Theory of Cold Stabilization
Cold stability is a method of separating unstable natural ionic salts (potassium: K+, calcium: Ca2+, bitartrate: HT-) from wine. After fermentation, but prior to bottling, cold stability is conducted to prevent the tartaric salt, bitartrate (HT-), from precipitating out of the wine when stored and/or chilled post-bottling.
Precipitation occurs due to the instability of tartaric acid in its bitartrate salt form, which is known as potassium hydrogen tartrate (KHT) as a supersaturated solution. KHT is commonly referred to as potassium bitartrate or cream of tartar, and accounts for much of wine's sourness or tartness (Butzke 2010). Instability occurs when the concentrations of potassium (K+) and bitartrate (HT-) bind to form the KHT product that exceeds solubility in the wine (Iland et al. 2004), thus precipitating out of the wine.
The pH of wine should be checked prior to cold stabilization as pH affects the efficacy of stabilization techniques. A higher pH wine (>3.6) will precipitate out more KHT in comparison to a lower pH wine (<3.6) (Church 2004). This is due to the percentage of the HT- present in the wine at a particular pH. The amount of tartaric acid in HT- form, as a function of pH, is illustrated by the acid stability curve seen in Figure 1.
Figure 1 was provided courtesy of Dr. Roger Boulton in 2012, University of California, Davis.
As seen in Figure 1, the maximum concentration of bitartrate (HT-) exists at pH 3.6. At such pH, there is much available substrate to bind with K+ to form the KHT product. As a result of KHT precipitation, the titratable acidity will decrease. However, changes in pH are a result of the initial pH prior to cold stabilization. A direct relationship between pH value and percentage of HT- occurs. If the initial pH of the wine, prior to cold stabilization is below pH 3.6, then the pH will decrease as KHT precipitates out of solution. If the initial pH is above pH 3.6, then the pH will increase as KHT precipitates out of solution (Iland 2004).
Changes to the wine (e.g., blending or fining) will affect the KHT holding capacity of the wine (Church 2004). The presence of colloidal materials, such as mannoproteins, pectins, and other polysaccharides, also affect the precipitation of KHT. Solubility of KHT also depends on alcohol content. KHT is more soluble in water than in alcohol, so it is more soluble in juice than in wine, hence the tendency to precipitate from wine (Butzke 2010).
To appease consumers, winemakers often cold stabilize wines to obtain a flawless, clear visual final product. If a wine is untreated and chilled to a low temperature in the consumer's refrigerator, the unstable KHT crystals may precipitate out of the wine and settle at the bottom of the bottle or glass. White colored crystals are commonly referred to as "wine diamonds" because of their characteristic shine, similar to diamonds. Consumers often confuse these harmless crystals with glass fragments. KHT crystals may also appear smaller and darker in color, which may also offend consumers that are unaware of the harmless precipitate. Though the crystals pose no health risk, the continuous concern from consumers require winemakers to find a method for cold stabilizing their product.
In general, it is more common for white wines than red wines to be cold stabilized for the following reasons (Church 2004):
- White wines are bottled earlier than red wines so KHT crystals have less time for precipitation.
- Consumers commonly store white wines at colder temperatures, which increases the risk for bottle precipitation.
- Crystals are typically more noticeable in white wines than in red wines.
Methods of Cold Stability
Various methods for stabilizing wines are discussed below. These methods include a traditional stabilization by cooling, contact seeding, the use of an electrical field (Electrodialysis), and inhibition methods by product addition.
Stabilization by Cooling
As temperature impacts solubility, KHT crystals are more likely to form and precipitate out of wine by chilling the wine to lower temperatures for a defined period of time, usually up to three weeks (Butzke 2010). The wine is chilled to just above the freezing point because KHT solubility is reduced at a lower temperature. Alcohol concentration affects the freezing point of wine. As alcohol increases, the freezing temperature decreases, hence having an effect on stabilization. For example, the freezing point of a 10% ethanol by volume solution is approximately 25°F, 12% is 23°F, and 14% is 21°F (Butzke 2010). Therefore, higher alcohol wines must be chilled to a more extreme temperature to facilitate precipitation.
