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Stickiness New concept in grape and wine tannin analysis.

There is an incredible range of sensory descriptors that refer to red wine tannin. Words such as silky, velvety, dusty, green or hard have all been used by winemak-ers, wine writers and consumers from around the world when referring to tannins in wines. What is the underlying chemistry that leads to these descriptors?

Developing a standardized vocabulary of mouthfeel descriptors (1) and the chemistry responsible for these descriptors (2) is essential if we are to effectively manage red wine mouthfeel in the vineyard and winery.

Our goal as tannin researchers is to understand the chemistry of astringency so that viticulturists and winemakers can manage mouthfeel and astringency more effectively.

Red wine tannins are derived primarily from the skins and seeds of grapes and are extracted during fermentation and maceration. (3) The concentration of this extracted tannin plays a significant role with regard to astringency, but it does not tell the entire story--anecdotes abound regarding wines that have similar tannin concentration but have textural mouthfeel characteristics that are inexplicably different.

For example, the transformation of red wine mouth feel during micro-oxygenation is an indication that wine matrix components (Figure 1), while important in tannin perception, do not fully explain mouthfeel descriptors. In order to better understand tannins within wine, we need to develop a methodology to determine not only the tannin concentration but also how active that tannin is.

Astringency and mouthfeel management

Astringency in red wine is considered to be a result of the precipitation of salivary proteins by tannins and/or the binding of tannins to our oral mucosa. (2) Tannin structure and subsequently the astringency of wine is modified throughout the various winemaking processes and proceeds through development in the bottle of the finished product.

Astringency perception in wine may be modified in several ways that we can broadly refer to as mouthfeel management (Figure 2). The "wine matrix" plays a role with acidity, alcohol concentration, residual sugar, mannoprotein and polysaccharides all contributing in various ways to the rnouthfeel of a wine.
                                Tannin

"Wine Matrix"      Concentration      Composition

* Acidity             * Amount    * Skin/Seed
* Ethanol                         * Size distribution
* Residual sugar                  * Color Incorporation
* Mannoproteins                   * Tannin oxidation
* Polysaccharides

Figure 2. The management of red wine mouthfeel generally
encompasses the management of the attributes shown above.


Acidity, for example, in wine increases both bitterness and astringency, while an increase in alcohol concentration reduces perceived astringency while increasing bitterness and contributing to the "body" of a wine. (2), (4), (5) Residual sugar contributes to the sweetness of wine, which attenuates astringency.

Yeast-derived mannoproteins and grape-derived polysaccharides interact with tannins in wine and can inhibit their astringent qualities. In a perfect world, knowing the tannin composition and concentration would tell us all we need to know about the perceived astringency it would produce, but wines are never that simple.

Research has shown that astringency correlates with tannin concentration, In general, the higher the tannin concentration, the more astringent the wine becomes. (6) However, tannin compositional effects such as skin/seed tannin proportion, polymer size distribution, color incorporation and degree of oxidation are all considered to be of critical importance to understanding the sub-qualities of astringency in wines. (2)

Skin tannins are less bitter and astringent than seed tannins and greater proportions of skin tannins have also been found to correlate with higher perceptions of quality and price point. (7), (8) The size of tannin polymers also has an effect with larger (higher molecular mass) tannins correlating with an increase in perceived astringency. (9)

Recently, a great deal of attention has been given to the incorporation of anthocyanins (the phenolic responsible for color in red wine) into the tannin polymer. Referred to as pigmented polymer, these tannin-anthocyanin products are thought to be less astringent. (8) Tannin oxidation during elevage and development in bottle is generally considered to influence perception by modifying tannin interaction through structure and shape modification. (10)

Protein-tannin interaction, stickiness concept

In order to better understand the role of tannin structure in red wine astringency, it is helpful to understand how tannin interacts with other molecules, including proteins. (11), (12), (13), (14), (15), (16)

From previous studies on the interaction of tannins with model proteins,17,1839 a three-stage process can be proposed where the initial stage of interaction is consistent with a hydrophobically-driven interaction (a chemical force that drives the molecules together). (20) Subsequent stages including aggregation of tannin-protein complexes (the second stage), and precipitation of aggregates (the third stage). Based upon current evidence, this process includes both hydrophobic interactions and hydrogen bonding, with the bottom line effect being that tannin likes to "stick" to protein.

