Grape skins
Grape skins are the outer layers of the grape berry and play a central role in determining the colour, structure, aroma, and ageing potential of wine. In winemaking, grape skins are the primary source of phenolic compounds, including anthocyanins, tannins, and many aroma precursors, and their interaction with grape juice during fermentation and maceration strongly influences wine style.[1]
Anatomy and composition
The grape skin forms part of the berry’s exocarp and consists of several layers of cells protected by a waxy cuticle. This outer surface contains natural yeasts, epicuticular waxes, and compounds involved in defence against dehydration and ultraviolet radiation. Beneath the cuticle, skin cells contain vacuoles rich in phenolic compounds and aroma precursors.[2]
Key chemical constituents of grape skins include anthocyanins (in red and black varieties), flavanols and tannins, flavonols, and glycosylated aroma compounds. The relative proportions of these compounds vary widely by grape variety, climate, vineyard conditions, and ripeness at harvest.[3]
Role in wine colour
In red wines, grape skins are the exclusive source of anthocyanins, the pigments responsible for red, purple, and blue hues. These compounds are extracted into the must during fermentation and subsequently interact with tannins and other phenolics to form more stable colour complexes over time.[4]
In white winemaking, skins are usually removed prior to fermentation to avoid colour and phenolic extraction. However, extended skin contact in white wines can produce deeper colour and phenolic texture, as seen in so-called orange wines.[5]
Extraction and maceration
The transfer of skin-derived compounds into wine occurs primarily during maceration, when grape skins remain in contact with juice or fermenting must. Factors influencing extraction include temperature, alcohol concentration, duration of contact, cap management techniques, and grape skin thickness.[6]
Anthocyanins are generally extracted early in fermentation, while tannins and other phenolics are extracted more slowly and are enhanced by rising alcohol levels. Winemakers manage skin contact carefully to balance colour intensity, tannic structure, and aromatic expression.[7]
Varietal and viticultural influences
Skin thickness and composition are strongly variety-dependent. Thick-skinned varieties such as Cabernet Sauvignon or Tannat tend to produce wines with higher tannin and colour intensity, while thin-skinned varieties such as Pinot Noir yield lighter-coloured, more delicate wines.[8]
Viticultural factors such as sunlight exposure, water availability, crop load, and disease pressure also affect skin development and phenolic maturity. Sun-exposed skins typically contain higher levels of flavonols and more stable colour compounds.[9]
Aromatic contribution
Many important aroma compounds originate in grape skins, either as free volatiles or as bound precursors released during fermentation and ageing. Terpenes, norisoprenoids, and thiol precursors are commonly associated with skin tissues and contribute to varietal aroma expression.[10]
The degree of skin contact therefore has a direct impact not only on structure and colour but also on aromatic complexity, particularly in aromatic grape varieties.[11]
See also
References
- ↑ Jancis Robinson, Oxford Companion to Wine, Oxford University Press, 17 Sept. 2015. ISBN 9780198705383.
- ↑ Michael G. Mullins, Alain Bouquet, Larry E. Williams, & 0 more, Biology of the Grapevine, Cambridge University Press, July 30, 2007. ISBN 9780521038676.
- ↑ Pascal Ribéreau-Gayon, Yves Glories, Alain Maujean, Denis Dubourdieu, & 1 more, Handbook of Enology, Volume 2: The Chemistry of Wine - Stabilization and Treatments, Wiley, 31 Mar. 2006. ISBN 9780470010372.
- ↑ Cheynier et al., Wine Chemistry and Biochemistry, Elsevier, 2012, ISBN 9780123884381.
- ↑ Stevenson, The Sotheby’s Wine Encyclopedia, 5th ed., DK, 2011, ISBN 9780756686840.
- ↑ Roger B. Boulton, Vernon L. Singleton, Linda F. Bisson, Ralph E. Kunkee, & 1 more, Principles and Practices of Winemaking, Springer, 31 Oct. 1998. ISBN 9780834212701.
- ↑ PhD Jackson, Ronald S., Wine Science: Principles and Applications, Academic Press Inc, 14 April 2020. ISBN 9780128161180.
- ↑ Jancis Robinson, Jose Vouillamoz, Julia Harding, & 0 more, Wine Grapes: A Complete Guide to 1,368 Vine Varieties, Including Their Origins and Flavours, Ecco, 1 Nov. 2012. ISBN 9780062206367.
- ↑ Markus Keller, The Science of Grapevines: Anatomy and Physiology, Academic Press Inc, 19 Jan. 2015. ISBN 9780124199873.
- ↑ Andrew L. Waterhouse, Understanding Wine Chemistry, Wiley, 19 Aug. 2016. ISBN 9781118627808.
- ↑ Peynaud, Knowing and Making Wine, Wiley, 1984, ISBN 9780471881491.