One common method of reducing tartaric acid in wine is “cold stabilization”, also called “cold crashing”. This is performed by chilling the wine, holding at a lower temperature for a period of time, and racking the wine while cold.
This has 2 primary effects:
- Reduce tartaric acid, as excess acid precipitates as crystals
- Clear the wine, as the reduced temperature will induce larger particles to precipitate, and the acid crystals will pull some particles with them as they precipitate.
Effect #2 is always useful. However, Effect #1 may or may not be a benefit:
- If the wine is high in acid, cold stabilization may reduce the acid, making the wine more palatable while avoiding or reducing use of chemicals to lower the acid level.
- If the wine is not high in acid, too much acid may be removed, producing a flabby, tasteless wine.
Why Does This Work?
Particles are held in suspension in the wine, up to a point at which a given particle type is saturated. For example, with tartaric acid, the wine can hold a certain amount in suspension, but if more acid is present, it will fall out of suspension as tartrate crystals.
This is temperature dependent, e.g., a warmer liquid will be capable of holding more of a particle type in suspension.
When the temperature is reduced the saturation threshold is reduced, e.g., the wine cannot hold as much tartaric acid in suspension. The excess acid forms crystals which drop to the bottom of the container or adhere to the walls. This has the beneficial side effect of attracting other large particles, so the wine clears faster and possibly better.
How to Do It
Cold stabilization is performed by chilling the wine to a specific point, maintaining that temperature for 1 to 2 weeks, then racking off the sediment before wine warms up. The method is quite easy.
However, having the facilities to execute the process may be difficult, depending on batch size and what winemaker has available.
When I lived in Upstate NY, the temperature on my porch ranged from 33 F (0.5 C) to 38 F (3.3 C) for most of December through March, so I put carboys on the porch for a week. This was true unless there was a serious cold snap, e.g., the outside temperature dropped below 10 F (-12.2 C), so I watched the temperature and did not experience problems.
An acquaintance had a large refrigerator from which he removed all racks. It fit 2 carboys so he’d put 2 in the fridge for 3 to 4 weeks each.
Another option is to move the wine into gallon / 4 liter jugs and put as many in the fridge as there is room. This is not as convenient, but it works.
A more labor intensive option is to build a “chill box”, an enclosure in which 1 or more carboys can be placed along with dry ice. I have read of others doing this, but haven’t done it myself, so I cannot provide any advice.
It’s not necessary or even advisable to perform cold stabilization on all wines.
White wines from grapes grown in colder climates typically have high acid levels and often benefit from cold stabilization. High acid reds may benefit, while lower acid wines (white, red or fruit) may be damaged as the acid reduction may produce flabby, tasteless wine (as mentioned above).
Note that cold stabilization works only if there is excess tartaric acid — if other acids (such as malic) are high, this may not produce a useful result.
The following advice is based upon my personal experience and my memory of Chemistry 101/102 and Physics 101/102.
32 F (0 C) or less – Some folks recommend reducing the temperature below the freezing point of water. The freezing point of wine is not a single value. It varies by alcohol content and other constituents, including sulfite. Various sources make different claims regarding the freezing point of wine — the points that appear to be the most realistic is that 10% ABV wine freezes at 25 F (-3.9 C) while 12.5% ABV wine freezes at 22.5 F (-5.3 C).
Freezing damages wine, so I’d not use a temperature below 30 F (-1.1 C). Since I don’t go below freezing, the actual freezing point of wine doesn’t matter to me. Anyone choosing to cold stabilize below 27 F (-2.8 C) should be cautious.
33 F (0.5 C) to 40 F (4.4 C) – In my experience this produces the best results, e.g., provides the best reduction in acid, and the wine is in no danger of freezing. Closer to 33 F is produces the best results, but anything below 40 F is sufficient.
41 F (5 C) to 50 F (10 C) – This is next best, e.g., it may reduce some of the acid. If the wine is not highly acidic, this is probably a good choice.
51 F (10.6 C) to 60 F (15.6 C) – This temperature range is unlikely to reduce the tartaric acid levels much unless the level is very high.
Some bottled wines, when refrigerated for longer periods, may develop tartrate crystals. While these detract from the wine’s appearance, they are not harmful. Cold stabilizing in this temperature range may produce some crystals, which helps prevent crystals from forming in the bottle while not drastically reducing the overall acid.
My cellar is fairly steady at 58 F (14.4 C) during the winter months, so all my fall & winter wines reside at this temperature for months. I make it a point to rack sometime after each wine has spent at least a month at this temperature. Given that I make wine from kits, fruits, and hot weather grapes, acid levels are not a problem. I do find that natural clearing works well, and believe the lower-than-room temperature helps in this respect.
I cannot recall any of my wines forming crystals in the fridge. This may be because of crystals dropping unnoticed in the sediment during bulk aging OR the wines simply don’t have high enough levels of tartaric acid that the reduced temperature caused some to fall out of suspension in the fridge.
My experience is that bulk aging wine in the mid to upper 50’s may be beneficial, but more importantly, does no harm.
Caution – do not use low temperature cold stabilization unless you are sure you wine needs it. If the wine tastes fine, e.g., it doesn’t taste acidic, think before you do. It’s easy to get into a yo-yo cycle of reducing acid too much, adding acid back and raising it too high, then reducing the acid …
An experienced winemaker told me that wine makes itself. We provide guidance and correction, and should let the wine alone as much as possible.