**1. Introduction**

All around the world in wine production many sensory defects, physicochemical instabilities, and a few toxic compounds can appear, which usually results in a decrease of the wine quality and/or safety, being responsible for economic losses for the wine industry. Nowadays, the origin of most wine defects and processing conditions that favour their formation are well identified. At the same time, many viticultural and technological solutions are available to completely or partially avoid their formation, and various additives and fining agents are allowed to be used to avoid their formation or remove the compounds, or their precursors, responsible for the instability [1]. Frequently, the question is not how to remove the defect or their precursors, but how to remove it without changing the wine sensory profile.

The most troublesome defects for wine producers are those occurring after wine bottling, as the intervention at this stage is rather limited and therefore preventive actions are the most efficient and sometimes the only strategy. After wine bottling the main external factor that can influence wine stability and the kinetics of the chemical reactions and interactions of the wine constituents is the temperature that can significantly affect for example the solubility of tartrate salts, induce colour

changes in red and white wines, and the formation of protein haze. Also, the redox potential, mainly determined by the levels of dissolved oxygen in the wine can significantly influence the wine shelf life. Before bottling, depending on the wine defect to be treated two main stabilisation strategies are currently used: subtractive and non-subtractive. For the first approach fining agents are used that can interact with the compound(s) responsible for the wine defect or their precursors removing them from the wine. In the second type of strategy, an additive is added to the wine that interacts with the compound(s) responsible for the wine defect affecting their ability to form crystals as in the case of carboxymethylcellulose in the tartaric instability [2] or decrease the vapour pressure of the compound(s) responsible for an undesirable odour for example like chitosan in the case of the ethylphenols responsible for the '*Brett character*' [3].

In the next sections (2, 3, 4, and 5) the main wine defects and their stabilisation treatments currently allowed and used will be overviewed as well as the stabilisation treatments currently under research. For a matter of simplicity and systematisation, the wines defect that can occur in wine production are divided into four main groups according to their impact on the wine sensory quality and safety: (1) visual sensory defects; (2) off-odours and taints; (3) taste and tactile defects (4) safetyrelated defects. The wine visual defects are concerned with changes in wine limpidity and undesirable colour changes that can occur during wine production and especially after wine bottling. These defects can be due to the formation of precipitates related to solubility issues, the formation of precipitates related to the colloidal instability of some wines components, the reaction between wine components, or due to the oxidation of wine phenolic compounds.

Wine aroma significantly determines consumer acceptability [4], being extremely complex as it is the result of the cumulative effect of a diverse group of volatile compounds present at levels ranging from fractions of ng/L up to mg/L. These compounds can interact with the olfactory epithelium to generate a sensory perception [5, 6]. The levels of these volatiles are dependent on both viticultural [7] and oenological practices [8, 9]. Although several hundreds of these volatile compounds can be present in wines [10], only a few are present at levels above the perception threshold and thus being responsible for characteristic odours [11]. During wine production, several off-odours and taints can cause severe quality problems. Off-odours are considered to be the occurrence of any atypical odours resulting from compounds formed by the deterioration of the wine, including chemical reactions and microbial spoilage; whereas, taints result from external contamination of the wine as a result of exposure of grapes to contaminating environments or migration of compounds form packaging materials contaminated by either synthetic chemicals or chemicals produced or transformed by microbial action [12]. Sometimes, compounds that impart a positive aroma character in one instance may cause an off-odour when present in the wrong context or at high levels. Besides, faults in one wine may not be undesirable in another: for example, the complex oxidised bouquets of Sherries, the fusel odours of Port wine, and the baked character of Madeira wine. Some faults, such as a barnyard odour, generated by ethylphenols, may be considered pleasingly 'rustic,' or part of the terroir character of certain wines. The evident presence of ethyl acetate in the aroma of wine is also usually considered a fault. However, in expensive Sauternes, it appears to be acceptable (or ignored). Nevertheless, there is general agreement among most wine professionals as to what constitutes an aromatic fault in table wines. These wine defects, their origin, and their sensory impact are discussed in Section 2. Balancing the oral sensations of wine is one of the most demanding tasks for a winemaker since a distinguishing feature of superior wines is the harmony achieved among these seemingly simple sensations. Indeed, imbalances created by excessive acidity,