The precipitation of salivary proteins by tannins has been considered to create a loss of lubricity in the mouth, which is responsible for the sensation of astringency. Studies have shown that tannins isolated from skins interact differently with proteins than tannins extracted from seeds. (20)

Tannin structure variation has been shown to influence the extent of hydrophobic interaction and it can be quantitatively measured. It is hypothesized that the extent of hydrophobic reaction between tannin and salivary protein is related to the ability of hydrophobic surfaces to interact (Figure 3).

To conceptualize tannin stickiness, the following is proposed. A young tannin that would be typical of a pre-veraison grape would have a more rod-like structure that would have more surface available for interaction with salivary protein.

However, more modified tannins in matured wines would contain more complex intramolecular bonds and structure thus developing a more globular shape due to structure modification, resulting in less available surface for interaction. The goal of our research is to develop analytics that can more routinely measure the ability of tannin to interact with a hydrophobic surface (hereafter referred to as stickiness).

Recent developments in determining the shape of tannin structures provides supporting evidence for this shape transformation. (10) As stated above, tannin shape modification with red wine age is postulated to ultimately lead to a reduction in the surface available for interaction between tannin and protein, resulting in the tannin being less sticky (Figure 3). This transformation is consistent with sensory evaluation of older wines and the "softening" effect that occurs. With this interaction in mind, a high performance liquid chromatography (HPLC) method was developed to measure tannin stickiness.

In an effort to measure stickiness, and based upon earlier findings, (20) an HPLC method that measures the extent to which tannin interacts with an HPLC column, was developed. (21) The HPLC method consists of a polystyrene divi-nylbenzene column that would only allow for hydrophobic interaction s--the same chemical interaction during the initial stage of tannin-protein interaction, and was modified from a previously published method. (22) After initial success, this HPLC method was further modified and tested. (23)

To acquire stickiness information for the tannin in question, each sample is analyzed at four different column temperatures, and from this temperature response, the stickiness of the tannin towards the HPLC column can be cal-culated. (24) In addition to stickiness, this HPLC method can also provide concentration in formation. (23)

Tannin stickiness is unique and different than concentration

Operating under the hypothesis that stickiness is a property of the tannin rather than a function of its concentration in the system; it was predicted that stickiness would not be affected by concentration. To investigate this, wine or tannin solutions were serially diluted across a large concentration range (from 7408-225 mg/L) and the results showed that regardless of tannin concentration, the stickiness did not vary (-4905 [+ or -] 153 J/mol). This finding is critical because it shows that tannin stickiness is an intensive property--an attribute that is independent of concentration.

The above finding was strengthened by the standard addition of exogenous tannins to a wine. In this experiment, the stickiness of the resultant tannin was found to be dependent on the fractional contribution of the stickiness of individual tannins as opposed to their concentration. This finding indicates that this new analytical method not only measures the concentration of tannin within the sample, but how sticky that tannin is. With this finding, we decided to examine this new stickiness parameter with initial experiments designed to show the potential significance of tannin stickiness.

In order to explore the potential applications of this method in a winemaking application, a gelatin fining trial was conducted. The results were consistent with prior observations that the gelatin preferentially removed higher molecular mass tannins, or stickier tannins. In other words, not only did the concentration of tannin decline, but also the stickiness of the remaining tannin. This finding is in contrast to a dilution experiment where concentration is effected while stickiness is not.

