How do climate and irrigation affect wine tannin?


In a previous post, we talked about what scientists have learned recently about tannin structure and activity. Tannin is one of the key determinants of wine color and flavors. In last week’s post, we also discussed the importance of microclimates in determining wine and grape quality composition.

This raises a question: Can we link wine sensory indexes with vineyard health indexes? In particular, is there a relationship between tannin sensorial properties, climate and vineyard management practices?

Fruition Sciences conducted a study in 2015 with Dr. Kennedy (UC Davis) and James Campbell (Cal State Fresno) to answer these questions. We focused on the effect of site-specific climate and irrigation practices on maturation profiles and tannins properties, namely sugar accumulation rate, tannin size, pigmented tannin and tannin activity.  

Study design

Our sample included 32 Napa wineries with a focus on Cabernet Sauvignon grapes. 66 blocks were monitored in total across the wineries. For each block, on top of classical measurements (berry weight, sugar, etc.) we took the following measurements on five dates:

  • Anthocyanins
  • Tannin concentration
  • Tannin size
  • Tannin activity

Our chosen sites exhibited stark contrasts in climate conditions. For valley floor sites, temperature difference over 24 hours was 29 degrees Celsius on a hot day like September 9. On that same day, the difference was only 15 degrees Celsius for mountainous areas. This marked difference in temperatures likely resulted in variations in berry development and proanthocyanidin accumulation across sites, according to a 2012 study by Cohen et al. It also affected differently color degradation.

We also varied the irrigation treatments to compare the impact of high-stress and low-stress regimes. The chart below shows the gap in water stress levels post-veraison between the two regimes on mountain sites, as measured by Sap Flow sensors.

Water deficit regimes.png

Impacts on wine tannin

We had several interesting findings on the correlations between various tannin measures. Pigmented tannins are positively correlated with monomeric anthocyanin and inversely correlated with tannin molecular size. In addition, tannin size and tannin pigmentation during berry maturation affect tannin activity.        

In terms of irrigation impacts, water deficit index variations seem to affect the onset of sugar accumulation and coloration. Meanwhile, climate variations across vineyard sites seem to affect pigmented tannin, tannin size and activity. Compared to berries on valley floor sites, those on mountainous sites have higher anthocyanin, smaller tannin size, and lower tannin activity.

This study illustrated the importance of understanding microclimate variations in your vineyard and adjusting your irrigation practices accordingly. Winegrowers can now develop a granular view of temperatures and measure plant-based responses to irrigation using Terra Clima and Sap Flow sensors, respectively.

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  1. Would like to ask: how does deficit irrigation effected onset of sugar accumulation and color? Was it beneficial or detrimental to deficit irrigate after veraison?

    1. Hello Carol,

      Typically a moderate level of water deficit pre-veraison hastens the onset of sugar loading into the berry. This is what we observed in our study. This is also reported through many scientific articles published around 2004-2007 (for instance you can look at the work published by the team of Pr. Matthews at UC-Davis among others). More recently, new studies have shown how sugar transport is affected not only by water deficit but also by a combination of abiotic factors, including day and night heat stress on top of water deficit. The design of our study in Napa – mixing vineyards with large vs. small temperature variations for a similar vine water deficit profile- has helped understanding the relative impact of temperature variations vs. water deficit variations on sugar transport to the berries.

      The same can be said about color accumulation which occurred earlier by 7-10 days in response to moderate water deficit applied pre veraison. Such field observations are in agreement with several scientific studies also recently published on that topic (for instance from Italy).

      Regarding your second question: No, it was not detrimental to irrigate after veraison following a period of moderate water deficit pre veraison. In fact, in the context of the study in Napa in 2015, this strategy was beneficial. Many winemakers and vineyard managers reported that it prevented from observing important berry dehydration pre-harvest, which is considered detrimental for its bad impact on flavor profile and color degradation (as well as really high brix). On top of that, having applied a moderate water deficit pre-veraison boosted the potential for higher flavonoids content at harvest.

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