Potassium Bicarbonate and Cold Hardy Hybrids
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What a difference a year makes. A late, cool wet spring and summer have many wineries thinking about a high acid harvest. While harvest is still a little ways away for many northern vineyards, the potential for high acid fruit remains.
Hopefully some warm weather will give us a boost. With the potential of high acid fruit, many winemakers will use, or consider using, chemical de-acidification techniques to help tame the sourness in their wines.
The most common of these agents is potassium bicarbonate. Other chemical de-acidification products include calcium carbonate and double salt (Acidex). This article will focus on potassium bicarbonate. (Calcium carbonate can create bottle instability issues that are difficult to resolve. Double salt require extra steps and does not always work as intended.)
Potassium bicarbonate, KHCO3, neutralizes the two hydrogen ions available on tartaric acid. The byproducts are water and CO2. A 0.9 g/L addition of potassium bicarbonate results in a 1 g/L reduction in titratable acidity. Under typical wine conditions and normal use, potassium bicarbonate neutralizes only tartaric acid.
It is best used on high acid and low pH wines. Potassium bicarbonate will cause an increase in pH when used. It also adds potassium to the wine that will require cold stabilization. It is therefore ideal to maintain a pH of less than 3.65 in order to have pH reduction during cold stabilization. Wines above pH 3.65 will experience an increase in pH during cold stabilization.
Using potassium bicarbonate properly requires more than an understanding of the basic chemical principles. The following five factors should be considered when using potassium bicarbonate:
A significant amount of CO2 can be generated during potassium bicarbonate additions. If the wine is chilled at the time of addition, much of the CO2 will remain dissolved in the wine. This will require the use of nitrogen sparging to remove excess CO2.
Excess CO2 in table wine can lead to light effervescence and an overly crisp palate. However, a certain amount of residual CO2 is often desired in wines. Reds, depending on style, are often bottled at 400 ppm CO2, and whites can be bottled with levels as high as 1000 ppm. Typically 600 to 800 ppm of CO2 is satisfactory for most white wines.
Overly carbonated wines can create bottling problems as the wines may degas and foam at the filler, slowing bottling operations. A carbodoseur is a simple and handy tool that can be used to measure dissolved CO2.
If sparging is necessary, set up a nitrogen sparging assemble and regularly measure the CO2 concentration. Consequently, CO2 can be added using the same process for wines that are considered flat. Nitrogen sparging can strip SO2. Sulfur measurements and adjustments are necessary after treatment.
The other aspect of dissolved CO2 in wine is the threat of foam over. If adding potassium bicarbonate to warm wine, make sure to have an extra 25% or more headspace to account for any potential foam ups. When wine that was cool when treated is allowed to warm, be careful on transfers and further treatments as the dissolved CO2 can off gas very quickly.
Potassium bicarbonate can contribute to tartrate instability in wine. Cold stabilization following treatment is recommended.
Having the time and tank space to do multiple cold-stabilization treatments is difficult for many wineries. With properly chilled tanks and sufficient time, the double chill process could be an option.
Timing of Addition
When to add potassium bicarbonate depends a lot on the type of wine being produced. Many wineries add potassium bicarbonate sometime after fermentation. Activity in the winery has slowed down at this point and more attention can be given to the process.
Waiting until any blending has been conducted also gives clear need of whether to use potassium bicarbonate. One difficulty with adding potassium bicarbonate at this point is predicting the outcome of cold stabilization.
Cold stabilization has the potential to reduce acidity by 0.5 g/L to 2.0 g/L depending on the wine. In an ideal world the potassium bicarbonate would be added after cold stabilization to acquire the exact level of acidity.
Wineries wanting control over the acid reduction can consider potassium bicarbonate additions after cold stabilization and then treating the wine with a tartrate prevention product. This would allow for optimal TA reduction from both cold stabilization and potassium bicarbonate additions. Many tartrate prevention compounds are relatively new products and should be tested cautiously.
Adding potassium bicarbonate early in the process has its advantages. There is some thought that adding potassium bicarbonate early minimizes flavor and aroma losses. Wines with very low pH may benefit from a light pH boost. Low pH musts with high SO2 added at crush can create difficult fermentation conditions. At a pH of 3.0 a little SO2 goes a long way. Red wines that are targeted for malo-lactic fermentation may need a little pH help. This is especially true if alcohol levels rise above 14% abv.
Regardless of when potassium bicarbonate is added, always run a bench test. When adding potassium bicarbonate, start with a partial addition. Adding more potassium bicarbonate is easy and preferable to readjusting the acidity or pH from an over addition.
Changes in pH
Bicarbonate additions will raise the pH of the wine. How much the pH will shift will be different for every wine and is difficult to predict. All treatment should first be bench tested. Always remember to check and adjust FSO2 after treatment. Increases in pH will reduce the effectiveness of sulfur and wines may experience microbial stability problems if too much of a pH shift is experienced. I don’t recommend wines with a pH of 3.5 or higher to be treated with potassium bicarbonate.
Over use of potassium bicarbonate can lead to undesirable sensory attributes. Potassium bicarbonate can lend a salty taste to wine if too much is used. Additions above 1 g/L can begin to impart this character. Potassium bicarbonate changes the acid ratio of wine due to its selective neutralization of tartaric acid. Many cold climate varietals, particularly during cool seasons, have a high amount of malic acid relative to tartaric acid. The reduction of tartaric acid leaves creates a wine whose sourness is mainly from the harsher malic acid.
Wineries faced with high acid musts this harvest may want to consider using potassium bicarbonate to adjust acidity. If you use it, proceed with care.
Nick Smith is an enologist at the University of Minnesota Horticultural Research Center in Chanhassen.