This may be an unpopular opinion, but I don’t think there is any way to “sell” wine diamonds to the average wine consumer. I’ve heard so many explanations for why they exist in a wine bottle:
- The wine is all natural.
- Wine diamonds are special.
- It’s not glass, it’s acid.
- We just go with the vintage.
At the end of all the explanations, and sometimes we need those excuses because physical chemistry is hard, the reality is that most American consumers still expect brilliantly clear white and rosé wines. Otherwise, they are likely to believe something is wrong with the wine.
Red wines… little bit of a different story. However, I’m finding more and more red-hybrid wine producers cold stabilizing as reassurance policies for their wines.
So let’s talk about cold stabilization. Through my experience, I’ve found winemakers cite misnomers, short cuts, and misunderstandings centralized around the cold stabilization process. Hopefully, this will debunk a few of those myths for you!
Why do wines need to be cold stabilized?
To be clear, I’m talking about grape-based wines in relation to cold stability. Grapes – as a fruit – contain tartaric acid as its primary organic acid. Given the high concentration of this unique acid in [grape] wine, it acts as a component to form the solid, insoluble crystal, potassium bitartrate.
Tartaric acid, the primary acid in grapes, is an organic, weak acid. Weak acids have their protons (abbreviated H+) and anions (abbreviated A-) in an equilibrium. This means the proton and anion can separate and come back together, and that both molecular species can exist at any one time.
The separation or connectiveness of the protons/anions is based on pH. It is this chemical property, that protons separate or come back to the molecular backbone, that gives organic acids their buffering capacity, or resistance to pH changes when acids or bases (alkalines) are added to the wine.
Based on its molecular structure, tartaric acid has two protons that it can hold onto or that can be separated from the primary molecular structure. For this reason, there are three potential molecular forms of tartaric acid:
- Tartaric (sometimes referred to as H2T in the literature): In this molecular form, both protons are attached to the primary structure of tartaric acid. This is also known as the undissociated form of tartaric acid. In the abbreviation, H2T, the “H2” represents the fact that the two protons are still connected to tartaric acid, or T. In this form of tartaric acid, the acid does not carry a positive or negative charge.
- Bitartrate (the bitartrate anion, sometimes referred to as HT– in the literature): This is the partially dissociated form of tartaric acid in which one proton is separated from the primary molecular structure of tartaric acid. Here, there is only one “H” in the abbreviation because one proton has been separated and another is still attached to the primary molecular form of tartaric acid. In the bitartrate form, the molecule carries a negative charge, hence the “-“ sign in the abbreviation.
- Tartrate (the tartrate anion, sometimes referred to as T2- in literature): This is the fully dissociated form of tartaric acid in which both protons are separated from the primary molecular structure of tartaric acid. Note that in the abbreviation, T, there is no “H” to represent the fact that both protons have been removed from the molecule. Furthermore, the primary molecular structure has a “2-minus” charge, which is also noted in the abbreviation.
In wine, the bitartrate anion represents about 50% to 70% of the tartaric acid (Waterhouse et al. 2016). It exists at its maximum concentration somewhere around the pH of 3.60 to 3.65 (Iland et al. 2012). Within this pH range, bitartrate can easily react with the positive ion, potassium (K+), which also naturally exists in wine at various concentrations. (Note: “K” is the atomic symbol for “potassium.”)
When the positive-potassium ion (K+) and negative-bitartrate ion (HT–) join, they create the insoluble salt, potassium bitartrate, typically abbreviated KHT in the literature. Note that the ionic salt, potassium, is positively charged, while bitartrate is negatively charged. If we all remember that age-old elementary school lesson we were told about birds and bees, we’ll recall this basic chemistry principle: Opposites attract!
The crystals that we visually see in wine are solid KHT salt precipitates (deposits). When the KHT falls out of the liquid wine, a drop in wine titratable acidity (TA) is observed. Changes in pH can increase or decrease depending on the wine’s pH prior to KHT’s precipitation (Waterhouse et al. 2016).