A set of wines was analyzed in order to gain insight into the variation of stickiness in a set of commercial Bordeaux varietal wines. California wines were sourced directly from the winery and Bordeaux wines were purchased from a wine shop. This data set varied by age and region, and the analytical results are shown in Figure 4.

The set of wines showed that tannin concentration and stickiness varied considerably and that there was a broad range of stickiness values possible at any given concentration. There was also a poor correlation (r = 0.24) when stickiness values were plotted against concentration values, which reinforces the finding that tannin concentration and tannin stickiness are independent and unique variables.

Sensory relevance of tannin stickiness Tasting wines with similar tannin concentrations and different stickiness values--and conversely tasting wines with different tannin concentrations but similar stickiness values has been the focal point of preliminary uncontrolled sensory sessions, with promising results. The researchers have worked in collaboration with winemaking teams in California and found good correlations between analytical values and sensory descriptors, albeit with a limited data set.

Conceptualizing the perceptual difference between tannin concentration and stickiness under wine tasting conditions is hypothesized as follows (Figure 5). The concentration of tannin in wine is related to the astringency maximum while stickiness is related more to its duration. This variation in astringency time-intensity is hypothesized to be related to variations in astringency sub-qualities.

Preliminary sensory experiments have shown that tannin stickiness correlates with distinct tactile qualities in wine; however, further more controlled sensory studies are required. Linking sensory data to these analytical results is the next phase of this research.

Funding from the American Vineyard Foundation (AVF) and the California State University Agricultural Research Institute (CSU-ARI) has been secured and will involve Dr. Hildegarde Heymann at the University of California, Davis. We are currently building a large data set of wines in order to target wines with specific stickiness attributes that can then undergo sensory studies.

Summary

This method shows promise with regards to processing decisions that influence tannin composition including grape maturation, pressing, racking, micro-oxygenation, elevage, fining and filtering. The fining trial and exogenous tannin addition experiments in this study showcased the applicability of this method to practical winemaking scenarios, and current projects including extended maceration and additional fining trials.

The novel information that this approach captures has the potential to demystify some of the more frustrating issues of tannm, astringency and overall mouthfeel quality. This new approach may help winemakers to better identify the influence of various winemaking practices on tannin stickiness, and identify wine mouthfeel attributes objectively.

Caption: Figure 1. Generalized view of the role that tannin concentration and composition, in conjunction with wine matrix elements, play in mouthfeel description.

Tannin compositional effects such as skin/ seed tannin proportion, polymer size distribution, color incorporation and degree of oxidation are all considered to be of critical importance to understanding the sub-qualities of astringency in wines.

Caption: Figure 3. Model conceptualizing tannin hydrophobic interaction with salivary protein. In this model, a grape-derived tannin (left), a proline repeat that is particularly abundant in salivary proteins (middle) and an aged tannin (right). Purple ovals indicate regions available for surface interaction.

Tannin stickiness is concentrationindependent and therefore adds to the analytical information avaiLabLe.

Caption: Figure 4. Analysis of wines with groups separated by numbers; Paso Robles Bordeaux wines (1), research-scale California Cabernet Sauvignon wines (2), 2009 Bordeaux wines (3), 2012 North Coast estate-grown barrel samples (4).

Caption: Figure 5. Time-intensity graph indicating astringent response to tannins and with proposed effect of tannin concentration and stickiness on sensory response.

Wineries that are interested in participating in this study and would like to submit samples for analysis, are encouraged to contact Dr. James Kennedy in the Department of Viticulture & Enology at Fresno State (jaken-nedy@csufresno.edu).

Bibliography

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BY Matthew R. Revelette, Ralph S. Yacco and James A. Kennedy, Department of Viticulture & Enology, California State University, Fresno, Calif.
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Title Annotation:WINEMAKING
Comment:Stickiness New concept in grape and wine tannin analysis.(WINEMAKING)
Author:Revelette, Matthew R.; Yacco, Ralph S.; Kennedy, James A.
Publication:Wines & Vines
Date:Jan 1, 2015
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