Cold Stabilization

 

We can use Cold Stabilization to reduce the formation of sediments (i.e., prevent the precipitation of crystals when bottles are stored at low temperatures for extended periods) and reduce the amount of tartaric acid in wine.

Theory

Tartaric Acid (H2T) dissociates into Bitrate (HT-) and Tartrate (T2-) depending on the pH and the temperature of the wine. The chart shows the distribution at 25 oC.

In the presence of Potassium ions (K+), of which there is plenty in wine, Bitartrate combines to form Potassium Tartrate (KHT). At high concentrations, Potassium Tartrate will crystallize and fall out as sediment. The concentration at which crystallization happens (i.e., the wine becomes unstable) depends on the pH, the temperature, and the alcohol content of the wine. The ability of a wine to hold KHT in solution increases the higher the pH, the higher the temperature, and the lower the alcohol. Consequently, tartrate crystals can form in the bottle when the wine is cooled down or stored for a long time. The crystals show up at the bottom of the cork and as sediment in the bottle. While they are not harmful or degrade the wine, some consider their appearance as crystalline sediment unattractive.


Practice

The ideal temperature T to create rapid precipitation of tartrate crystals in oC is in approximate terms: T= -A/2 + 1, where A stands for the % alcohol level in the wine (e.g., if A=13%, then the ideal temperature is minus 6.5 oC or 20.3 oF). To get there, we need a glycol-cooled vessel. To experiment, we built such a vessel: it is a 30 gal steel tank with copper cooling coils on the outside and both inserted into a plastic drum holding cooling fluid that sits inside an insulated wood box. The cooling liquid inside the copper coils is Propylene Glycol cooled down by our chiller (Kreyer Chilly Max). The picture shows the components on the right and the fully assembled Cold Stabilization unit on the left.

The cooling fluid in the plastic drum needs to be at least 20% propylene glycol in water (which has a freezing point of -8 oC or 18 oF) or 20% ethanol in water (which has a freezing point of -9 oC or 15 oF).

Cold stabilization takes longer (days instead of hours) if the wine is cooled down to only 32 oF or slightly above. The advantage of 32 oF+ is that water can be used instead of glycol. We can accelerate the crystallization of tartrate by seeding the process with a small amount of Potassium Tartrate (KHT) powder.

We need to take special care to limit the wine's exposure to oxygen during cold stabilization. At these low temperatures, wine can absorb oxygen more rapidly and thus may age faster. This is especially important when cold stabilization takes longer and is done in a tank that is not completely air-tight. To mitigate oxidation, we fill the airspace in the tank with Argon and seal the lid.

We cold-stabilize during the winter months. Our process is:

  1. Take a barrel sample and measure critical parameters (pH, TA, phenolics)

  2. Rack the wine from the barrel into the two 30-gallon cooling tanks. If necessary, top them up.

  3. Expose the cooling tanks to cold winter nights until they cool down to around 45 oF.

  4. Clean the barrel and keep it ready for a refill.

  5. Cool the tanks down to 35 oF (using water in the cooling drum), then add 10g of Potassium Tartrate powder (KHT) to each tank.

  6. Wait for seven days to let the KHT crystallize

  7. Take a test sample and check whether Total Acidity has dropped enough. If yes, rack the treated wine back into the barrel; if no, go back to step 6.

  8. Clean the tartrate sediments out of the cooling tanks.

For more details on Cold Stabilization, search for “cold stabilization” on the Pen State Extension website: https://extension.psu.edu/ . For a detailed description of the chemistry, read pages 352-360 in Yair Margalit's Concept in Wine Chemistry, 3rd edition (http://www.amazon.com/Concepts-Wine-Chemistry-Yair-Margalit/dp/1935879812 ) – this is also the source of the above chart.

Cold Stabilization has two negatives: The wine needs to be chilled down significantly, which takes a fair amount of energy and makes the wine vulnerable to oxidation (oxygen solubility in wine increases with low temperature). There are two alternatives to handle excess KHT: Electrodialysis (which, due to the complex machinery required, needs to be outsourced) or adding Tartrate Crystal Inhibitors.

Example

We only tried cold stabilization once – in late 2015 on a barrel of 2012 cabernet, which we judged to have too much acidity. The attempt was not entirely successful. After three weeks at a temperature between 35-40 oF we measured only a slight increase in pH, and instead of tartrate crystals at the bottom of the tank, we found dark-red sediment. We never figured out what happened.

 

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Last updated: June 2, 2022