Confused about ions separating from a molecular structure? Don’t worry; this is A LOT of chemistry, and it can be confusing. If you find yourself a bit lost in this summary, I recommend becoming a DGW Insider or Elite Member to review the “pH Explained” Winemaking Lesson, which breaks this concept down further while focusing on good pH monitoring techniques.
But wait! What does tartaric acid chemistry have to do with cold stability?
Simply, everything!
Tartaric acid crystals form in wine and precipitate to the bottom of the tank, barrel, or bottle because of bitartrate’s chemical properties in wine.
Because the tartaric acid concentration and pH of the wine dictate a wine’s cold stability properties, any time the wine’s acid is changed, the cold stability property needs to be checked. Changing a wine’s acid structure includes directly adding an acidulent or deacidification agent, blending, or adding any additional ingredients that affect the acid of the wine.
Okay, so the wine has tartaric acid. If the wine is white or rosé, it’s likely that the wine requires cold stabilization. So, just go through the cold stabilization process, right?
Well, not totally.
The reason why I don’t typically blanket or recommend cold stability processes is because
- not all wines require cold stabilization,
- the cold stabilization process selected can have an impact on the wine’s quality and longevity, and
- the effect of a cold stabilization process is unknown. In this last point, this means that just because a winemaker treats a wine with a cold stabilization process, it does not guarantee the wine is cold stabilized. This is because cold stabilization is dependent on the wine’s chemistry at a given temperature. Thus a wine cold stable at 45°F may not be cold stable at 40°F, and so on and so forth.
Let’s break down these 3 points.
In order for potassium bitartrate crystals to form in wine, three things need to be true:
- The wine must contain adequate concentrations of tartaric acid. For grape wines, as this is the primary acid, they do.
- The wine must contain adequate potassium ions. This can be variable from wine-to-wine.
- The equilibrium to produce crystals must be favored. Again, this is variable from wine-to-wine, and the equilibrium is affected by temperature. Cold stability at one temperature does not guarantee cold stability at another temperature.
Some grape wines are naturally cold stable the entire time through their production. Others, are not. This is why testing to determine if a wine is cold stable is so regularly recommended by winemaking experts. The need of cold stabilization is not a given.
Again, going through a cold stabilization does not guarantee the wine is cold stable. And, to reiterate, this is why analytical testing is so regularly recommended by winemaking experts and enologists.
The Effect of the Cold Stabilization Processes
There are several ways a wine can get cold stabilized:
- Natural cold stabilization
- Contact seeding
- Addition of a tartrate inhibitor
Traditional Cold Stabilization
Natural cold stabilization and contact seeding processes are considered traditional cold stabilization operations. Both will have an effect on the pH and TA of the wine after completing the cold stabilization process.
Natural cold stabilization does not refer to putting wines out into the winter cold temperatures. Natural cold stabilization is a defined winemaking operation in which the wine temperature is dropped and held at a consistent temperature for a specific period of time. Again, before calling this operation complete, the wine should be tested to ensure it is cold stable at a specific temperature.
In comparison, subjecting wines to environmental, cold winter temperatures does not often provide the same effect on the wine. The primary reason why subjection to environmental temperatures is variable in its ability to cold stabilize wines is because of the regular and daily fluctuations in temperature that are expected between daytime and nighttime cycles.
Figure 1 shows variation in crystal formation associated with numerous wines in production. A rapid and sustained chilling process is required to increase the number of nucleation sites (microscopic crevices in the wine that initiate crystal growth) required for crystallization (Zoecklein et al. 1999). During natural cold stabilization operations, this occurs. However, when subjected to environmental temperatures, which are uncontrollable in how the temperature drops and if that temperature sustains, the adequate chilling kinetics may not be reached to sustain full bitartrate precipitation.
Furthermore, many winemakers fail to finalize a blend before subjecting wines to environmental temperatures. Later production processes (e.g., wine blending) may render those early “cold stabilization” processes void even if each individual wine had been subjected to the outside cold temperatures. This is due to the alteration of wine pH and acid structure, which influence bitartrate precipitation, upon blending.
