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Sulfur dioxide, or SO2, is a very common additive in winemaking and is used to inhibit microbial activity (primarily bacteria) as well as prevent oxidation. It should also be noted that sulfur dioxide is a natural byproduct of yeast fermentation so even if you are not adding SO2 there will almost always be small amounts present in wine.
Some links for further reading:
If alcohol is a preservative, why do winemakers add sulfur dioxide? (source: Wine Spectator)
The Bottom Line on Sulfites in Wine (source: Wine Folly)
Our use of SO2 is very minimal in comparison to most commercial wineries. Currently we utilize SO2 during three stages of the winemaking process. We endeavor to reduce and elimante earlier additions in our future vintages by incorporating more rigorous protocols, sorting, etc prior to crush.
We occasionally will add 10 parts per million (ppm) SO2 when the grapes are picked or received at the winery to help inhibit any initial bacterial activity. We want healthy native yeast strains to populate and take over fermentation from the start.
We always add 20 ppm after the wine has finished both primary and malolactic fermentation. This addition is for both microbial stability (inhibiting bacterial growth) as well as helping prevent oxidation.
From time to time, we do a small 5 ppm SO2 addition right before bottling. This final addition is mainly to help prevent oxidation.
This SO2 protocol is applicable to all of our wines, meaning most of our wines have a total SO2 addition of 35 ppm. For comparison, many wineries will add a total of 50-100 ppm on average.
Tartaric acid is the main acid found in grapes. Winemakers can also decide to add tartaric acid, usually to increase the acidity of juice or wine. But… we utilize tartaric acid a bit differently here.
Tartaric acid and potassium are both naturally occurring in grapes, and in Texas we currently see higher than average potassium levels. Tartaric acid has the ability to bind to potassium to form potassium bitartrate, which precipitates out of the wine in the form of wine crystals. (You might notice these crystals in the punt of some of our white wines.)
In red grapes, potassium is primarily found in the skins and seeds of the fruit. During fermentation, when the juice is in contact with the skins and seeds, potassium levels in the juice increases, which also causes the pH to rise. Due to the higher potassium we see in grapes from Texas, we occasionally see a big increase in pH during fermentation of our red wines. High pH can cause stability issues in the wines down the road, affecting microbial stability, color, taste, and mouthfeel.
tl;dr High potassium in grapes leads to high pH levels in wine, and high pH negatively impacts wine stability. Tartaric acid can help precipitate potassium out of solution, so that we don’t get those big increases in pH. This is the exploration of a short term solution as we continue to dial in our viticulture so that our potassium levels reach the crush pad at lower levels. We are also learning to live with higher than university prescribed pH.
Some links for further reading:
Making [red] wine from fruit high in potassium (source: Penn State Extension Wine and Grapes U.)
Winemaking with high pH, high TA and high potassium fruit (source: Australian Wine Research Institute)
This past vintage we experimented with adding 0.5g/liter of tartaric acid to two red lots. We were not adding in order to influence pH at an early stage, we knew this addition would buffer out with the potassium and fall out of the juice prior to fermentation. Our interest was whether it would help mitigate the large swings in pH post malolactic fermentation that are typical in our region. Ultimately our goal is to address the high potassium levels in the vineyard.
Wines we added tartaric acid to:
Hoover Valley Sangiovese/Grenache blend (0.5 g/liter)
Robert Clay Vineyard Merlot (0.5 g/liter)
Bentonite is a natural clay that is used in winemaking to ‘fine’ wine.
The AWRI explains that “the purpose of adding a fining agent preparation to wine is to soften or reduce its astringency and/or bitterness; remove proteins capable of haze formation; or reduce colour by the adsorption and precipitation of polymeric phenols and tannins. The fining agent reacts with wine components either chemically or physically, to form a new complex that can separate from the wine” (Fining Agents, n.d.).
Suspended proteins in wines are what primarily cause wine to be cloudy or hazy. When a wine has too many suspended proteins, these proteins can cause too much “mouthfeel” and can muddy the wine. What that means is that the elevated level of proteins can very much distract from the wine by causing too much “noise” in terms of both mouthfeel and aromatics.
When we add a small amount of bentonite clay prior to bottling, that bentonite binds to some of the proteins and the bound bentonite and protein then settle to the bottom of the tank. This helps clarify the wine and brighten both mouthfeel and aromatics.
Some links for further reading:
Fining Agents > Bentonite (source: Australian Wine Research Institute)
We added bentonite to a select few white wines. We ferment many of our whites in concrete tanks and have found that the proteins do not precipitate as well as they do in wines fermented in stainless steel or oak. As you will notice some of these wines remain somewhat cloudy as clarity isn’t our goal in this process. It is our hope in the future to let some of these wines age longer in order for these particles to fall out on their own.
Wines we added bentonite to:
2021 Grand Gestures (1 g/liter)
2021 Beautiful Vulnerability (1.5 g/liter)
2021 Don’t Forget to Soar (1 g/liter)