In the initial stages of chilling, KHT crystals rapidly precipitate out of solution. Over time, precipitation slows due to the reduction in KHT saturation level as seen in Figure 1.
Chilling methods such as refrigeration, use of insulated tanks, or exposure to low winter temperatures can be used to effectively lower the temperature of wine. Insulated tanks may save money in comparison to the cost of refrigerating the cellar (Dharmadhikari 1994). Moving tanks outside or exposing cellars to winter weather may be economically reasonable, but may not be convenient or effective in years that have warmer winter temperatures or variable warming periods.
Stabilization by cooling can be time consuming and costly for winemakers. However, it is the traditional and most common method practiced by many winemakers in the wine industry (Church 2004).
Contact seeding involves the direct addition of potassium bitartrate (KHT) powder to a wine to create KHT nucleation sites. Nucleation is the process of initiating crystal growth by generating sites to form crystals. Formation of crystals is influenced by the quantity of KHT powder, crystal size, contact time, and temperature of seeding (Zoecklein et al. 1995). The amount of KHT used for seeding must create a supersaturated wine solution in order to force crystal formation, which is insoluble in wine. Concentration of seed additions may be up to 4 g/L, but this is not always required for stabilization (Zoecklein et al. 1995).
Zoecklein et al. (1995) recommends seeding at any temperature, but should be equal to the desired stability temperature. Seed should be completed in tanks with regular agitation to increase surface area exposure of the crystals, which influences the rate and success of crystallization (Zoecklein et al. 1995). By mixing tanks, KHT precipitation is enhanced because more nucleation sites for crystal development are created (Zoecklein et al. 1995).
The main disadvantage of contact seeding is cost associated with purchasing KHT. Anecdotal reports also indicate seeding can diminish wine quality. Others believe this is the most cost effective and practical way of cold stabilizing wines.
Electrodialysis (ED) is the separation of ions using a charged membrane to enhance diffusion. This process is often applied to desalinate salt water, but can also be used to remove bitartrate ions from wine using the same principles. In wines, positively charged ions (K+, Ca2+) are separated from the negatively charged bitartrate ion (HT-) by pumping wine across selective membrane sheets under the influence of a charged current (Wilkes 2006). As the wine passes through an electrically charged chamber, those ions migrate towards their opposing ionic membranes (i.e., K+ and Ca2+ to a negatively charged electrode and HT- to a positively charged electrode) while the wine itself continues to flow into a second holding chamber. The resulting wine has a minimized concentration of HT- ions, which makes it less likely to form and precipitate KHT crystals. Hence the wine is more cold stable.
ED was shown to be energy-efficient, maintain wine quality, reduce wine volume loss, and reduce processing time from several weeks to days (Fok 2008). However, ED uses more water than other cold stabilization practices and requires insulated tanks (Fok 2008). It may also be expensive for small boutique wineries.
Inhibition Techniques by Product Addition
Carboxymethyl Cellulose (CMC)
Carboxymethyl cellulose (CMC) is a long-chain cellulose gum with attached carboxymethyl (-CH2COOH) groups to its chain. It is water soluble and physiologically inert. CMC functions as an inhibitor of crystal growth by eliminating nucleation sites, restricting further crystal growth. This reaction is microscopic. Addition of CMC causes no known changes in pH, titratable acidity (TA), tartaric acid concentrations, or organoleptic effects.
Not all CMC polymers are created equal and differ slightly among suppliers. There are several commercial suppliers that sell CMC products.
