The long months post-harvest require regular attention by cellar staff and winemakers to ensure that wine quality is upheld through storage conditions. Wine stability, while somewhat nebulous, is essential to obtain in order to ensure the wine’s quality will be upheld post-sale. Below is a list of cellar maintenance practices that are recommended in preparation before the growing (and bottling) season.
Monitor Sulfur Dioxide Concentrations
Now (i.e., the winter and spring months) is a good time to regularly check sulfur dioxide concentrations of wines sitting in tanks and barrels waiting to get bottled. At minimum, wines should be checked once a month for free sulfur dioxide concentrations. Some winemakers opt to check barreled wines every other month in order to minimize frequently opening the barrel.
Proper sanitation and sampling is required for best analytical results:
- Use clean sampling bottles when taking wine samples
- Make sure that you sanitize any valves or sampling ports before and after releasing a sample from a tank. At the very least, you can use a food-grade alcohol solution spray or a citric acid-sulfur dioxide mix as a sanitizing agent.
- Properly clean and sanitize wine thieves or other sampling devices each time you use it to take a sample from a barrel or the top of tank. Warm water is not enough to sanitize a wine thief. We recommend using a citric acid-sulfur dioxide mix for quick dipping in between barrel sampling.
For wines that have completed primary fermentation and/or malolactic fermentation, maintaining a molecular free sulfur dioxide concentration is helpful to reduce the risk of yeast and bacterial spoilage. For a review on sulfur dioxide and making sulfur dioxide additions, please refer to this Penn State Wine Made Easy fact sheet.
It is essential to clean and sanitize your wine thief in between sampling from barrels. Photo from: Denise M. Gardner
Cold (Tartrate) Stabilization
Cold stabilization is often utilized to avoid the precipitation of tartrate crystals, which is common in instable wines at cooler temperatures.
In 2012, Virginia (Smith) Mitchell, now head winemaker at Galer Estate Winery, wrote a primer on cold stabilization techniques available for wine producers: http://extension.psu.edu/food/enology/analytical-services/assessment-of-cold-stabilization This primer covered everything from how to analyze for cold stability to the use of carboxymethylcellulose (CMC) to avoid tartaric acid crystallization in wine.
Prior to putting a wine through cold stabilization, it is worth the time and effort to analyze the wine for cold stability. Not all wines end up having cold stabilization problems. For those wines that do not, going through the cold stabilization process can actually minimize wine quality by stripping out delicate aromas and flavors, or altering taste or mouthfeel attributes of the wine. This doesn’t touch upon the amount of wasted time and effort to cold stabilize wines that are otherwise cold stable.
The above report recommends several testing procedures to ensure tartrate stability of a wine.
With the relatively warmer 2015-2016 winter, many winemakers may need to turn to artificial chilling in order to cold stabilize their wines properly. Again, this could be used as an argument to test wines prior to cold stabilization to minimize the use of electricity and to better manage the flow of wines in and out of the cold stabilization tank.
Wines that do undergo cold stabilization will likely have changes in pH and titratable acidity (TA) that can ultimately affect other parameters of the wine: protein (heat) stability, color, sulfur dioxide concentrations, and volatile acidity. It is prudent to check these components analytically following the cold stabilization process.
Protein (Heat) Stabilization
Proteins in wine can elicit hazes in wines post-bottling that may be off-putting to some consumers. While the proteins cause no effect on wine quality, they do cause an alteration in the appearance of the wine. Some varieties, like Gruner Veltliner, have naturally high concentrations of proteins, and, therefore, require a more aggressive approach to protein fining. Other varietals, however, may not require protein fining with bentonite at all.
Wines should undergo protein (heat) stability after they are cold stabilized due to the fact that cold stabilization will affect the acidity (pH and TA) of the wine, and therefore, alter protein stability properties of the wine. Again, winemakers are encouraged to check the wine for protein stability prior to treating a wine with bentonite.
Bentonite is a fining agent used to bind any proteins in a wine that would otherwise be considered unstable. However, if the addition of bentonite is unnecessary (i.e., the wine is protein stable and does not provide a component for bentonite to bind to, bentonite can bind to other components in the wine, most specifically: aroma and flavor active compounds. While this has been shown in the research literature, it is unclear how detrimental the loss of aromatic compounds is to the wine (Marchal and Waters 2010). Additionally, bentonite additions have been noted to strip color out of rosé and red wines (Butzke 2010).
A summary from UC Davis on heat stability testing can useful to understand the positive points and limitations of protein stability testing. Protocols for heat stability tests can be found here from Dr. Bruce Zoecklein. Additionally,ETS Labs has provided a small summary of how to interpret heat stability results, which can be helpful for wineries that are not used to reading analytical results on this test.
Additionally, wineries can submit wines to ISO-accredited labs for a bentonite trial in which the lab pinpoints the exact concentration of bentonite needed to heat stabilize the wine. This may be helpful to avoid making too little or too much bentonite additions, which costs time and labor in the winery.
Bench trials may be needed to determine how much bentonite is needed to obtain protein stability of your wine. Remember to use the same source and lot of bentonite in both your bench trials and commercial application. Photo from: Denise M. Gardner
Finally, if wineries are conducting their own bench trials, they are encouraged to use the same lot of bentonite in both the trials and the commercial application (Marchal and Waters 2010). This is due to the natural variability associated with most bentonite products. Finally, unless otherwise stated by the supplier, bentonite should always be blended in chlorine-free, hot (60°C, 140°F) water (Butzke 2010), and allowed to cool to room temperature so that the bentonite can swell. Allowing the slurry to cool will ensure that the wine is not exposed to a hot slurry.
Butzke, C. 2010. “What Should I use: sodium or calcium bentonite?” In: Winemaking Problems Solved. Christian E. Butzke, Ed. Woodhead Publishing Limited and CRC Press, Boca Raton, FL. ISBN: 978-1-4398-3416-9
Marchal, R. and Waters, E.J. 2010. “New directions in stabilization, clarification and fining of white wines.” In: Managing wine quality, volume 2. Andrew G. Reynolds, Ed. Woodhead Publishing Limited, Great Abington, UK. ISBN: 978-1-84569-798-3
Iland, P., N. Bruer, A. Ewart, A. Markids, and J. Sitters. 2012. Monitoring the winemaking process from grapes to wine: techniques and concepts, 2ndedition. Patrick Iland Wine Promotions Pty. Ltd., Adelaide, Australia. ISBN: 978-0-9581605-6-8.
Penn State Extension Wine Made Easy: Sulfur Dioxide Management:http://extension.psu.edu/publications/ee0093
Penn State Extension: Assessment on Cold Stabilization:http://extension.psu.edu/food/enology/analytical-services/assessment-of-cold-stabilization
UC Davis: Heat Stability Testing:http://wineserver.ucdavis.edu/pdf/attachment/88%20stability%20tests%20and%20haze%20formation%20.pdf
Virginia Tech: Protein Stability Determination in Juice and Wine (1991):http://www.apps.fst.vt.edu/extension/enology/downloads/ProteinS.pdf
ETS Labs: Interpreting Heat Stability Tests:https://www.etslabs.com/assets/PTB011-Interpretation%20of%20Heat%20Stability%20Results%20and%20Turbidity%20Readings.pdf