To: Virginia Vintners
From: Bruce Zoecklein
Subject: Potassium Bitartrate Stability, Winery Planning and Design Manual Available
Tartaric acid species in solution are available in three forms, as undissociated tartaric acid (H2T) as bitartrate ions (HT-) and as tartrate ions (T=). The percentage of tartrate present as HT- is maximum at pH 3.7, generally as a precipitate. Therefore, winemakers are concerned with the potential for bitartrate precipitation and preventing "tartrate casse" formation in the bottle.Crystallization depends on (1) the concentration of the salt and other components that may be involved in the crystallization equilibrium; (2) the presence of nuclei upon which crystalline growth may occur; and (3) the presence of complexing factors that may impede crystal growth. In general, a certain level of supersaturation is necessary for adequate nucleation. Once nucleation has occurred, further crystal growth results in precipitation.During alcoholic fermentation, KHT becomes increasingly insoluble, resulting in supersaturation. Thus, bitartrate stability is frequently achieved naturally. Potassium bitartrate stability is also achieved by chilling (with or without seeding), ion exchange, or combinations of both. In conventional cold stabilization (chill proofing), wines are chilled to a selected low temperature in order to decrease KHT solubility. The optimum temperature needed for bitartrate stabilization is:
Temperature (EC) = % ethanol (vol/vol), divided by 2, minus 1.
KHT precipitation occurs in two stages. During the initial induction stage, the concentration of KHT nuclei increases due to chilling. This is followed by the crystallization stage, where crystal growth and development occur. During conventional chill-proofing, precipitation is most rapid during the first 12 days. After the initial period, KHT precipitation decreases due to decreased levels of KHT saturation. Temperature fluctuations during cold stabilization may have a significant effect on reducing precipitation rates, because of the effect on the speed of nucleation. Without crystal nuclei formation, crystal growth and subsequent precipitation cannot occur. Therefore, simply opening the cellar doors in the winter, although cost effective, may not be ideal for KHT precipitation. Because of the potential for increased absorption of oxygen in wines held at low temperatures, alternatives have been sought.
Complexing factors can greatly affect KHT formation and precipitation. As a result, wine (especially red wine) may remain supersaturated longer than a corresponding alcohol-water solution. Wine can support a supersaturated solution of KHT because portions of the tartrate, bitartrate, and potassium ions may be complexed or bound with other components and thus resistant to reaction and precipitation. Metals, sulfates, proteins, gums, and polyphenols may form complexes with free tartaric acid and potassium, thus inhibiting formation of KHT. The complexes are mainly between polyphenols and tartaric acid in red wines, and between proteins and tartaric acid in whites. In a study of white wines, it was found that sulfates were the most important factor in stability, next to potassium and tartrate. This influence would appear to be due to complex formation between sulfate and potassium. Almost one-half of the sulfate in white wines, and 100% of the sulfate in red wines, is thought to form complexes with potassium ions. Red wine pigments can form complexes with tartaric acid. As pigment polymerization occurs, the holding capacity for tartaric acid diminishes, resulting in delayed precipitation of KHT. Pectins and other polysaccharides such as glucans, produced by Botrytis cinerea, may inhibit bitartrate crystallization, due to crystal adsorption preventing further growth. Thus, each wine, because of its unique composition, will achieve unique solubility and equilibria under imposed temperature conditions.
Wine Processing and Complexing factors
There is a close relationship between wine fining and KHT stabilization. For example, condensed polypheonols interfere with bitartrate precipitation, suggesting that removal of a portion of the polyphenols, by addition of protein fining agents prior to cold stabilization, may enhance subsequent KHT precipitation. Cold stabilization (chill-proofing) may result in precipitation of both KHT and wine proteins. In white wines, proteins may affect tartrate holding capacity, thus inhibiting precipitation. As white wine phenols oxidize and polymerize, binding and co-precipitation with proteins can occur, affecting tartrate holding equilibria and stability. Bentonite fining may decrease the tartrate holding capacity by reducing both proteins and phenolics. Additionally, if a bitartrate unstable wine has a pH below 3.65, chill-proofing causes a downward shift in pH that may enhance protein precipitation (see below). Some winemakers elect to bentonite fine during bitartrate stabilization, which allows KHT crystals to help compact bentonite lees.
Bitartrate Stabilization and Changes in Titratable Acidity and pH
Wines with initial pH values below 3.65 show reductions in pH and titratable acidity (TA) during cold stabilization, because of the generation of one free proton per molecule of KHT precipitated. The pH may drop by as much as 0.2 pH units, with a corresponding decrease in TA of up to 2 g/L. By comparison, KHT precipitation in wines with pH values above 3.65 results in higher pH levels, and corresponding decreases in TA. This is the result of removal of one proton per tartrate anion precipitated. The above values represent ranges seen in practice and may vary.
METHODOLOGY FOR ESTIMATING COLD STABILITY WILL BE DISCUSSED IN THE NEXT ENOLOGY NOTES.
Winery Planning and Design Manual Available
Winery Planning and Design Workshop Proceedings Available. Proceedings of the Winery Planning and Design Workshop conducted in July are available. The 104 page proceedings covers establishing a business plan, and winery design considerations, including gravity flow, winery tank selection, sanitation, etc. Send $45 payable to
Dr. Bruce Zoecklein
Department of Food Science and Technology
Virginia Tech (0418)
Blacksburg, VA 24061
Proceedings are used to support our enology graduate education efforts.
Subscription to Enology Notes. All past Enology Notes and Vintner's Corner newsjournals are posted on the Enology-Grape Chemistry Group's web site at: www.fst.vt.edu/zoecklein/index.html. Enology Notes are different in content from this subscription-based Vintner's Corner newsjoural. To be added to the Enology Notes list serve send an email message to firstname.lastname@example.org with the word "ADD" or "REMOVE" in the subject line.
Dr. Bruce Zoecklein
Associate Professor and Enology Specialist
Head, Enology-Grape Chemistry Group
Department of Food Science and Technology
Virginia Tech, Blacksburg VA 24061
Enology-Grape Chemistry Group Web address: www.fst.vt.edu/zoecklein/index.html
Phone: (540) 231-5325
Fax: (540) 231-9293