By: Denise M. Gardner
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.
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.
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, 2nd edition. 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
By: Denise M. Gardner
Wine protein chemistry is a daunting subject to tackle for a wine novice, and can also be a challenge for those who have been in the industry for years.
Proteins exist in wine in a state of charge: positive, negative, or neutral. The stability, and charge, of proteins is determined by the protein’s isoelectric point (or pI), in which the net charge of the protein is 0. Proteins are most unstable at their pI. Isoelectric points are based on pH, and alterations of pH in the wine will ultimately affect the charge of proteins in the wine. Alterations to wine chemistry (i.e. cold stabilization, acidification or deacidification, blending, and use of some fining agents) may alter protein stability of a given wine.
[Production Note: Due to the fact that protein stability is based on pH, winemakers should allow a wine to go through cold stabilization before testing and treating for protein stability. This is because alterations to the wine’s pH and titratable acidity (TA) will occur as a result of cold stabilization.]
Factors that affect protein stability
In general, lower pH wines have less protein stability problems than higher pH wines. This is a consideration for those producing lower pH (<3.4) wines. It is possible that the wines may not need bentonite additions.
The only way to evaluate if a wine is a protein stable is through a heat/protein stability test. For a protocol on protein stability, please refer to Bruce Zoecklein’s “Wine Analysis and Production” book (page 469-473) or via this online protocol. Most experts recommend following an ethanol-based test for better accuracy affiliated with protein stability. Additionally, wineries can contract certified wine labs to run protein stability tests for them, in addition to determining a rate for bentonite additions.
Tannin content can also affect the stability of wine proteins. As some tannins bind with proteins, having a higher concentration of tannins in the wine may make the wine more protein stable. This is one why some wine grape varieties, like Pinot Noir or some hybrids, have more protein instability problems than others. Additionally, white and rosé wines are more likely to carry protein instability problems as they are typically lower in tannins available to bind to proteins, but other factors contribute to this generality.
Some wine grape varieties may also be high in wine proteins, which dictate a need for more aggressive protein fining treatments. Gruner Veltliner and native varieties like Concord and Niagara anecdotally tend to have more consistent issues related to protein stability than other varieties. For Gruner Veltliner, juice treatment with bentonite additions is often recommended. Native varieties tend to require a treatment with Sparkolloid.
Be forewarned that protein stability can alter for a given grape variety each vintage year; a wine harvested in one vintage year may be protein stable for that particular year, but not so for a second, consecutive year. Protein stability varies with vintage season, and should be evaluated on each wine annually.
Why is bentonite used to treat protein instabilities?
Bentonite is a clay-based fining agent used in winemaking to bind and remove some wine proteins through precipitation or acts as a clarifying agent. Winemakers should note that there is a lot of variation between bentonite products and suppliers, and some products may be more effective compared to others. Winemakers should also ensure that the bentonite they are using is strictly for wine additions.
Bentonite is negatively charged, and will, therefore, bind with positively charged proteins in wine. It is possible to treat a wine with bentonite and still have protein stability problems by:
- A concentration effect in which case all of the proteins in the treated wine have not been bound by bentonite.
- A charge effect in which negatively charged proteins have not been bound and, therefore, remain in the wine.
In contrast, the fining agent Sparkolloid offers alternatives to winemakers as it is primarily positively charged and will bind to negatively charged proteins. Winemakers are always recommended to do bench trials before treating a wine with any fining agent.
Like many fining agents, bentonite may have some alternative side effects including aroma/flavor and color stripping. While it is sometimes preferred to add bentonite without testing a wine for protein stability, winemakers run several risks towards the wine including:
- No knowledge towards the protein stability of the wine even after treatment of bentonite. (Keep in mind that some wines require 10+ pounds per 1,000 gallon additions of bentonite.)
- Unnecessary stripping of aroma/flavor composition or wine color when there is no protein instability issue in the wine. Depending on the wine, this can come as a detriment to wine quality.
- Potential off-flavor development in aroma-neutral varieties.
- Economic losses for the winery if unnecessary additions of bentonite are made to a wine without protein instabilities (i.e. waste of product). Or a waste of resources if the wine hazes in the bottle even after a bentonite addition was made during production. If no analytical evaluation to reassure the winery that the addition was successful at stabilizing the wine, the time and cost affiliated with un-bottling, treating, and re-bottling wine can be significant. [Note: Protein stability tests are not a 100% guarantee, but provide a winemaker with more information than guessing a wine’s protein stability]
- Unnecessary use of employee time for fining if the wine does not require a bentonite addition.
Practical Directions for Wineries
Wineries should consider routine analysis for protein stability to avoid guesswork affiliated with bentonite additions. Some wineries may find they are making unnecessary bentonite additions or find a need for making larger bentonite additions. After a fining treatment, wines should be re-evaluated for protein stability to ensure that the treatment has worked properly.
When adding bentonite to any wine, it is recommended that the fining agent be hydrated in warm, chlorine-free water. Bentonite additions should be thoroughly mixed into a wine for approximately 15 minutes, and racked after a week’s settling time. Most wine suppliers will provide preparation and use instructions for wineries unfamiliar with bentonite treatments.
More Practical Information
This article does not touch upon a lot of the theory, chemistry, preparation of bentonite, and wine treatment for protein stability issues. For more information available online, please consider the following resources:
“Fining Agents for Use in Wine” from The Wine Lab
Wine Analysis and Production by Bruce W. Zoecklein et al. (ISBN: 0-8342-1701-5)
Winemaking Problems Solved, edited by Christian E. Butzke (ISBN: 978-1-84569-475-3 or 978-1-4398-3416)