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*Wine Stabilisation: An Overview of Defects and Treatments*

astringency, or bitterness, among others, are often the first deficiencies noted by a panellist [13]. Since it is very important to understand the factors that affect the sensory properties responsible for wine taste and tactile imbalance, to allow the wine industry to be able to control these sensory characteristics of the final product from the first production stages. The main wine taste and tactile defects are astringency, bitterness, and acidity imbalance. Wine astringency is caused by excess proanthocyanidins in young red wine, it is considered a tactile sensation [14], responsible for the 'drying', 'roughing' and 'puckering'. The ageing process reduces astringency due to oxidation and will be less evident in aged wines. Bitterness is a taste perception related to phenolic compounds with low molecular weights such as flavonol aglycones, especially myricetin or quercetin [15] as well as to monomeric or small phenolic flavanols [16]. Bitterness, astringency, and acidity could change depending on the oenological practice. In Section 4 the sensory impact of excessive astringency, bitterness and acidity will be highlighted, as well as the stabilisation/

In Section 5, the formation of the main potential toxic compounds that affect wine safety is overviewed. These compounds are present in wines due to the activity of bacteria and moulds in the wine production and due to the bad hygienic sanity of

**2. Origin of visual sensory defects and strategies for wine stabilisation**

The two most important visual sensory defects that affect wine quality occur during the storage of bottled wine. These are the tartrate instabilities and protein instability. Other instabilities that can occur include red wine colour colloidal instability, pinking of certain white wines produced from white grape varieties, browning of white and rose wines, and oxidation of all wines as well as metallic

The crystallisation of potassium hydrogen tartrate or calcium tartrate salts may occur in wines where their concentration product exceeds their solubility product [17, 18]. The formation of these crystals results in the formation of deposits at the bottom vat and sometimes in the bottled wine. Prevention of tartaric precipitation in bottled wine is essential as consumers associate its occurrence with poor production conditions or an exogen wine material. Methods to prevent tartaric crystals precipitation include metatartaric acid, cold stabilisation, and electrodialysis [19]. The addition of mannoproteins obtained from the hydrolysis of the yeast cell wall was authorised by the European Community since 2005 [19]. Mannoproteins inhibit the crystallisation of tartrate salts by lowering the crystallisation temperature [20] preventing the occurrence of precipitates in wine [21]. Arabic gum can also have some effectiveness to prevent tartaric instability, as they are protective colloids [22], and more recently sodium carboxymethylcellulose for white wine (since 2009) and potassium polyaspartate (since 2015) were authorised [19].

Precipitation of soluble proteins in bottled wines can also occur and this results in the formation of an amorphous haze or deposit. This defect occurs frequently in white wines or wines with low polyphenol content. It is rare in wines with relatively high levels of flavonoid phenols, particularly tannins, which complex and

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

the grapes used for winemaking, respectively.

instabilities such as iron and copper instability.

correction strategies.

**2.1 Tartrate instabilities**

**2.2 Protein instability**

*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*

responsible for the '*Brett character*' [3].

due to the oxidation of wine phenolic compounds.

changes in red and white wines, and the formation of protein haze. Also, the redox potential, mainly determined by the levels of dissolved oxygen in the wine can significantly influence the wine shelf life. Before bottling, depending on the wine defect to be treated two main stabilisation strategies are currently used: subtractive and non-subtractive. For the first approach fining agents are used that can interact with the compound(s) responsible for the wine defect or their precursors removing them from the wine. In the second type of strategy, an additive is added to the wine that interacts with the compound(s) responsible for the wine defect affecting their ability to form crystals as in the case of carboxymethylcellulose in the tartaric instability [2] or decrease the vapour pressure of the compound(s) responsible for an undesirable odour for example like chitosan in the case of the ethylphenols

