**4.1 Acidity**

Organic acids are the main responsible for sourness and able of modifying this sourness sensation in wines producing a pleasant and refreshing sensation [130]. However, when present at high levels they are responsible for an unpleasant acidity. Therefore, it is generally accepted that too much acidity will taste excessively sour and sharp, while wines with too little acidity will taste flabby and flat and present a less defined flavour profile [131]. Organic acids contribute to the tartness and mouth-feel properties of wine. Tartaric acid is the main organic acid in wine, which, at high levels (>5 g/L), is responsible for an unpleasant taste. Other acids include malic, citric, fumaric, succinic, pyruvic, α-ketoglutaric, lactic, and acetic [3]. However, different organic acids have different sensory properties, and the impact of organic acids is therefore not only linked to total acidity and pH, but to the specific levels of each acid in the wine [132]. The perceived sourness was imparted by L-tartaric acid, D-galacturonic acid, acetic acid, succinic acid, L-malic acid, and L-lactic acid and was slightly suppressed by the levels of chlorides of potassium, magnesium, and ammonium [16]. Acidity adjustment is the reduction or increase in titratable acidity so that the resulting wine will be acceptable. Acidity adjustment can be performed by the addition of an approved acid, the chemical deacidification with approved salts, and using ion exchange resins, either cation, anion or both, electromembrane processes and by biological deacidification. Tartaric acid is commonly used to increase the titratable acidity and reduce the

**187**

*Wine Stabilisation: An Overview of Defects and Treatments*

microorganisms are unable to metabolise it at wine pH [133].

pH in the wine industry, because of its stability and the fact that yeast and other

The reduction of titratable acidity by the addition of carbonate salts such as calcium carbonate, can be done in one of two ways, the first, is a direct addition which is not recommended as it results in wines which are unstable with respect to calcium tartrate, the second is to treat only a wine portion. This process causes the pH to increase up increase to 4 or 4.5 at the end of the addition. The tartaric and malic acids are primarily in the ionised forms. The precipitation of their calcium salts is favoured and this also lowers the calcium levels. This method is referred to as the double-salt and it may be used to reduce the total acidity of high-acid grape musts before fermentation. The precipitation is primarily that of calcium tartrate and under certain circumstances the coprecipitation of calcium malate [2]. Ion exchange

resins, either cation exchange alone or as a combination of anion and cation exchange, can also be used to change wine acidity [19]. In red wine for example the acid reduction can be achieved by using LAB strains. MLF refers to the conversion of malic acid to lactic acid and CO2. This secondary fermentation usually takes place after the AF. The benefits of MLF is the acidity reduction and simultaneously add the complexity of aroma and taste and provides a more microbiological stable wine [2].

One of the most important sensations and a quality attribute is astringency. Gawel et al. [134] presented a structured vocabulary derived by a panel of experienced wine tasters that describe the astringent sub-qualities of red wines, such as velvety, drying, puckering, or roughing. Astringency is mainly a tactile sensation [135] not a taste because it can be perceived in regions of the oral cavity where there is no taste receptor [96, 136, 137]. The major mechanism proposed to astringency perception is the interaction and precipitation of salivary glycoproteins, namely by tannins generating a loss of lubrication [136]. Vidal et al. [138] showed in model solutions that astringency perception of proanthocyanidins increases with their mean degree of polymerisation (mDP) and their percentage of galloylation [139]. Oligomeric proanthocyanidins have been described as inducing lower roughness than the more polymerised molecules, whereas an increase in galloylation has been associated with a higher perceived drying and roughing astringency [139]. However, other wine phenolic compounds, such as flavonols, phenolic acids, or anthocyanins, can also play an important role in astringency

Bitter perception in wines is related to phenolic compounds with low molecular weights as well as to monomeric or small phenolic flavanols [16]. Concerning the latter, they have been described for a long time as the main contributors to the bitterness generated by flavonoid phenols [140]. Monomeric flavonoid phenols are primarily bitter but as the molecular weight increases upon polymerisation, astringency increases more rapidly than bitterness. It has also been shown that chiral difference between the two major wine monomeric flavanols produces a significant difference in temporal perception of bitterness: (−)-epicatechin is significantly bitterer and has a significantly longer duration of bitterness in the mouth than

Protein fining agents could induce some sensory changes. Astringency and bitterness of wine can decline due to its interaction with tannins. The fining process

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

**4.2 Astringency**

development [139].

(+)-catechin [140].

**4.3 Bitterness**

## *Wine Stabilisation: An Overview of Defects and Treatments DOI: http://dx.doi.org/10.5772/intechopen.95245*

pH in the wine industry, because of its stability and the fact that yeast and other microorganisms are unable to metabolise it at wine pH [133].

The reduction of titratable acidity by the addition of carbonate salts such as calcium carbonate, can be done in one of two ways, the first, is a direct addition which is not recommended as it results in wines which are unstable with respect to calcium tartrate, the second is to treat only a wine portion. This process causes the pH to increase up increase to 4 or 4.5 at the end of the addition. The tartaric and malic acids are primarily in the ionised forms. The precipitation of their calcium salts is favoured and this also lowers the calcium levels. This method is referred to as the double-salt and it may be used to reduce the total acidity of high-acid grape musts before fermentation. The precipitation is primarily that of calcium tartrate and under certain circumstances the coprecipitation of calcium malate [2]. Ion exchange resins, either cation exchange alone or as a combination of anion and cation exchange, can also be used to change wine acidity [19]. In red wine for example the acid reduction can be achieved by using LAB strains. MLF refers to the conversion of malic acid to lactic acid and CO2. This secondary fermentation usually takes place after the AF. The benefits of MLF is the acidity reduction and simultaneously add the complexity of aroma and taste and provides a more microbiological stable wine [2].
