While the issue is quite challenging to address in an established vineyard, processing grapes from high pH fruit, or fruit that has the potential to create a high pH wine (>3.70), as a result of high potassium (K or K+) concentrations is also a challenge for the winemaker.
Concentrations of 22 – 32 mmol/L K+ (860 – 1,279 mg/L K+) are considered “normal” ranges for wine grapes (Somers 1977, cited byMpelasoka et al. 2003), while ranges in the 27 – 71 mmol/L K+ (1,056 – 2,776 mg/L K+) are considered “high” (Somers 1975, cited by Mpelasoka et al. 2003) and may lead to potential winemaking problems. Grapes and juice that come in with high levels of potassium can lead to a series of difficulties for winemakers including:
- High potassium concentrations can cause large increases in pH during primary and malolactic fermentations, which drive the finished wine into a high pH (>3.70) range.
- Color hue, intensity, and stability of red wines can be negatively affected.
- High pH wines produced throughout the Mid-Atlantic may lead to negative perceptions associated with taste and mouthfeel of both white and red wines.
- As pH is a big driver in wine stability, higher pH’s will have impacts on the microbial stability (both in terms of microflora and inhibition of growth), sulfur dioxide levels and efficacy, color stability of red and rosé wines, stability of tartaric acid, and protein stability.
- Higher pH’s leads to an increase in oxidative potential, which may cause premature oxidation for young wines.
Figure 1: Color instability problems associated with 2013 Chambourcin wines. The sample on the right is from our Biglerville (Adams County) research site, which we later discovered is associated with high potassium in the fruit and wine. The sample on the left is from our North East (Erie County) site, which is more representative of the color hue and intensity associated with Pennsylvania-produced Chambourcin. Photo by: Denise M. Gardner
Although the articles listed below are not peer reviewed, previous attention has been given to high potassium winemaking issues. Some of the content relayed in these 2 articles will not be discussed in this blog post:
- Really, really high pH remedies from Wines & Vines: a discussion on potassium concentrations increasing the pH of wine and utilization of ion exchange if the problem is not prevented
- High pH and high potassium wines produced in Colorado from White Hall Vineyards: Includes a discussion pertaining to malic acid concentration in high pH fruit. (Author’s note: This article discusses adding tartaric acid prior to fermentation, but not exceeding a TA of 8.0 g/L while hitting a pH of 3.60, ideally. While the practice of analytically checking your additions is encouraged, and will be discussed throughout the duration of this blog post, please note that sampling procedures and tartaric acid settling time will greatly influence your juice TA after tartaric acid addition.)
A problem for winemakers is that unless potassium uptake and management is addressed in the vineyard, they will likely have to deal with having high potassium-based fruit for several years. However, winemakers are encouraged to work with their growers, as this is a relatively newer viticultural issue that the Mid-Atlantic is facing, and it may take several years to stabilize before results are seen in incoming fruit from the vineyard.
In regions like Australia, which frequently experience high K concentrations in their fruit and wines, making tartaric acid additions to the juice, pre-fermentation is often recommended to lower the pH of the must/juice and precipitate some of the potassium as it binds to tartaric acid during primary fermentation. While a 2 g/L of tartaric acid addition to must/juice is a common recommendation for acidulating musts, it may not be enough in order to alter the effects of high potassium concentrations in the fruit. In these cases, a higher addition rate of tartaric acid, such as 4 – 6 g/L of tartaric acid, may not be out of the question.
It should be noted that must/juice acidification will have chemical and sensory implications to the finished wine. If the winemaker is aiming to produce a specific style, making large tartaric acid additions pre-fermentation may not be conducive with the desired and finished wine style. However, when dealing with high potassium issues, and hence, high pH issues, larger tartaric acid additions pre-fermentation seem to be helpful in stabilizing red wine color and improving the flavor of red wines. For those that would prefer a lower TA (<6.0 g/L tartaric acid), deacidification following malolactic fermentation of red wines is recommended. While there are limitations on deacidification practices, including the degree to which a winemaker can deacidify, this action may help improve mouthfeel and decrease the perception of acidity (sourness) in the finished wine.
