As Predicted, Record Grape Tannins Require Winemakers to Vigilantly Track Fermentations.

Stratospheric grape tannins require finesse and fine tuning to balance extraction in 2017.

by Doug McKesson

October 2, 2017—Seasonably cool conditions over California over the past couple of weeks allowed the late ripening varieties to recharge and rehydrate. Thankfully brix accumulation slowed down, and in many cases returned to pre-heat wave levels.

This has caused the color (total anthocyanin) to also drop a bit as well, which is more likely a function of rehydration as opposed to actual color loss. Let us remember that as of today, we are 141 days since bloom and—more importantly just 59 days since veraison in Napa Valley. 10-year average for Cabernet Sauvignon is 143 and 70 days post-bloom and post-veraison, respectively. October 4th, marks the early window for optimum color and tannin levels. If you are harvesting grapes with less than 1200 total anthocyanin and tannins are still 1400-1500, range you will be challenged to make a high quality Style III wine in 2017. 

How Can I Predict my Wine Quality from my Grape Numbers?
After pouring over 15 years of analysis data, we are seeing remarkably consistent tannin extractions year over year. Over the long term, extraction averages for tannin is 95% of the grape numbers. Complex anthocyanin varies more due to seasonality, but averages 75% of the grape numbers. If we apply this logic to our 2017 grape averages, which are currently 1500 tannin and 293 complex, we could predict that we will get 1425 Tannin and 216 complex anthocyanin at drain down.

Figure 1. (Top) 2017 Daily temperatures vs historical averages record temperatures in St. Helena (Bottom) Huglin Heat Summations for 2017 compared to past years for St. Helena weather station. 

Click the image for the live version on our website.

Not only is this average tannin level outside of our ideal or maximum tannin threshold, the ratio of complex to tannin is too low for a Napa Valley luxury Cabernet Sauvignon. These numbers are simply reinforcing what we already know (or perhaps refuse to admit): Things just are not physiologically ready quite yet. Because of the late veraison and brutal late August early September heat, our sense is that grape maturity is not aligned with the Gregorian calendar. A few ill-timed record temperature heat spells in August and early September has also affected the grape chemistries. Things appear to be calming down.  Ideally another week to 10 days will allow things to get back in line and we cross the crucial DVH milestone.
If you don’t have to pick due to poor vine or fruit condition right now, then do not! Keep the water flowing through the short and moderate warmup this week and grab another grape sample every 7 days to track the maturity.

This Week’s Weather
Currently we are looking at clear skies with breezy conditions across portions of our region through the day due to a strong northerly gradient. Temperatures are forecast to fall just short of yesterday with 60s and 70s expected in almost all locations. Similar conditions can be expected tomorrow. A ridge will rebuild into our region for the second half of the week leading to a rebound in temperatures.

Right now the weekend looks to be warm and sunny in all areas -- upper 70s to mid 80s at the coast with mid 80s to mid 90s inland.

Figure 2. Current weather conditions and 7-day forecast for St. Helena, CA as of 12:00 PM Monday, October 2, 2017. Click the image for the live version on our website.

A seasonably cool, dry air mass remains over the region this morning as a broad mid/upper level trough sits over the intermountain west and stretches westward toward the West Coast. Meanwhile, as high pressure builds over the eastern Pacific, pressure gradients over the region remain tight and have resulted in breezy to gusty winds in the hills/higher elevations. This is especially true in the north and East Bay hills/mountains where wind gusts have been in excess of 45 miles per hour through the night along with relative humidity values at 30% or lower. Thus, the critical fire weather threat persists over these regions, especially at or above 1000 feet in elevation. These offshore winds have also held overnight temperatures up in portions of Napa County.

The overall weather pattern aloft will change very little through Tuesday with seasonably cool temperatures, breezy to locally gusty winds and mostly clear/sunny conditions. Wind speeds are forecast to diminish slightly late tonight into Tuesday as a vigorous short-wave disturbance digs down the backside of the aforementioned mid/upper level ridge. This too will bring slightly cooler overnight conditions to the region with a reinforcing shot of cold air advection. 

The embedded disturbance and the broader trough is then forecast to shift further inland late in the week as a ridge of high pressure builds along the West Coast. This will result in a gradual warming trend with daytime temperatures reaching back into the upper 70s to lower 80s at the coast and into the upper 80s or lower 90s inland by Friday. These conditions along with dry weather are then likely to persist into the upcoming weekend as well.

