**2.5 Enzymatic and non-enzymatic oxidation**

One of the most frequent oenological problems in winemaking is premature wine oxidation, especially the oxidative spoilage of young white wines causing wine browning [34]. During winemaking and bottle-ageing wine, components react with oxygen [35]. Moderate oxidation of red wines phenolic compounds can contribute positively to the red wine colour stabilisation and decrease wine astringency, nevertheless, excessive oxidation can have negative effects on wine quality [36]. Wine oxidation generally results in wine colour changes, an important sensory attribute that is the first to be appreciated by consumers. Today the market wants white wines with a citrine colour, almost colourless, except for those white wines fermented in oak barrels or wines with some ageing time. In rosé wines, many colours can be found on the market, since the 'Provence style', with a slight salmon colour, until rosé wines with the colour of open coloured red wines like 'Palhete', wines produced with white and red grapes and with some maceration. In the red wines, many styles and colours can be found, from the faint colour of Pinot Noir wines to the wines produced with Alicante Bouschet or Vinhão grape varieties that yield wines with intense red colours. The fast colour change in a white or rosé bottled wine is normally the result of an oxidative problem.

The deleterious browning reaction in must and wine occurs due to the oxidation of phenolic compounds and can start as soon the grapes are crushed due to the polyphenol oxidase activity. Polyphenol oxidase with tyrosinase and catecholase activity are natural enzymes present in grape berry. They can catalyse the oxidation of monophenols to *o*-diphenols and further oxidation to orthoquinone. In wine hydroxycinnamates and flavanols, such as caffeoyltartaric acids and catechin, respectively [37] are oxidised to the corresponding quinones. Further reaction of the quinones can result in the formation of a brown colour, especially that of catechin than can yield by dimerisation the yellow dehydrodicatechin B [38] Another problem can arise when grapes are affected with *Botrytis cinerea* [39] and the resulting must become contaminated with laccase enzyme. Laccase catalyses the oneelectron oxidation of a broad range of compounds including substituted phenolics to the corresponding radicals [40]. Wine phenolic acids, catechins, anthocyanins, tannins, and stilbenes are converted into the corresponding quinones, which often react further to dark coloured polymers [41]. The latter are generally insoluble in water and precipitate out from must and wine. Grape polyphenol oxidase is sensitive to low concentrations of SO2 being inactivated, but laccase is more resistant to SO2, and it may be present in the final wine [22], while polyphenol oxidase rarely survives the fermentation process [42]. After fermentation, with the enzyme removed or inactivated, oxidation reactions in white wine are based on non-enzymatic pathways, where Fe (II) is oxidised to Fe (III), producing hydrogen peroxide, and the following reaction where Fe (III) coordinates with catechols and oxidises them to semiquinones [43]. Then the semiquinones disproportionate to form reactive electrophilic quinones and these reactive compounds have a key impact on wine chemistry, by degrading several colour and flavour substances [35]. Reactions of oxidation products with flavonoids are well known, and some of the products are pigmented. When tartaric acid is oxidised to glyoxal, the resulting bridged product

continues to react, creating a xanthylium product that absorbs in the visible region, and may contribute to the yellow hue of oxidised wines [44].

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

#### **2.6 Pinking**

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 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,

**179**

**Figure 1.**

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

*Wine Stabilisation: An Overview of Defects and Treatments*

and Riesling has been shown [49]. To avoid the pinking problem there are various preventive or curative oenological treatments, including adding PVPP or PVPP associated with bentonite or increasing the redox potential using ascorbic acid in

**3. Off-odours and taints and strategies for their mitigation and wine** 

Uncontrolled or undesirable microbiological activity developed in the wine can be responsible for several wine spoilage problems. These defects are diverse in origin and chemical compounds involved impact as well on the wine sensory quality.

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

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

the pre-bottling stage [46].

**stabilisation**

**3.1 Wine off-odours**

and Riesling has been shown [49]. To avoid the pinking problem there are various preventive or curative oenological treatments, including adding PVPP or PVPP associated with bentonite or increasing the redox potential using ascorbic acid in the pre-bottling stage [46].
