Cold Stabilization is used to reduce the formation of sediments (i.e. to prevent the precipitation of crystals when bottles are stored at low temperatures for extended periods) and to reduce the amount of tartaric acid in wine.
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 crystallise and fall out as sediment. The concentration at which crystallisation happens (i.e. the wine becomes unstable) depends on the pH, the temperature and the alcohol content of the wine. The ability for wine to hold KHT in solution increases the higher the pH, the higher the temperature and the lower the alcohol. As a consequence tartrate crystals will 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, their appearance as crystalline sediment is considered unattractive.
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. 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 which itself sits inside an insulated wood box. The cooling fluid 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).
If the wine is cooled down to only 32 oF or slightly above, then cold stabilisation takes longer (days instead of hours). This may be preferable because then water can be used as cooling fluid in the plastic drum. The crystallisation of tartrate can be accelerated by seeding the process with a small amount of Potassium Tartrate (KHT) powder.
Special care has to be taken to limit the wine’s exposure to oxygen during cold stabilisation; at these low temperatures, wine is prone to absorb oxygen more rapidly and thus age faster. This is especially important when cold stabilisation takes longer and is done in a tank which is not completely air-tight. To mitigate oxidation, we fill the airspace in the tank with Argon and seal the lid.
We cold-stabilise during the winter months. Our process is:
- Cool down the barrel from the usual 55 oF in the cellar to around 45 oF by taking it out and exposing it to the cool nights in winter.
- Take a barrel sample and measure key parameters (pH, TA, phenolics)
- Rack the wine from the barrel into the two 30 gallon cooling tanks.
- Clean the barrel with the barrel washer and burn a sulfur pill inside it to keep it clean during the cold stabilisation process.
- Cool the tanks down to 35 oF (using water in the cooling drum) then add 10g of Potassium Tartrate powder (KHT) to each tank.
- Wait for 7 days to let the KHT crystallise
- Test whether the TA has dropped enough. If yes, proceed to step 8, if no, go back to step 6.
- Rack the treated wine back into the barrel and take a full set of measurements (pH, TA, SO2, phenolics)
- Replenish the sulfur (SO2) level with KMBS as needed, top up the barrel and close it up.
- Clean the tartrate sediments out of the cooling tanks.
For more details on Cold Stabilization see the following document written by Virginia Smith: http://extension.psu.edu/food/enology/analytical-services/assessment-of-cold-stabilization. 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 important negatives: The wine needs to be chilled down to ~20 dF 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: one is Electrodialysis (which due to the complex machinery required needs to be outsourced) and Addition of Tartrate Crystal Inhibitors.
We only tried cold stabilisation 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 3 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: February 28, 2018