In the wine industry, “controlling astringency” is also known as “modulating astringency”. The correct modulation of astringency will be a key point in obtaining a wine with a good balance of tactile sensations.
Making balanced wines at a tactile level involves taking into account variables that influence the level and perception of astringency, the main points to be taken into account being the following:
- – Tannicity of the compounds present in the wine.
- – Increased unctuousness that decreases the perception of tannicity.
- – Consumer sapid profile.
Before learning how to influence and balance wine, we begin by learning how astringency is perceived during wine consumption.
The buccal epithelial cells are covered by a mucosa rich in proteins with a lubricating effect. During the consumption of wine, with variable tannin content, aggregates are formed as a result of the reaction between the proteins present in the saliva and the tannins in the wine. This causes a decrease in lubrication and a modification of the structure of the mucosa and, therefore, a sensation of dryness of intensity proportional to the capacity of reaction of the tannins (concentration and type) with the salivary proteins.
Illustration 1. Astringency perception mechanism.
Determination of astringency.
There are a large number of methodologies, assays and models for the study of tannin-protein interactions. Many of these methods are based on the use of model tannins such as catechin, epicatechin, EGCG, EGC, tannic acid…or fractions of different degree of polymerization obtained from grape seeds and skins.
Due to the variability in the influence of phenolic compounds on the perception of astringency, the determination of absorbance at 280 nm is not an adequate parameter for its determination. Its high complexity for analytical determination, due to being a tactile sensation influenced by a combination of chemical and personal aspects, has led to multiple studies to facilitate its determination. Possibly, the most widely used analytical method in oenology to determine astringency, especially in those trials involving the use of commercial tannins or the choice of fining agents, is the so-called Gelatin Index. Designed by Professor Glories (1984), it consists of placing the wine in contact with cold gelatin for 3 days. The tannins are then determined by acid hydrolysis, giving a % of astringency which is related to the tannins capable of causing the sensation of astringency.
On the other hand, more recent tests carried out by the Institut Français de la Vigne et du Vin indicated that the most astringent tannic compounds have an absorption peak at 230 nm, making this DO a point of measurement and thus determining the sensation of astringency of the wines. For this purpose, the absorption spectra of 21 red wines were determined, showing a higher correlation of the OD at 230 nm (R2 =0.71) than that present at an OD of 280 nm (R2 =0.56) when these samples faced a tasting panel for the determination of astringency.
Consumer perception of astringency.
Astringency is a tactile sensation and, therefore, related to the specific receptors of each consumer, which conditions this individual perception of astringency.
Tannic compounds react mainly with the proteins present in saliva with a higher proline content, which account for around 70 % (SarniManchado et al., 1999) of the proteins in human saliva; however, these values vary in different individuals. On the other hand, studies related to the individual perception of astringency have focused on studying the individual salivary flow rate, which according to Müller et al. (2010), were in a range of 696 𝝻l ± 312 𝝻l, also confirming the personal variability of this aspect.
All this implies an important variability between different individuals, since the perception of astringency will have a strong dependence on the flow of saliva and the particular content of proline-rich proteins, which will increase the complexity of the objective determination of the astringency of a wine.
Elements and mechanisms that increase and decrease astringency in wine
Tannins: Wine tannin composition
The tannic structure of the wine is defined at the time of maceration, maturing during the harmonization of the wine, whether during its time in barrel, tank or bottle. The grape variety, the maceration time, the extraction protocol will have an influence on the tannic component and thus on the basic astringency of the wine.
The grape variety used during vinification will influence the tannin content of the wine both in terms of the tannin content present in the skins and pips and the ease of release during vinification.
Table 1. IPT content of different varieties (Watrelot, A.A, 2021).
Within the detailed study of tannins, the size, concentration, as well as the presence of galloyl groups will influence the perception of astringency.
Monomers are less astringent compounds than polymerized tannins, with astringency increasing as the degree of polymerization increases. However, the passage of time and polymerization of tannins leads to a decrease in the astringency of wines due to a decrease in the number of active molecules present. For this reason, in determining the astringency of red wines, both the size of the tannins and their concentration must be evaluated.
Regarding the type, grape seed tannins with variable galloylation indexes will be more astringent than grape skin tannins that do not have these groups particularly reactive to saliva proteins. This implies that the winemaking protocol in which extraction during maceration is intensified in the first phases and decreased in the final phases, which are more extractive due to the presence of ethanol, will have an influence on the lower presence of grape seed tannins and, therefore, on the modulation of astringency.