In some cases, winemakers may only have access to environmental temperatures. If this is the case for your operation, testing is key to ensure the wine is cold stable. Also, the consideration of tartrate inhibitor additions could be helpful.
Contact seeding is essentially a rapid “natural cold stabilization” process. Natural cold stabilization takes weeks to complete. Wines usually reach cold stability within 10 – 14 days of a temperature being held during the natural cold stabilization process. In contrast, contact seeding takes place over a few hours. With contact seeding, an excess amount of potassium bitartrate is added to the wine with turbulence (mixing) to facilitate the precipitation of potassium bitartrate. At the same time, the wine is rapidly chilled to a specific temperature. After a few hours, the wine is filtered off of the crystals, as contact seeding crystals are small and require filtration removal. Again, going through this process does not guarantee the wine is cold stable. The wine must be tested to ensure cold stability.
Tartrate Inhibitor Cold Stabilization
Both natural cold stabilization and contact seeding methods change the pH and the TA of the wine. Wines with excessively high TA’s and relatively low pH’s can benefit from these two traditional cold stabilization processes by reducing the TA (sourness) of the wine without negatively affecting the pH.
However, other wines may not be good candidates for traditional cold stabilization methods. This includes
- wines with pH’s at or above 3.60,
- wines with ideal TA values or sourness perception, or
- wines that undergo a myriad of latent production operations (e.g., sparkling wines).
Furthermore, for wineries dependent on winter temperatures to cold stabilize wines, given the variability in results, further cold stabilization may be required for a given wine. Finally, for some wines that are notorious for crystal development (e.g., Concord), multiple methods of cold stabilization may be required.
Tartrate inhibitors provide an alternative to traditional cold stabilization methods. A tartrate inhibitor is a substance (ingredient) added to the wine that physically blocks the formation of perceptible potassium bitartrate crystals. Tartrate inhibitors include many gums and some mannoproteins. The most common tartrate inhibitors in the U.S. market include carboxymethylcellulose (CMC) and potassium polyaspartate (KPA, known commercially as “Zenith”). Most have maximum allowable doses, which vary by supplier, and therefore, are only effective up to that dose. This means that like traditional cold stabilization processes, the addition of tartrate inhibitors also comes with limitations.
The use of a tartrate inhibitor “freezes” (pun intended) the acid chemistry. This means that the wine can go from not-cold-stable to cold stable without changing the pH and TA of the wine.
However, the use of a tartrate inhibitor requires the winemaker to reconsider the operational order of the wine’s production. In traditional cold stabilization processes, a winemaker would conduct those operations ideally after the final blend is determined, but before bentonite fining for protein stability. In the case of tartrate inhibition, it is most likely added in between rough/fine filtration (i.e., plate-and-frame, lenticular, or crossflow filtration processes) and sterile filtration. In other words, the tartrate inhibitor is typically one of the last ingredients added to a wine prior to bottling. This means that the wine must undergo blending, protein stabilization, and some level of filtration before the tartrate inhibitor is added to the wine. Additionally, winemakers must adhere to the mixing and stabilization time requirements of the tartrate inhibitor to reduce any issues with sterile filtration and post-bottling potassium bitartrate precipitation.
The use of a tartrate inhibitor really requires winemakers to pay attention to production and operational details, including operational order. For those winemakers that miss the details, filtration clogs or cold unstable wines result.
Furthermore, the addition of the tartrate inhibitor also requires winemakers to pay attention to protein and colloidal (non-crystalline, microscopic components) stability of the wine. Winemakers absolutely have to test the wine for protein stability and adjust protein stability adequately if they plan on adding a tartrate inhibitor to the wine. Failure to do so can result in cloudy wines post-bottling.
Finally, always remember: the addition of a tartrate inhibitor does not guarantee the wine is 100% cold stable. Like with traditional cold stabilization processes, the wine can be tested to determine its cold stability properties.