In the next sections (2, 3, 4, and 5) the main wine defects and their stabilisation treatments currently allowed and used will be overviewed as well as the stabilisation treatments currently under research. For a matter of simplicity and systematisation, the wines defect that can occur in wine production are divided into four main groups according to their impact on the wine sensory quality and safety: (1) visual sensory defects; (2) off-odours and taints; (3) taste and tactile defects (4) safetyrelated defects. The wine visual defects are concerned with changes in wine limpidity and undesirable colour changes that can occur during wine production and especially after wine bottling. These defects can be due to the formation of precipitates related to solubility issues, the formation of precipitates related to the colloidal instability of some wines components, the reaction between wine components, or

Wine aroma significantly determines consumer acceptability [4], being extremely complex as it is the result of the cumulative effect of a diverse group of volatile compounds present at levels ranging from fractions of ng/L up to mg/L. These compounds can interact with the olfactory epithelium to generate a sensory perception [5, 6]. The levels of these volatiles are dependent on both viticultural [7] and oenological practices [8, 9]. Although several hundreds of these volatile compounds can be present in wines [10], only a few are present at levels above the perception threshold and thus being responsible for characteristic odours [11]. During wine production, several off-odours and taints can cause severe quality problems. Off-odours are considered to be the occurrence of any atypical odours resulting from compounds formed by the deterioration of the wine, including chemical reactions and microbial spoilage; whereas, taints result from external contamination of the wine as a result of exposure of grapes to contaminating environments or migration of compounds form packaging materials contaminated by either synthetic chemicals or chemicals produced or transformed by microbial action [12]. Sometimes, compounds that impart a positive aroma character in one instance may cause an off-odour when present in the wrong context or at high levels. Besides, faults in one wine may not be undesirable in another: for example, the complex oxidised bouquets of Sherries, the fusel odours of Port wine, and the baked character of Madeira wine. Some faults, such as a barnyard odour, generated by ethylphenols, may be considered pleasingly 'rustic,' or part of the terroir character of certain wines. The evident presence of ethyl acetate in the aroma of wine is also usually considered a fault. However, in expensive Sauternes, it appears to be acceptable (or ignored). Nevertheless, there is general agreement among most wine professionals as to what constitutes an aromatic fault in table wines. These wine defects, their origin, and their sensory impact are discussed in Section 2. Balancing the oral sensations of wine is one of the most demanding tasks for a winemaker since a distinguishing feature of superior wines is the harmony achieved among these seemingly simple sensations. Indeed, imbalances created by excessive acidity,

**174**

astringency, or bitterness, among others, are often the first deficiencies noted by a panellist [13]. Since it is very important to understand the factors that affect the sensory properties responsible for wine taste and tactile imbalance, to allow the wine industry to be able to control these sensory characteristics of the final product from the first production stages. The main wine taste and tactile defects are astringency, bitterness, and acidity imbalance. Wine astringency is caused by excess proanthocyanidins in young red wine, it is considered a tactile sensation [14], responsible for the 'drying', 'roughing' and 'puckering'. The ageing process reduces astringency due to oxidation and will be less evident in aged wines. Bitterness is a taste perception related to phenolic compounds with low molecular weights such as flavonol aglycones, especially myricetin or quercetin [15] as well as to monomeric or small phenolic flavanols [16]. Bitterness, astringency, and acidity could change depending on the oenological practice. In Section 4 the sensory impact of excessive astringency, bitterness and acidity will be highlighted, as well as the stabilisation/ correction strategies.

In Section 5, the formation of the main potential toxic compounds that affect wine safety is overviewed. These compounds are present in wines due to the activity of bacteria and moulds in the wine production and due to the bad hygienic sanity of the grapes used for winemaking, respectively.
