Cold Stabilization
Cold stabilization is the process of chilling wine prior to bottling in order to hasten the crystallization and precipitation of potassium bitartrate (cream of tartar) and thereby prevent “wine diamonds” from forming in the bottle. Home winemakers tend not to be overly concerned about these tartrate deposits because they know they are harmless. However, commercial winemakers go to considerable effort to prevent them. If they don't, customers will return wine when they find “glass” or “particles” in the bottle.
Preliminary points
- Cold stabilization only works for potassium bitartrate instability. There are other possible crystalline salts (calcium tartrate, calcium oxalate) but their precipitation is not assisted by cold temperatures. You can cold stabilize your wine and still end up with “wine diamonds,” although this will be relatively rare.
- Changes to a wine (e.g., blending, fining) will affect its bitartrate holding capacity. Therefore cold stabilization is better done when other changes to the wine are complete.
- Higher pH wines drop more bitartrate.
- White wines are more likely to be cold stabilized because (a) they are bottled earlier and have less time for natural precipitation, (b) they are commonly stored at colder temperatures, and (c) their sediment is more noticeable. However, commercial wineries usually cold stabilize reds as well.
- Red wines undergo many more changes in the bottle than white wines—and therefore can drop their bitartrate years later. As the tannins change and color oxidizes in an aging red wine, bitartrate that was previously bound to these phenols is newly available for crystallization and becomes part of the sediment.
The technique of cold stabilization
Over the course of a year or two, potassium bitartrate will crystallize and precipitate even at moderate temperatures. Because potassium bitartrate is less soluble at colder temperature, chilling speeds up the process—the colder, the faster. Commercial wine makers chill large tanks of wine through refrigeration. They aim for about one week at -5.5°C or two weeks at -3.9°C.
The Vancouver Island home winemaker usually opts for a less precise method, either:
- Several months at a very cool cellar temperature; OR
- One month in a fridge or outdoors in the coldest part of our winter. Winter elsewhere can be too cold—wine can freeze, in which case you end up with a broken carboy and a mess.
You can assist with process of bitartrate crystallization and stabilization by:
- Seeding with finely powdered cream of tartar. Add one gram per liter and stir thoroughly. You don’t need to be precise about additions. The cream of tartar will not dissolve in the wine, which is already supersaturated with it. It simply acts as small crystals, the nuclei, which will attract the bitartrate in the wine, thereby creating bigger crystals, which fall to the bottom. Getting crystals started is the hardest (most energy intensive) part of crystallization. Seeding jump-starts the process.
- Stirring regularly. This increases the contact between growing crystals and the wine. But don’t introduce oxygen and thereby endanger the wine.
- Filtering the wine while it is cold. Filtering captures the small crystals that may still be in suspension.
Commercial wineries have other techniques they can use to remove bitartrate, but they are not feasible for the home winemaker.
Cold stabilization and acid reduction
People often say the wine is “dropping acid” when bitartrate precipitates. This is because it takes 1.00 gram of tartaric acid to make 1.26 grams of potassium bitartrate. Although not a lot of new potassium bitartrate is formed during cold stabilization (most already exists in solution), it turns out that cold stabilization can reduce TA in a wine about 0.3 grams per liter. And the magic is that pH also goes down a bit, due to the removal of positive potassium ions.
Sources
Ron S. Jackson (1994). Wine Science: Principles and Applications (Academic Press), 283-84.
Bruce Zoecklein (1988). “A Review of Potassium Bitartrate Stabilization of Wines,” Virginia Cooperative Extension Service, Publication 463-013. Available in .pdf format at http://www.fst.vt.edu/extension/enology/PotBitar.pdf
See also the FAQ: Should I fine my wine before cold stabilization?
Drafted by Rod Church, January 24, 2004.