**4.2 Astringency**

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

water La Guerche et al. [123]; 0.04 μg/L [123]; in red wine). 1-octen-3-ol (olfactory detection threshold in the water of 2 μg/L and red wine, 40 μg/L, La Guerche et al. [123], however, has also been found on rotten grapes and the musts made from them and is stable during AF so that an occurrence of this compound in a wine can be caused by mould growth on the grapes as well as by contaminated cork stoppers [124]. Lisanti et al. [125] showed that in the red wine the potassium caseinate and grape seed oil treatments decreased the level of geosmin by 14% and 83%, respectively, while in the white wine, the AC and the grape seed oil were able to decrease the level of geosmin by 23% and 81%, respectively. However, after estimating the olfactory impact of the volatile compounds by OAVs (concentration/odour perception threshold), only the treatment with grape seed oil was able to decrease the relative contribution of geosmin in the profile of the odour active compounds, in

Wine can accidentally be contaminated with styrene when trace amounts of the styrene (XLII, **Figure 1**) are released during wine storage in polyester tanks reinforced with fibre glass [126]. Also, occasionally styrene contamination has been detected in wine in contact with synthetic closures [127]. The taste threshold for styrene in water has been reported as 22 μg/L [128] but may be higher in wine. An amount higher than 100 μg/L (the generally accepted threshold of sensory perception), styrene can modify the wine sensory characteristics by imparting a taste of plastic and adhesive. Wagner et al. [129] found in German wines values ranging

**4. Origin of taste and tactile sensory defects and strategies for wine** 

defects noted in the sensory perception of wine quality [13].

The wine imbalances by acidity, astringency, or bitterness, are often the first

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

**186**

both wines.

from 0 to 19 μg/L.

**stabilisation**

**4.1 Acidity**

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 development [139].

### **4.3 Bitterness**

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 (+)-catechin [140].

Protein fining agents could induce some sensory changes. Astringency and bitterness of wine can decline due to its interaction with tannins. The fining process directly occurs from the precipitation of proanthocyanidins by these protein fining agents and it is influenced by the chemical characteristics of the protein used. The interactions between proanthocyanidins and protein fining agents depend on molecular weight, amino acid composition and surface charge density of the proteins used [141–144].

Different proteins are used for wine fining such as gelatine, egg albumin, isinglass, and casein/potassium caseinate. Different types of gelatine remove different amounts of proanthocyanidins (9–16%) depending on the wine phenolic composition and structural characteristics of the proanthocyanidins and on the gelatine composition and characteristics [142, 143]. It has been generally thought that proteins bind primarily high polymerised tannins as well as high galloylated tannins, and therefore are preferentially removed [141], but some recent work showed that each of the different proteins (gelatine, egg albumin, isinglass, casein) and different size fractions of the same protein class interact differentially with different sizes of tannins [142, 143]. Regardless, allergen labelling may make wine fining with any of the animal-derived products impractical although some effort has been made to evaluate plant-derived proteins [144]. Recent studies of wine astringency demonstrated that tannins must be different two-fold for a trained panel to be able to successfully differentiate the wines [145]. Further, since some of the polymeric pigments can precipitate with protein there is the risk of losing stable colour [146]. As mentioned previously a higher astringency intensity is directly associated with a higher concentration of proanthocyanidins with a higher mean degree of polymerisation [147]. During ageing, astringency perception becomes softer, the reasons for the change in wine astringency could involve a decrease in proanthocyanidin concentration accompanied by a decrease in proanthocyanidins structural changes [148].

Therefore, the phenolic composition could be modulated during the winemaking steps (maceration/fermentation, stabilisation (fining) and ageing) and consequently, it allows the modulation of wine astringency and/or astringency sub-qualities as well as the wine bitterness.