Therefore, astringency will increase with the degree of tannin polymerization and the amount of galloyl units present in the proanthocyanidins (Vivas et al. 1996; Herderich & Smith, 2005).
Micro-oxygenation.
Micro-oxygenation (MOX) involves the controlled addition of small amounts of oxygen to a wine system, either during fermentation or in the initial months following fermentation. The effect of increased oxygen exposure in red wine can contribute to stabilization of wine color and improved flavor and aroma. One of the impacts of MOX treatment may be to produce changes in tannin structure that mimic the changes produced during aging, thus changing the perceived astringency of the wine.
Serving temperature:
Through studies carried out to determine the influence of temperature on the sensory attributes of red and white wines, it was determined that:
Depending on whether the wine is white or red, different aspects of flavor and mouthfeel emerge that capture our interest. In the case of white wine, it is argued that a lower serving temperature typically attenuates sweetness and enhances acidity and freshness. On the other hand, serving red wine at room temperature enhances its aroma, while reducing the perception of bitterness and astringency leading to a pleasant perception of more structured wines.
Oily compounds that reduce astringency.
Once the tannin content has been defined during maceration, the harmonization process will influence the modulation of astringency, with wine refinement being a stage where we can continue to influence it.
The increase in the unctuosity of the wine implies a decrease in the perception of astringency, especially linked to a greater presence of polysaccharides. Polysaccharides, especially mannoproteins, reduce the reactivity of grape tannins with saliva proteins, preventing them from denaturing and thus precipitating (Pozo-Bayón et al., 2009). Mannoproteins are mannose glycosylated proteins in which their variable protein fraction gives them specific characteristics. Mannoproteins with a high protein fraction have a greater capacity to fix tannins (Escot et al., 2003) and therefore to reduce the sensation of astringency.
Agrovin tools for controlling astringency
The complexity of the perception of astringency in wines allows its modulation to be worked on at different points, having particularly useful tools at the time of harmonization and refinement of wines, such as the increase of polysaccharides in the medium with an important influence on the perception of astringency.
- – Contribution of inactive yeasts rich in polysaccharides in combination with β-glucanase enzymes to increase mannoproteins in wine. In this aspect, the combination of Enozym Glucan with main activity β-1,3-1,6 glucanase with Super Bouquet MN, with around 20% of free mannoproteins, will increase the unctuousness and decrease the perception of astringency.
- – The supply of pure mannoproteins with a high protein fraction, such as Mannoplus ND, limits the mechanism of denaturation of saliva proteins and the consequent decrease in the lubrication of the oral mucosa.
- – Microoxygenator DosiOx range: the controlled and precise supply of small quantities of oxygen produces the anthocyanin-tannin polymerization as well as producing a greater tannic roundness and increasing the structure.
- – Spirit Toppings produce a rapid release of compounds that reduce the aggressive notes of the wine in the mouth, mainly due to an elegant release of polysaccharides. The polysaccharide-tannin bonding causes green notes related to C6 compounds, bitterness and astringency present in the wine to be almost totally diminished, depending on the matrix from which they are produced.
Conclusions:
- – The characteristics of salivary proteins and tannins present in wine play a crucial role in the way they interact in the oral cavity, thus influencing the level of astringency experienced when consuming wine.
- – During the aging process, a small percentage of hydrolyzable tannins are extracted from barrels or oak chips; however, these compounds are unlikely to contribute significantly to astringency on their own.
- – Transformations in tannin structure, both during grape fermentation and wine aging, modify the interaction of tannins with salivary proteins, thus impacting wine astringency.
- – Differences in the size and hardness of the precipitate formed with saliva tannin-protein binding can affect perceived astringency.
- – Higher tannin concentrations have been found to be directly related to an increase in perceived astringency. In addition, variations in tannin structures have been shown to exert a significant influence on both protein binding efficiency and perceived astringency.
Sources:
- – Ross, CF; Weller, K. Effect of serving temperature on sensory attributes of red and white wines. Sens. Stud. 2008 , 23 , 398-416.
- – McRae, J. M., & Kennedy, J. A. (2011). Wine and grape tannin interactions with salivary proteins and their impact on astringency: A review of current research. Molecules, 16(3), 2348-2364. https://doi.org/10.3390/molecules16032348.