**2.4 Metallic instabilities**

Two metal-dependent instabilities can also result in the formation of wine turbidity or deposits. Excess levels of iron (5–20 mg/L) in wine (white or red) can lead to the formation of precipitates with phosphates and tannins resulting in instabilities (iron (III) phosphate [white casse] or tannate [blue casse]). Application of arabic gum or casein/potassium caseinate or citric acid (< 1 g/L) is used to prevent this instability [30]. Copper instability occurs only in white wines, initially as a white haze, and later as a reddish-brown amorphous precipitate, develop upon storage of bottled wine with excess copper (> 0.5 mg/L), under strong reducing conditions and in the presence of SO2. This metal instability is caused by a reaction of metal traces, mainly copper from machinery, pesticide residues, or treatment with copper sulphate for the treatment of reductive off-flavours [31]. The OIV recommends a maximum copper content of 1.0 mg/L in wines [32]. The application of arabic gum is used to prevent this instability [30]. The protective colloids prevent metal precipitation but do not eliminate the copper. Exchange resins such

**177**

*Wine Stabilisation: An Overview of Defects and Treatments*

as polyvinylimidazole-polyvinylpyrrolidone copolymers with selective binding of metals such as copper or iron have been developed [33]. The occurrence of iron and copper-related precipitations are much less usual due to the reduction of the level of these metals in the wines all-around the world by the use of stainless steel vats and wood barrels in the wine production, vat taps, and plastic polymers in the winery

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 nor-

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

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

hoses, press machines and filling machines.

mally the result of an oxidative problem.

**2.5 Enzymatic and non-enzymatic oxidation**

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

the use of some mannoproteins [25].

arabic gum could prevents wine colour instability [22].

**2.3 Colour colloidal instability**

**2.4 Metallic instabilities**

precipitates proteins during wine production. The most important proteins that have been related to wine protein instability are pathogenesis-related proteins of *Vitis vinifera* that include the chitinases and thaumatin-like proteins as described by Tian et al. [23]. The formation of wine protein haze is a multifactorial process with several factors known to influence the process, such as storage or wine ageing temperature, pH, ionic strength, wine protein composition, organic acids, ethanol, phenolic compounds, metals, and sulphate content; however, other important factors remain unidentified, such as the non-proteinaceous component(s) usually named X factor [24]. These proteins can be slowly denatured and aggregate throughout wine storage, forming a light-dispersing haze; therefore, this phenomenon needs to be prevented by removing them from the wine, usually by fining, before wine bottling [25]. Bentonite fining is the most used process to avoid protein instability in white wine, with the dose used being preferentially determined previously by stability tests [25]. However, bentonite fining can have a detrimental effect on wine quality, for example, by the removal of colour and aroma compounds [26]. Therefore, alternative techniques to bentonite fining have been studied, such as ultrafiltration, the addition of proteolytic enzymes, flash pasteurisation, other adsorbents, zirconium oxide, natural zeolites, chitin and chitosan, carrageenan and

Colour instability of some red wines can cause product depreciation. In the last years, excessive precipitation of colouring matter has been observed at the bottom of the storage tanks and even in bottled wines [27]. This precipitation occurs along with the natural evolution of red wine during the storage period and results in a considerable modification of its colour and limpidity that is undesirable in terms of visual perception and loss in taste and flavour [22]. This precipitate may happen after a few months of ageing. It may be sometimes gelatinous and strongly red coloured. If the red wine is aged and bottled, the precipitate may occur later in the form of a thin leaf, lining the inner side of the bottles [28]. A method commonly used to reduce wine turbidity and stabilise the colouring matter is fining by the addition of proteins (albumin, casein, or gelatine) that promotes flocculation or precipitation before bottling [29]. However, in some cases, the precipitate formation can still be observed in the later stages of the ageing process. The addition of

Two metal-dependent instabilities can also result in the formation of wine turbidity or deposits. Excess levels of iron (5–20 mg/L) in wine (white or red) can lead to the formation of precipitates with phosphates and tannins resulting in instabilities (iron (III) phosphate [white casse] or tannate [blue casse]). Application of arabic gum or casein/potassium caseinate or citric acid (< 1 g/L) is used to prevent this instability [30]. Copper instability occurs only in white wines, initially as a white haze, and later as a reddish-brown amorphous precipitate, develop upon storage of bottled wine with excess copper (> 0.5 mg/L), under strong reducing conditions and in the presence of SO2. This metal instability is caused by a reaction of metal traces, mainly copper from machinery, pesticide residues, or treatment with copper sulphate for the treatment of reductive off-flavours [31]. The OIV recommends a maximum copper content of 1.0 mg/L in wines [32]. The application of arabic gum is used to prevent this instability [30]. The protective colloids prevent metal precipitation but do not eliminate the copper. Exchange resins such

**176**

as polyvinylimidazole-polyvinylpyrrolidone copolymers with selective binding of metals such as copper or iron have been developed [33]. The occurrence of iron and copper-related precipitations are much less usual due to the reduction of the level of these metals in the wines all-around the world by the use of stainless steel vats and wood barrels in the wine production, vat taps, and plastic polymers in the winery hoses, press machines and filling machines.