Observations This Week: Tannins trending way above normal.
Tannin levels are some of the highest on record for all late ripening Bordeaux varieties. 2017 will likely wind up as the most tannic vintage recorded, surpassing 2013, the current record holder. Average grape tannin for Cabernet Franc and Cabernet Sauvignon are 1303 ppm and 1503 ppm respectively. Grape tannins in Malbec (1587 ppm) and Petit Verdot (1419 ppm) are the 2nd and 3rd highest tannin vintages respectively.

However, grape samples assayed last weekend showed slight downward tannin movement  This could be the start of the trend we have been waiting for.

Figure 3. Box Plot of Grape Tannin Levels 2002-2017

After a drop in total and complex anthocyanins after the heat earlier this month, we are seeing a bit of a recovery in grape samples this week. However they have not all rebounded enough in many areas to alleviate our concern over color levels. This is especially worrisome because on average we only extract 70% of the complex anthocyanin from the grape, yet we average close to 100% tannin extraction. This has the potential to be a tannic vintage.

Overall fruit condition is OK, with varying degrees of shrivel observed in hillsides and areas with afternoon exposure in Napa and Sonoma counties. 

Cabernet Sauvignon - Customize fermentation based on grape color levels.
Early returns showing high tannins, but variable anthocyanin levels. So far winemakers have been keeping tannin extraction down to 83% on the first few extractions. However, it is still early and past history shows that we extract nearly 100% of the grape on average, so we can’t get too confident that we will not extract all of this tannin. The most difficult dilemma for Cabernet Sauvignon in 2017 will be constructing the Standard Fermentation Plan (SFP) for low colored grapes. The real question is—"Should I use enzymes this year?" Enzymes work very well at liberating color and could be helpful to maximize the extraction from low anthocyanin grape tissues. However, this almost always increases tannin extraction, too. We predict that low color in some vineyards will definitely require pectolytic enzyme even though it will increase the rate of tannin extraction and effectively reduce the optimum tannin-extraction window significantly. If one is to add enzyme during cold soak, we suggest sampling at inoculation, then again at 18-20 brix. These numbers will help us predict the rate of tannin and complex anthocyanin extraction early in the fermentation, and allow us to make specific customizations for temperature control and cap management. Please contact us for specifics and up-to-the-minute trends that we are seeing.

Figure 4. 2017 Cabernet Sauvignon Tannin Extractions (Through October 1, 2017)For highly colored vineyards, we would consider not using enzymes for the first couple of fermentations at this point.

Extraction is volatile in this early stage of the Cabernet harvest, and appears to be driven by vineyard location and grape chemistry. Some vineyards are overflowing with inky black color, while others still have pale, tough grape tissues. This is a vintage where grape analysis will serve as a direct predictor of your extraction capacity.

Figure 5. 2017 Cabernet Sauvignon Complex Anthocyanin Extractions (Through October 1, 2017)The key to Cabernet Sauvignon extraction is (per usual) minimizing tannin by capturing the color and complex early. CAVEAT: If grape color is low, it is always easier to fix our mistakes by making a smaller wine, rather than over-extracting.

A low Complex to Tannin ratio will (almost) never "catch up" after 12-14° Brix. If your ratio is not ideal at this point and you have already extracted 60% of your grape Total Anthocyanins, make your drain and press decision using tannin as your primary criteria. By all means, avoid that siren song that calls us to chase for more complex anthocyanin by pushing the extraction further than we should. The likely outcome will be a monster high tannin wine with comparatively low color. We advise switching gears and making a smaller Style IV blending wine to combine with bigger, more extracted wines.

Merlot — Hot August nights spell another down year for a once prolific grape.
Once again, lightly colored grapes are yielding sub-par wine this vintage. 2017 marked the lowest total anthocyanin levels in Merlot grapes over 24° brix in history—just 986 mg/L. Even though we are on average extracting 80% of the total color from the grape, there just isn’t enough ‘feedstock’ to fuel the complex anthocyanin machine. We can look no further than extraction percentage of complex anthocyanin—150 ppm Complex from a potential of only 180 ppm.

Figure 6. 2017 Merlot Tannin Extractions (Through October 1, 2017)

Figure 7. 2017 Merlot Complex Anthocyanin Extractions (Through October 1, 2017)

One cannot say that growing season weather isn’t playing a role here. Rapidly declining color development in Merlot absolutely must be tied to the overall, and especially overnight increase in temperatures we have been observing and discussing for the past three growing seasons. What does the future hold for Merlot in context of California luxury wine?

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