Wine Trials at PSU
During the 2015 harvest, our research team confirmed that a couple of our varieties that annually had high pH problems came from sites or locations with high potassium retention in the fruit. This did not necessarily correlate with high potassium concentrations in the soil.
From our Biglerville (Adams County) research vineyard, our Merlot contained 1,682 mg/L K+ and Cabernet Sauvignon contained 1,668 mg/L K+ in the 2015 growing season. Both samples were taken from the must and analyzed by atomic absorption analysis at Enartis USA – Vinquiry.
Based on previous research from Somers (1975), both musts were considered high in potassium. The following (Tables 1 and 2) show additional harvest parameters for our Merlot and Cabernet Sauvignon musts in the 2015 season.
As previous yeast strain selection, malolactic bacteria selection, and standard (2 g/L) tartaric acid addition trials did not seem to improve color stability or flavor of the wines in past harvest years, we took the approach at comparing 3 different tartaric acid addition rates (2 g/L, 4 g/L, and 6 g/L) to the Merlot pre-fermentation and two rates (4 g/L and 5 g/L) of tartaric acid to the Cabernet Sauvignon based on previous recommendations made in the Australian literature. There were fewer treatments on the Cabernet Sauvignon due to decreased yields in 2015. Please note that these treatments were not replicated and, therefore, we have not provided any statistical parameters.
For the Merlot, the 2 g/L addition rate of tartaric acid acted as the “control,” as previous years indicated no differences in pH or TA by the end of MLF between wines fermented without tartaric acid added pre-fermentation and a 2 g/L addition treatment. There was no designated “control” for the Cabernet Sauvignon fermentations.
Table 1: 2015 Pennsylvania Merlot must chemistries in 2015; pH and titratable acidity (TA) were adjusted pre-fermentation (i.e., pre-inoculation) and given at least 3 hours of settling time before inoculation with ICV-GRE yeast
Figure 2: Pre-Fermentation tartaric acid addition (2 g/L, 4 g/L, and 6 g/L) trials to 2015 Merlot must. This image shows the wines during malolactic fermentation. Photo by: Denise M. Gardner
Table 2: 2015 Pennsylvania Cabernet Sauvignon must chemistries in 2015; pH and titratable acidity (TA) were adjusted pre-fermentation (i.e., pre-inoculation) and given at least 3 hours of settling time before inoculation with ICV-GRE yeast
The following table (Table 3) shows the differences in pH and TA for each pre-fermentation tartaric acid addition treatment following primary fermentation and MLF for our Merlot wines in the 2015 vintage year.
Table 3: 2015 Merlot wine chemistries (pH, TA, volatile acidity, and alcohol concentration) post-primary fermentation and post-MLF (fermentation trials were not conducted in replicate)
Trends were similar in the 2015 Cabernet Sauvignon wines, as shown inTable 4.
Table 4: 2015 Cabernet Sauvignon wine chemistries (pH, TA, volatile acidity, and alcohol concentration) post-primary fermentation and post-MLF (fermentation trials were not conducted in replicate)
While we do not quite have an explanation for the rise in TA from post-primary fermentation to post-MLF in the 5 g/L tartaric acid addition treatment in the Cabernet Sauvignon wine, we did note that post-bottling, most of the TA’s slightly decreased across all treatments in both Merlot and Cabernet Sauvignon wines. A decrease in TA would reduce the perception of sourness even further. This decrease was likely due to better removal of dissolved carbon dioxide within the wines due to the fact the wines had been moved (i.e., racked, transferred and bottled) more routinely prior to bottling.
