22-Sep-09 08:32 | Eric Stafne

Mark Chien Grape and Wine Educator from Penn State sends out a very informative newsletter.  In his last one, there was an attachment with great stuff from Bruce Zoecklein in Virginia.  I have copied a portion below:

Sensory Significance. What is the sensory significance of specific phenols? All phenols are both bitter and astringent. Bitterness is the result of access to membrane-bound receptors, and is likely limited by molecular size.

Generally, polymerization, or binding together, increases astringency to a point, due to an increased number of possible hydrogen binding sites. Monomeric flavonoids (relatively low molecular weight single flavonoid phenols) are primarily bitter; however, upon polymerization, their astringency increases more rapidly than their bitterness (Noble, 1998).

The relative proportion of bitter to astringent phenolic compounds is important with regard to textural quality. The ratio of bitterness to astringency helps to explain the concept of hard vs. soft tannins. Grapes yielding intense and balanced wines are also characterized as having a high anthocyanin to tannin ratio (Cheynier et al., 1998).

If there is insufficient color (anthocyanins), there will not be enough for bonding with non-colored phenols, such as tannins. There is never a disadvantage in getting the darkest color possible, but there is a notable disadvantage in having too much tannin.

The interactions of wine structural components, including phenols, are also an important feature governing wine texture as a function of the interaction with other structural elements, according to the following relationships (Zoecklein et al., 1995).

Suppleness Index: Alcohol – (Acidity + Tannin)
Palate Balance Equation
Sweet <–> Acid + Phenolics
Carbohydrates Organic acids Skin, seed, and stem phenol
Polysaccharides Barrel phenol
Ethanol Enological tannins
Volatile phenols

Wine structural/textural components interact in certain relationships depicted here in what I call the palate balance equation.

This inverse relationship suggests that an increase in the perception on one side decreases the perception of components on the other. The converse is also true.

With this in mind, it is easy to understand how the specific components of wine mouthfeel interact, and make some important winemaking inferences.

The sweet elements in a dry red wine are derived from carbohydrates, polysaccharides, and mainly ethanol. The acid elements are grape-derived organic acids.

The phenolic elements include input from these components: skin, seeds, stems, plus winemaker intrusion, such as barrels and tannin additions.

Sweet <–> Acid + Phenolics (tannin intensity, astringency, bitterness and dry tannins)

The phenolic elements in this relationship include the perceptions derived from the following: tannin intensity, astringency, bitterness, and dry tannins in a relationship well described by Delteil (2003). Some correlations and associations of astringency and bitterness are provided below:

Astringency
+ corr with grape and oak tannins
+ corr with acidity
0 corr with sugar
+ corr with VSCs (volatile sulfur compounds) and herbaceous compounds
– corr with alcohol up to 14%, + corr above 14%
+ corr with non-soluble solids

Bitterness
+ corr with ethanol
+ corr with grape and oak tannins, including immature seed tannins
+ corr with acid, specifically malic acid
+ corr with VSCs
+ corr with yeast in suspension
– corr with polysaccharides (reason for addition using high polysaccharide-producing yeast, yeast fining, and gums such as Gum Arabic)

Additionally, as viscosity and/or pH increase, the perception of astringency decreases, while bitterness is unchanged. Increasing the alcohol concentration increases the perception of bitterness, and reduces astringency. Astringency masks bitterness. This is an extremely important concept, as it relates to both aging and fining (see Zoecklein, 1988). It also helps to explain why even experienced sensory evaluators may have difficulty in distinguishing bitterness from astringency.

An understanding of the relationships among the various phenols and other wine components is essential to the understanding of textural quality of wines.

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