air. The wine is then racked under an inert atmosphere.

detected both in the pulp and in the skin of the white grapes [46].

Although it cannot be excluded that other compounds can be responsible for the appearance of a salmon colour in white wines from other grape varieties, the presence of the small number of anthocyanins in Chardonnay, Sauvignon Blanc,

The development of a salmon-red blush colour in white wines produced exclusively from white grape varieties is known as pinking, and the phenomenon is observed occasionally. It is perceived as an undesirable phenomenon by both wine consumers and the industry. Although with seasonal and regional variations, pinking has been observed worldwide, with predominance in white wines produced from *V. vinifera* L. grape varieties such as Chardonnay, Chenin Blanc, Crouchen, Muscat Gordo Blanco, Palomino, Riesling, Sauvignon Blanc, Sémillon, Sultana, and Thompson Seedless [46]. Pinking is mainly observed when white wines are produced under reducing conditions [47]. The pinking phenomenon is frequently observed after the bottling and storage of white wines or after alcoholic fermentation (AF) [48]. In wines made from Síria white grape variety, it was shown that the compounds responsible for the appearance of the salmon colour after bottling were due to the presence of small amounts of anthocyanins in the wine that could also be

continues to react, creating a xanthylium product that absorbs in the visible region,

To avoid the fast colour evolution, the winemakers use SO2 that due to their antioxidant and antioxidasic properties protect wine colour [45]. Unlike grape oxidases, which are inhibited by sulphites even at low levels, fungal laccases tend to be more resistant. The most effective treatment to eliminate the laccase activity in the must is heat treatment (2 min, 75°C). Ascorbic acid reduces and recycles quinones back to their original catechol forms, being generally used in pre-bottling. The presence of other nucleophiles, such as glutathione, 3-sulfanylhexanol and H2S, leads to the formation of additional products on different positions of the benzene ring [45], and such reactions should also prevent browning since the quinone is being quenched. There are also many technological solutions that when used can results in a more stable wine colour as, in white wines, a fast liquid/solid separation in the press machines, reducing phenolic acids by wine fining with PVPP, potassium caseinate/casein, isinglass, gelatine, patatin and pea protein. Winemakers need to be especially cautious when handling a cold wine, such as during cold stabilisation. Oxygen is more soluble at lower wine temperatures. However, the oxidation reaction speeds up when the temperature rises. As the cold wine warms up the greater amount of dissolved oxygen will contribute to serious wine oxidation. To minimise the adverse effects of oxidation during wine racking the winemakers employ several techniques such as, using SO2, using gentle pumps that minimise aeration, and checking hoses and fittings for leaks, and flushing hoses and containers with inert gas before wine racking. In modern winemaking, the inert gases are often used to minimise oxygen pickup in the head space of partially filled containers and during wine racking. The common inert gases used include; nitrogen, CO2, argon, and a mixture of these gases in various proportions. For economic reasons, the use of nitrogen and CO2 seems to be more common. To provide an inert gas cover over the wine surface in a partially filled container, CO2 or argon should be used. These gases are denser than the air and form an inert layer devoid of oxygen. The danger of oxygen exposure is greater during the wine racking. To minimise oxygen/air contact the system is purged with the inert gas. In the process of purging the inert gas is passed through the system such as hoses, transfer lines, equipment, and the receiving tank

**178**

**2.6 Pinking**

One of the main problems that can occur is the development of high levels of volatile acidity, mainly acetic acid (I, **Figure 1**). Acetic acid can be formed at the beginning of wine production (in grapes), during fermentation, and in the bottled wine as a bacterial or yeast metabolite [50]. High volatile acidity is associated with bad SO2 management or extreme wine exposure to oxygen that stimulate the growth of aerobic acetic acid bacteria (AAB), that increases acetic acid. This results in an olfactory sensory defect known as vinegar off-odour. Vinegary wines are typically sharply acidic with an irritating odour. Ideally, the content of acetic acid should not exceed 0.7 g/L in wine. Several methodologies, aiming to decrease excessive volatile acidity of acidic wines have been proposed [50], such as microbial stabilisation of the acidic wine followed by blending with other wines, reverse osmosis, nanofiltration, and biological removal of acetic acid through refermentation [22].

Acetaldehyde (ethanal) (II, **Figure 1**) in wine can impart some undesirable flavours, when above a certain level. The average values of acetaldehyde in white wine are about 80 mg/L, in red wine 30 mg/L and for Sherries wine 300 mg/L [51]. Acetaldehyde is an intermediate product of yeast fermentation; however, it is more commonly associated with ethanol oxidation, catalysed by the enzyme ethanol

**Figure 1.**

*Structure of the main wine off-odours and taints.*

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

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 to dissolved oxygen levels in the wine [64].

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

**181**

*Wine Stabilisation: An Overview of Defects and Treatments*

'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.

effective microbial control strategies in the winery [77].

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

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