Meta Tartaric Acid
Meta tartaric acid is formed by the structural esterification of tartaric acid through heating under controlled conditions (Dharmadhikari 1994). It is primarily recommended for wines that are sold and consumed within six months (Zoecklein et al. 1995). Precipitation inhibition is due to the coating of bitartrate crystals by meta tartaric acid (Zoecklein et al. 1995). This coating prevents crystal growth. Over time, the meta tartaric acid rehydrates and forms tartaric acid, which is susceptible to crystal growth (Dharmadhikari 1994). However, a higher esterification of the tartaric acid will give a longer period of tartaric stability. Meta tartartic addition is usually required right before bottling (Zoecklein et al. 1995).
Previous research has shown wines matured on lees tend to have greater tartaric stability (Theron 2007). One possible explanation to this observation is the production of mannoproteins, a major component of yeasts' cell walls, which is released during yeast lysis (Theron 2007). Mannoproteins have been shown to inhibit the formation of KHT by coating potential nucleation sites (Theron 2007). Without nucleation sites, the crystallization growth of KHT may not progress.
Gum Arabic is a complex compound made up of saccharides and glycoproteins, harvested from acacia trees, and can be in solid or purified liquid form (Church 2004). Gum Arabics serve many potential functions in wine, including color stabilization in young red and rosé wines, enhanced mouthfeel, reduced bitterness, and cold stabilization. The additive forms a protective coating over nucleation sites, inhibiting KHT crystal formation and precipitation (Church 2004). The Gum Arabic is added post filtration as the filtering process removes colloids from solution and destroys the protective cover supplied by the additive (Church 2004).
Detection of Cold Stable Wines
An analytical test should precede cold stabilization and also be conducted after a cold stabilization technique is applied to wine to verify the process. Various analytical methods to determine if the wine is cold stable are discussed below.
An estimation of KHT stability is based on the decrease in conductivity of the wine over a period of time (Iland et al. 2004). Potassium ions are primarily responsible for the conductivity in wine when the potassium bitartrate crystals form, the potassium ion concentration drops along with conductivity (ETS Laboratories 2012). Conductivity testing methods are based on temperature, time, and seeding. When the sample is chilled, the wine is seeded with KHT crystals. The naturally occurring KHT crystals attaches to these seeded crystals, which causes a drop in the wine's electrical conductivity (ETS Laboratories 2012). A change of less than 5% in electrical conductance over the testing period is considered stable (Zoecklein et al. 1995). A passing sample (stable wine) is only stable at or above the testing temperature (Zoecklein et al. 1995). However, the interpretation of the testing results is up to each winemaker's discretion. For potential conductivity test protocols, please see the following references:
- In Monitoring the winemaking process from grapes to wine: techniques and concepts by Patrick Iland et al. 2004. pp 82-83.
- Cold Stability Test for Wine by ETS Laboratories
- In Methodology for Estimating Cold Stability by Bruce Zoecklein et al. 1995. pp 234-6.
Potassium Concentration Product
The Potassium Concentration Product (KCP) method estimates the relative potassium bitartrate (KHT) stability of wine. This method indicates the concentration of the potassium bitartrate at a given temperature. The pH and the alcohol content of the wine will determine the percent of bitartrate ions (HT-), while the KCP value is estimated by the concentrations of bitartrate (HT-) and potassium (K+). This value is expressed as KCP x 10-5.
Work by Berg and Akioshi (1971) determined reference KCP values for different wine types using the KCP method. If the KCP value determined at the winery is lower than the reference value, the wine is most likely stable. If the KCP value is higher than the reference value, the wine is more likely to form KHT crystals. Though this method may be a quick and easy test for cold stability, the KCP method is only recommended for wines that undergo traditional cold stabilization methods. The KCP method is not recommended when product additions (e.g., CMC, mannoprotein, meta tartaric) are applied or when electrodialysis methods are used.
The freeze test involves freezing a wine sample until it is the consistency of a slushy, and then allowing the sample to thaw at room temperature. When completely thawed, if crystals are present in the sample, then the wine is considered unstable. If crystals are absent, then the wine is considered cold stable. This detection method is relatively quick and inexpensive, but has numerous disadvantages. The sample must be monitored continuously to avoid over freezing, and consistent results are difficult to achieve (Wilkes 2012). Results have not always been reliable in determining cold stability. Additionally, the sample size, sample shape, freezer, and particulates can affect freezing time and effectiveness of the test (Wilkes 2012).
