Wednesday, April 26, 2017

Wine styles: Skin-contact whites

In this series on white wine styles, I have treated Champagne and Sparkling wines, reductive wines, and wines made utilizing the process of hyperoxidation. In this post I cover skin-contact white wines.

Skin-contact white wines are recognized by a combination of their residence on the early part of the orange color spectrum, their earthy flavors, and enhanced mouthfeel. These characteristics are the result of macerating the skin of crushed and de-stemmed white grapes in their own juice (i) prior to pressing and (ii) under controlled time and temperature conditions (The procedure is generally carried out under cool conditions in order to limit the growth of spoilage organisms.).

While white juice fermented on their skins are, obviously, in contact with those skins, we differentiate those wines from the ones treated in this post both on the basis of time -- skin contact wines are macerated for between 2 and 24 hours while the fermented-on-skin wine is macerated for weeks to months -- and phase within the production process -- skin contact is a pre-fermentation process while its compatriot extends beyond that to fermentation and, in many cases, maturation.

Maceration refers to the release of constituents from the pomace following crushing and is facilitated by "the liberation and activation of hydrolytic enzymes from crushed cells." Substances extracted include: aromatic compounds, aromatic precursors, phenols and polyphenols, unsaturated lipids, nitrogen, and potassium. At high enough levels, these extractives will produce earthy flavors and enhanced mouthfeel in the wines and will contribute positively to the fermentation processes. The best results are obtained from fully ripe, aromatic grape varieties such as Gewurtztraminer, Riesling, Muscat, Viognier, Chenin Blanc, and Sauvignon Blanc.

What is the makeup of the grape berry skin and what role do the constituent parts play in the makeup of a skin-contact white wine? The berry skin consists of an outer layer with a wax-like coating (cuticle) and 6 to 10 layers of thick-walled cells (hypodermis) which accumulate phenolic compounds in fairly high concentrations as the berry matures (Dharmadhikari, McGlynn). The main components of the skin are phenols, aromatic substances, potassium, and other minerals.

Phenolic compounds are:
  • Responsible for the color of red grapes and wine
  • Involved in the oxidative browning of white wines
  • Contributors to taste and astringency through interactions with salivary proteins.
The two major classes of wine phenolic compounds are flavonoids (defined by a C6-C3-C6 skeleton consisting of two phenolic rings joined by a central, oxygen-containing ring -- Jackson) and nonflavonoids (possessing a C6-C1 or C6-C3 skeleton; all numbers following "C" are subscripts). The sources and roles of the phenolic compounds falling into these two classes are illustrated in the figure below and the relative concentrations of selected classes are provided in the table following.

Table 1. Generalized concentration of various phenolic compounds
present in wine
PhenolicWhite Wine (mg/L)Light Red Wine (mg/L)Full Red Wine (mg/L)
Hydroxycinnamic acids
Other nonflavonoids
Polymeric catechins
Source: Kennedy, et al., Grape and wine phenolics: History and perspective,
AJEV, 57(3), September 2006.

Skin contact increases the amount of hydroxycinnamates, gallic acids, and flavonoids. Flavonoids increase slightly with contact time but strongly with temperature. These compounds are of concern because they contribute to bitterness and astringency and also serve as substrates for oxidation in white wines. While there are elevated levels of astringency in skin-contact white wines, they are nowhere near as high as in red wines. First, even though tannin is extracted from the skin of the white grape, the lack of anthocyanins means that only tannin-tannin bonds are formed, a combination that is less soluble in alcohol. Second, during fermentation, most of the tannin will precipitate out, thus limiting its ability to negatively impact the wine's sensory characteristics.

Aromatic Substances
Aromatic substances are located in the skin and layers of cells immediately below it. Examples of these compounds include (Dharmadhikari):
  • 2-methoxy-3-isobutyl pyrazine -- imparts bell pepper odors to Cabernet Sauvignon and Sauvignon Blanc
  • 4-vinylguaiacol and 4-vinylphenol -- spicy, clove-like, and medicinal odors in some Gewurtztraminers
  • Terpenes -- can be found in Muscats and Rieslings.
Fermentation Benefits
While winemakers do not pursue skin-contact because of the benefits that it provides to the fermentation process, they gladly accept what is offered. Maceration (Jackson):
  • Improves juice fermentability and enhances yeast viability through its release of particulate matter, lipids, and soluble nitrogen compounds into the juice
    • Particulate matter provides surfaces for yeast and bacterial growth, adsorption of nutrients, the binding of toxic C10 and C12 carboxylic fatty acids, and the escape of CO₂
  • Improves the production of extra-cellular mannoproteins formed during alcoholic fermentation
    • When combined with reduced concentrations of carboxylic acid, facilitates malolactic fermentation by Oenococcus oeni.
When contrasted with a more traditional white wine, a skin-contact wine may exhibit lower levels of fruitiness and acidity. In addition, the familiar characteristics of your favorite varietal may be hidden behind a spicy character that may now be present.

Murli Dharmadhikari, Composition of Grapes,
Ron Jackson, Wine Science, Academic Press.
William McGlynn, Basic Grape Berry Structure, April 15, 2012,
Tim Patterson, White Wine Skin Contact, August/September 2013,
Nicola Tazzini, Polyphenols in Grape and Wines: Chemical Composition and Biological Activities, August 9, 2015,

©Wine -- Mise en abyme

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