The treatments within a varietal were also different sensorially, although this was not quantified. For example, in the Merlot, the first difference noted was the color. The Merlot wine that had been treated with 6 g/L tartaric acid had the most vibrant and red-hued color. The Merlot with a 2 g/L tartaric acid addition had a stronger purple-blue hue. We did not quantify these differences analytically. In terms of taste, the 6 g/L tartaric acid treatment had more noticeable and perceptible sourness, but many that tasted the wine agreed that it could be manipulated with some deacidification trials. The 2 g/L tartaric acid addition treatment tasted flat, had burnt rubber-like flavors and was relatively unappealing. It did not represent a typical flavor profile associated with Merlot. The 4 g/L and 6 g/L tartaric acid addition treatments had more noticeable red fruit flavors and less earthy characters.
What should you do in the winery if you think you have high pH wines as a result of high potassium concentrations in your grapes?
- Find out if potassium concentrations may be a culprit. Now is the time to find out what you are dealing with. In a previous blog post, Michela recommended getting petiole samples to determine vine nutrition. However, you can also test the fruit (must, juice) and the wine for potassium concentrations as well. We recommend sending your samples to an ISO accredited lab to confirm potassium concentrations in those wines that you believe may be suspect.
- Make tartaric acid additions pre-fermentation (pre-inoculation).With very high potassium concentrations, a 4 – 6 g/L addition of tartaric acid pre-fermentation may not be a detriment to wine quality. However, it is best to know the concentration of potassium you are dealing with before adding up to 6 g/L of tartaric acid pre-fermentation as this can have obvious effects on the wine’s taste and flavor as a finished wine (i.e., make the wine thin or overly sour).
- If you are unwilling to test to the potassium concentration, but have a wine with frequent high pH problems during production, use a 4 g/L tartaric acid addition pre-fermentation instead of 2 g/L. The 4 g/L tartaric acid addition rate is a relative “good guess” zone. Depending on the potassium concentration in your wine, this will either work or it will not work. If you refer to Tables 3 and 4, we can see that the 4 g/L addition rate was not a bad choice for the Merlot as it resulted in an ideal pH (3.63) and a workable TA (5.96 g/L), but for the Cabernet Sauvignon, the wine resulted in a high pH (>3.70) and a high TA (>6.00 g/L). This high pH, high TA situation can make the wine both difficult to manage for stability reasons (e.g., making applicable sulfur dioxide additions) while retaining a relatively sour taste.
- White wines can also suffer from high potassium. While the content of this blog post has focused on red wines and the effects of color stability and flavor associated with higher pH’s and high potassium concentrations, white wines can also be affected by high potassium concentrations. In most instances, high potassium can relate to a high pH in the finished wine, which makes the white wine difficult to stabilize or add proper sulfur dioxide additions in order to minimize microbial risk. Also, many of these wines have low TA’s, giving the white wine a fat, round, or flat mouthfeel (dependent on the variety). Stylistically, this may not be undesirable, but it is a sensory component that winemakers should be aware of that may occur in these chemical situations.
- Alter your pH and sourness post-malolactic fermentation. If the wine tastes too sour for your preference, the time to de-acidify is post-MLF with these wines. By that time, the color pigments will be fully extracted from the red skins and the flavors will be as optimal as they can be for the variety. For our Merlot wines, we made additions using (ironically) potassium carbonate, but calcium carbonate can also be used to de-acidify wines. I usually recommend Patrick Iland’s book for practical information on how to make de-acidification trials in wine. WSU also provides appropriate options and instructions for deacidifying wine.
Mpelasoka, B.S., D.P. Schachtman, M.T. Treeby, and M.R. Thomas. (2003) A review of potassium nutrition in grapevines with special emphasis on berry accumulation. Australian Journal of Grape and Wine Research. 9:154-168.
Somers, T.C. (1975) In search of quality for red wines. Food Technology in Australia. 27:49-56.
Somers, T.C. (1977) A connection between potassium levels in the harvest and relative quality in Australian red wines. Australian Wine, Brewing and Spirit Review. 24:32-34.