A Wine Consultant’s Observations on Cold Stabilization
My observations as a wine consultant have taught me two things:
- Many winemakers like to rely on recipe-driven operations without considering the impact their decisions will have on a wine. If a winemaker was taught to always use contact seeding on a wine variety, they are likely still doing it today and assuming the wine is always cold stable. In our pursuit to simplify the winemaking process, we have lost the context as to why some wines require cold stabilization and why others do not. Over simplification has simply led to poor winemaking habits. But the decisions to cold stabilize a wine, in addition to which cold stabilization process to choose, should not be formulaic. Winemakers can improve wine quality by choosing specific processes that are ideal for a wine’s quality.
If I put this in context of the vineyard: most spray programs are not 100% standard. Many vineyard managers have to adjust what to spray and when to spray dependent on the variation in annual weather. Vineyard managers are required to observe the parameters that affect the vines (e.g., weather) and adjust accordingly (e.g., what spray and when to spray). Furthermore, they must understand which sprays interact with one another, which can be combined, which need avoided or interval appropriate, et. cetera.
Winemaking processes are no different than this, but the rhetoric around winemaking often undermines the thought process that goes into winemaking operational decisions. One must observe what is going on with the wine (i.e., measure the chemistry, measure the microbiology) and then adjust the treatment (e.g., cold stabilization) as necessary.
- Many winemakers fail to have a standardized pre-bottling operational plan or a bottling operational plan. Ignorance truly is bliss here. If one does not know all of the potential things in bottle that are affected by our pre-bottling decisions, then a wine can easily get bottled. Every operational decision in a winery influences the wine’s chemistry, which, in turn, influences the wine’s quality and longevity in bottle. Avoiding the measurement of parameters like potassium bitartrate (cold) stability or protein (heat) stability does not mean that the wine is good to go; it simply means that those parameters are being ignored. This is also true of most wine analytes like dissolved oxygen (DO), pH, titratable acidity (TA), and sulfur dioxide (SO2) concentrations to name a few. All of these parameters change in a wine as a wine moves through production. Without recognizing that, winemakers are making decisions blind.
The most common issue I observe working with winemakers that have “tried tartrate inhibitors” is filtration issues that also lead to potassium bitartrate precipitation post-bottling. This requires me to reorganize pre-bottling and bottling operations for the winery. Most initially work with me indicating that the tartrate inhibitor clogged their filters. The reason for this is often the winemaker did not add the tartrate inhibitor at the appropriate time, they rushed through filtration, did not follow operational recommendations, or avoided recommended stabilization times.
Pre-bottling and bottling operations can be somewhat standardized for your operation. The order really doesn’t change that much. But things like stabilization time for certain processes or ingredient additions do play a role in when a winemaker should do certain things leading up to bottling.
If this is something that you struggle with, working with a winemaking consultant can provide instant help. For those that are not in the market for a winemaking consultant, consider joining the DGW Insider or Elite Membership programs. Such memberships provide access to pre-bottling timelines and checklists for wines in the following categories:
Furthermore, if you would like a more thorough understanding of cold stabilization, please check out the Winemaking Lessons, “Reviewing Cold Stability in Wine” and/or “Tartrate Inhibition.” DGW also provides downloadable protocols to Members and Clients on “Conductivity for Cold Stability,” and “Bentonite Fining and Colloid Stability Testing” (for when the winemaker plans on adding a tartrate inhibitor).
References
The above post was written with support from the following resources:
References
Iland, P., N. Bruer, A. Ewart, A. Markides, and J. Sitters. (2012.) Monitoring the winemaking process from grapes to wine: Techniques and concepts. 2nd Edition. ISBN: 978-0-9581605-6-8.
Waterhouse, A.L., G.L. Sacks, and D.W. Jeffrey. (2016.) Understanding Wine Chemistry. ISBN: 978-1118627808
Zoeckelin, B.W., K.C. Fugelsang, B.H. Gump, and F.S. Nury. (1999.) Wine Analysis and Production. ISBN: 0-8342-1701-5
The views and opinions expressed through dgwinemaking.com are intended for general informational purposes only. Denise Gardner Winemaking does not assume any responsibility or liability for those winery, cidery, or alcohol-producing operations that choose to use any of the information seen here or within dgwinemaking.com.