Cold stability is often considered an essential step in producing quality wine. Various production methods are used in the industry as a means to cold stabilize. Often, winemakers are looking for more economical or efficient solutions when cold stabilizing wines. Ultimately, the method chosen for cold stabilization can be regarded as a stylistic choice or preference, as each has recognizable benefits and disadvantages.
Product additions offer new options for winemakers during cold stabilization. As new products emerge, suppliers may find a secondary function of a product that helps prevent KHT precipitation. Bench trials are always recommended prior to commercial application, as wine chemistry differences can affect the efficacy of many products.
Additionally, analytical methods offer only an estimation of wine's cold stability. It is possible for winemakers to receive a "stable" result using one test, and an "unstable" result using another test for the same wine. For this reason, winemakers may choose to run several different tests or primarily use one test to achieve analytical results. Regardless of this choice, analytical testing ensures the reliability of a wine's cold stability, and should be used as part of the winery's quality control program.
References and Readings
Berg, H.W., M Akioshi. 1971. The utility of potassium bitartrate concentration product values in wine processing. Am. J. Enol. Vitic. 22:3: pp. 127-134.
Bower, P, C. Gouty, V. Moine, R. Marsh, and T. Battaglene. 2010. CMC: A New
Potassium Bitartrate Stabilisation Tool. Rep. no. 558. Winecheck.
Bosso, A., D. Salmaso, E. De Faveri, M. Guatia and D. Francheschi. 2010. The use of carboxymethylcellulose for tartaric stabilization of white wines in comparison with other oenological additives. Vitis. 49 (2), 95-99.
Boulton, R. 2012. Tartrate Solubility Curve. image.
Butzke, C. 2011. Wine Cold Stability: Assessments and Techniques. Purdue Wine Grape Team. Purdue University.
Butzke, C. 2010. Wine Cold Stability Issues. Purdue Extension. Purdue University.
Church, R. 2004. NW Winemaking Notes: Cold Stabilization. Nanaimo Winemakers.
Dharmadhikari, M. 1994. Methods for Tartrate Stabilization of Wine. Tech. Iowa State.
Enartis Vinquiry. 2012.
ETS Laboratories. 2012. Understanding Cold Stability Testing. Technical Bulletin.
Gusmer Enterprises, Inc. 2012.
Fok, S. 2008. PG&E Studies Electrodialysis for Cold Stability. Practical Winery & Vineyard Journal. Pacific Gas & Electric Co.
Iland, P., N. Bruer, A. Ewart, A. Markides, J. Sitters. 2004. Monitoring the winemaking process from grapes to wine techniques and concepts. Patrick Iland Wine Promotions PTY LTD, Cambelltown, SA 5074 Australia.
Lutin, F, and D. Bar. 2007. Keep it Natural! Adjusting the pH of food products without chemical additives thanks to Bipolar Membrane Electrodialysis. Ameridia, Division of Eurodia Industrie.
Patterson, T. 2009. Tartrate stabilization in a jug: mannoproteins prevent precipitation and save energy. Wines & Vines.
Wine Secrets. 2012 STARS Electrodialysis. image.
Theron, C. 2007. The Use of Mannoproteins for the Tartrate Stabilisation of Wine. Wynboer. WineLand.
Wilkes, E. 2006. Cold Stability: Anything But Stable." Lecture. Interwinery Conference. Mildura. Fosters Wine Estates.
Wilkes, E. 2012. Gum Arabic. Wine Chemical Dictionary.
Zoecklein, B., K. Fugelsang, B. Gump, and F. Nury. 1995. Wine Analysis and Production. Chapman & Hall, New York.
Prepared by Virginia (Smith) Mitchell, Penn State Food Science Undergraduate student