In the previous blog post, we provided an overview of common methods used in picking harvest dates. This blog post will dive more into how picking date influences tannin development and the linkage between tannin structure and the perception of wine astringency.
Condensed tannins found in berry skins and seeds result from the polymerization of small molecular units called flavan-3-ols. The concentration of flavan-3-ols peaks at veraison and there is no synthesis of flavan-3-ols after veraison. Because of this, the tannin potential of a wine is determined by environmental conditions affecting the vineyard before veraison.
Flavanols form co-pigments with anthocyanins and protect the wine colour against sulphur dioxide bleaching or pH changes. Thus, flavanols contribute to stabilizing wine colour, but they also play a direct role in wine sensory perception. In fact, flavan-3-ols polymers are the main contributors to bitterness and astringency.
The effects of tannin structure on the perception of astringency have been studied by Chira and her colleagues in 2011. After analysing the tannin properties of 23 vintages of the Cabernet Sauvignon-based wines of Ch Mouton-Rothschild, the authors found that older wines had lower polymer size along with weaker astringency perception than the young wines. These results suggest that the size of tannins may directly drive wine’s sensory properties.
In practice, the ratio of astringency to bitterness is low with low-molecular weight flavan-3-ols (found in seeds) and is high with high-molecular weight flavan-3-ols (found in skins). Recent measurements have shown that as fruit ripens, polymers of flavan-3-ols increase in size while at the same time astringency increases faster than bitterness. Thus, the timing of picking directly affects the astringency to bitterness ratio.
In that context, analysis of tannins’ structural properties is becoming more and more interesting to support the picking decision. We discuss two new methods of characterizing tannins’ astringency before and during winemaking.
Essentially, chemical analysis for astringency is based on the interaction between tannins and salivary proteins. Subsequent precipitation reduces the lubricating properties of saliva, leading to sensations of dryness, hardness, and constriction in mouth. In 2012, a team of Italian researchers led by Rinaldi has shown that tannins’ reactivity toward human saliva is a useful tool to measure the tactile sensation of astringency. The saliva precipitation index analyzes saliva before and after the binding reaction with tannins. This index measures red wine astringency and could be used to track fruit tannins’ sensory properties before picking.
In 2013, a team of Californian researchers led by Kennedy has developed the concept of tannin activity to characterize tannins’ sensory properties on fruit and on wine. The approach consists of measuring the energy needed to ‘unstick’ tannins from their bond to salivary proteins. Recent results on fruit tannin composition during ripening suggest that a reduction in the size of tannins correlates with a reduction in tannins’ activity, which directly affects the perception of astringency.
Sensory perception of astringency is influenced by tannins’ size and the relative contribution of seed versus skin tannins in the final wine. Picking date directly affects tannins’ polymerization and thus the distribution of tannins’ size, which in turn affect wine’s sensory properties. Measuring the salivary protein index or the tannins’ activity offers new ways to relate fruit tannins’ structural properties to wine’s sensory properties. If variations in pulp composition become less relevant to support picking decisions, the tracking of tannins’ structural properties could become more and more useful in predicting their sensory impact on wine composition.
The original version of this post was published here.
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