Skin-fermented white wines are treated in the same manner as are red wines; that is, fermentation of the juice in the presence of grape skins, seeds, and, in some cases, stems. The key difference between these wines and conventional white wines are the extractives from the skins, seeds, and stems (if stems are included). The key difference between these wines and skin-contact wines are the greater concentration of extractives from the skin and the extractives from the seeds and, if utilized, the stems.
The figure below illustrates the flavonoid phenolic compounds in wines and their sources. I have described the skin and its extractives in the previously mentioned post. I will provide a brief summary of the seeds and stems in the following.
Grape seeds are comprised of an outer seed coat, an endosperm, and an embryo, with the seed coat containing relatively large concentrations of tannin. Jackson stipulates that the predominant phenolics in seeds are the flavan-3-ols catechin, epicatechin, and procyanidin polymers (the latter a condensed tannin). The tannins in the seed walls are more polymerized, and contain a higher proportion of epicatechin gallate, than those in the inner portions. Phenol levels in the seed are higher than in the skin or stems (60% versus 20% each) but they are seldom extracted to their "full potential" during wine production due to the lipid coating which retards tannin extraction until alcohol content becomes a facilitator (Jackson).
Seed tannins weigh, on average, 3.5 - 5 mg per berry while skin tannins weigh in between 0.5 and 0.9 mg. Seed tannin polymers are shorter than skin tannin polymers (the longer the tannin chain the higher the astringency) yet seed tannins are perceived as harsher, greener, and more astringent due to a greater degree of galloylation.
Grape tannins accumulate during the first period of berry growth with skin tannin synthesis beginning earlier than seed tannin synthesis and then ending with the conclusion of the first phase of growth. Seed tannin synthesis continues into the early period of berry ripening before concluding. Both skin and seed tannins continue to mature during the berry ripening phase.
Skin tannins release early and easily but then plateau. Seed tannin release is slow, steady, and long and requires alcohol as a solvent. Tannin extraction will continue throughout fermentation with the ratio tilting in favor of seed tannin somtime during the process.
Grape stems are comprised of cellulose (approximately 30%), hemicellulose (21%), lignin(17%), tannin (16%), proteins (6%), and other constituents. As was the case for seeds, stem flavan-3-ols occur primarily as catechin, epicatechin, epigallocatechin, epicatechin-gallate, and condensed tannins (procyanidin oligomers and polymers).
In a study of wines made with varying levels of stem inclusion, Suriano, et al., came to the following conclusions:
- Wines vinified in the presence of stems were higher in tannins, flavans, vanillin, and proanthocyanidins
- Stems conferred more structure and flavor to the wines
- Stems facilitated must aeration thus promoting synthesis of higher alcohols and ethyl esters by the yeast.
Stem condensed tannins are also considered to be very bitter and astringent and fall between skin and seed tannins in size. Green stems should be avoided as it will take years for the wine in which it is included to mellow out (Pambianchi).
To summarize (Gil, et al.):
- Seeds and stems are major sources of phenolic compounds that condition the final composition of the wine
- Seeds release significant amounts of flavan-3-ol monomers as well as proanthocyanidins with relatively low mean degree of polymerization and a high percentage of galloylation
- Seeds also increase astringency and bitterness and generate a slight but significant decrease in ethanol content, probably through the release of potassium and water
- Stems also release flavan-3-ol monomers and proanthocyanidins but their composition differs from those of the seeds
- (+)-gallocatechin replaces (-)-epicatechin
- Procyanidins had a higher mean degree of polymerization than those from seeds and a higher percentage of prodelphinidins
- Stems significantly increased the pH and decreased the titratable acidity and ethanol content (probably through the solubilization of potassium and water) of the finished wine.
In my next post I will discuss two broad categorizations of skin-fermented white wines.
Mariona Gil, et al., Influence of Grape Seeds and Stem on Wine Composition and Astringency, Journal of Agricultural and Food Chemistry, August 2016.
Herraiz, et al., Effects of the presence of skin during alcoholic fermentation on the composition of new volatiles, Vitis 29, 1990.
Ronald Jackson, Wine Science, 3rd ed., Academic Press.
M. Jose Jara-Palacios, et al., Detailed phenolic composition of white grape by-products by RRLC/MS and measurement of antioxidant activity, Talanta, 2014.
James A. Kennedy, Grape and wine phenolics: Observations and recent findings, Ciencia e Investigacion Agraria 35, 2008.
William McGlynn, Basic Grape Berry Structure, articles.extension.org, April 15, 2012.
Daniel Pambianchi, Tannin Chemistry: Techniques, winemakermag.com, April/May 2011.
Jean-Marc Souquet, et al., Phenolic Composition of Grape Stems, Journal of Agricultural and Food Chemistry, May 2000.
S. Suriano, et al., Major phenolic and volatile compounds and their influence on sensorial aspects in stem-contact fermentation winemakingof Primitivo red wines, J Food Sci Technol 53, 2016.
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