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Coping with High-pH Wines
Addressing high-potassium concentrations in the fruit during winemaking operations
Processing wine from high-pH fruit can create potential difficulties for winemakers. Not only can stability issues such as color, sulfur-dioxide management and microbial content be problematic, but high-pH wines resulting from high potassium (K or K+) concentrations add additional challenges for winemakers.
Making wine with high-potassium fruit is not isolated to the eastern United States, and studies pertaining to the topic have been published globally. Concentrations of 22-32 mmol/L K+ (~860-1,279 mg/L K+) are considered “normal” ranges for wine grapes (Somers 1977), whereas ranges in the 27-71 mmol/L K+ (~1,056-2,776 mg/L K+) are considered “high” (Somers 1975). These higher ranges of potassium in the fruit tend to lead to potential winemaking problems throughout the duration of wine manufacturing. Such difficulties include:
• Large increases in pH during primary and malolactic fermentations, which drive the finished wine into a high pH (sometimes >4.20) range;
• Color hue, intensity and stability of red wines can be negatively affected;
• Longevity of wines may be decreased;
• Potential negative perceptions associated with taste and mouthfeel for both white and red wines;
• Wine stability problems such as 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;
• An increase in oxidative potential, which may cause premature oxidation for young wines.
Potassium and grapevines
High potassium problems recently have been addressed by many in the regional viticulture community. Dr. Tony Wolf, professor of viticulture and director of the Alson H. Smith Agricultural Research and Extension Center at Virginia Tech, published a change in potassium fertilization recommendations for vineyards in the July 2016 volume of Viticulture Notes, where he recommended reducing the 75 ppm (150 pounds per acre) rate found in the 2008 Wine Grape Production Guide for Eastern North America to 40 ppm (80 pounds per acre).
In September 2016, Dr. Michela Centinari offered solutions for reducing potassium uptake by the vines:
• When establishing a vineyard on a site with a high exchangeable potassium level, it can be useful to select rootstocks with lower potassium uptake capacity. The most common rootstocks that tend to uptake lower levels of potassium come from the Vitis berlandieri genetic background (Centinari 2016).
• After vineyard establishment, reducing vine vigor and foliage shading of grape berries can help reduce potassium uptake and translocation of potassium into the fruit, respectively (Centinari 2016). However, with excessively high potassium concentrations in the fruit, these vineyard-management strategies may not be enough to reduce potassium to acceptable levels for winemaking.
Research literature (Moss 2016) has recommended that growers carefully evaluate both vine and soil potassium levels before proceeding with potassium fertilization. In fact, in an earlier evaluation, it was found that within some soils containing the lowest concentrations of potassium surveyed, petiole analysis still resulted in adequate potassium concentrations within the vine (Beasley, Morton and Ambers, 2015).
The relationship with potassium uptake in wine grapes is complex. Professional viticulturists can give vineyard owners and managers specific recommendations for vineyard management techniques. However, even with better vineyard selection and management techniques, many winemakers in the eastern United States are facing high-pH fruit and wine challenges, some of which may be contributed by high potassium concentrations in the fruit. It should be noted that not all high-pH problems are associated with high potassium concentrations, and testing for potassium in vineyards is an important first step in resolving high-pH winemaking challenges.
Throughout our research work through Penn State University, we noticed that some of our most problematic fermentations tended to have high potassium concentrations in the leaf petiole during the growing season and in the fruit at harvest. Many of these wines resulted in large pH swings from the beginning of fermentation through the end of primary or malolactic fermentation. Red wines had uncharacteristic aroma and flavor profiles as well as odd color stability problems that were not amendable through post-fermentation acid additions. White wines were not quite stable through bottling, and many became undrinkable after a year in the bottle.
After several years of producing odd wines and sharing them with regional winemakers to troubleshoot the wines’ problems, it was apparent that the important question was: How do we make quality wines from grapes high in potassium or experiencing a high pH?
An approach to winemaking with high-potassium fruit
Dealing with high potassium in winemaking is not a new problem. Other than addressing the issue in the vineyard, winemakers can utilize techniques such as ion exchange and acid adjustments in the must to address wine quality consistency complicated by high-potassium fruit. However, from a practicality viewpoint, many regional winemakers do not believe ion exchange is a reasonable solution either due to difficulty finding available ion-exchange units or the associated cost.
Upon reviewing literature, Mpelasoka et al. (2003) noted the costs associated with commercial Australian wineries making regular pre-fermentation tartaric acid additions in an attempt to reduce potassium content in the juice to avoid a high pH by the end of fermentation. It is worth noting that potassium concentration influences buffering capacity and wine acidity. Additionally, the binding of potassium and precipitation of pot assium bitartrate is also influenced by pH; potassium bitartrate becomes more insoluble as ethanol concentrations increase during fermentation, and it is not unusual for titratable acidities (TAs) to decrease by the end of primary fermentation due to the precipitation of potassium tartrate (Iland et al. 2012, Schneider 2012). International wine consultant and expert Volker Schneider has referenced reducing potassium content through precipitation of potassium bitartrate: “…addition of… 2.0 g/L tartaric acid, one has to accept a… loss of approximately 500 mg/L potassium” (Schneider 2012). Nonetheless, acid chemistry should not be taken lightly, and winemakers struggling with the topic should refer to modern wine chemistry texts.
Because of the frequency of the high-pH problem, our research team at Penn State decided to run a trial on two red wine varieties that annually had pH greater than 4.0, color stability problems and had been previously tested for high concentrations of potassium in the petioles, fruit and previous wine vintages.
Merlot juice from the research vineyard at Penn State Fruit Research and Extension Center in Biglerville, Pa., contained 1,682 mg/L K+, and Cabernet Sauvignon contained 1,668 mg/L K+ in the juice prior to fermentation in the 2015 vintage. Both samples were taken from the must and analyzed by atomic absorption analysis at Enartis USA, but we did not have the results until after we made acid additions and inoculated for primary fermentation.
Both varieties contained potassium concentrations that were considered relatively high (Somers 1975), indicating that high potassium may have been a potential culprit for some of our previous winemaking challenges. The tables on page 61 show additional chemistry results for the Merlot and Cabernet Sauvignon musts pre-fermentation.
While we did not have replicate fermentations as we sought potential production solutions for winemakers, we attempted a production trial on both Merlot and Cabernet Sauvignon musts to assess the potential impact of higher than normal tartaric acid additions pre-fermentation. For the Merlot, in addition to a 2 g/L tartaric acid addition, which in previous vintages appeared to have little to no impact on the wine quality compared to no tartaric acid addition, we tested 4 g/L and 6 g/L additions, both of which had potential to decrease potassium concentrations. Due to lesser volume of the Cabernet Sauvignon, we only tested 4 g/L and 5 g/L tartaric acid additions.
Wines were all fermented in open-top fermentation bins, and primary fermentation was completed within approximately seven days. In hindsight, it would have been best to analyze pH and TA daily through primary fermentation to monitor changes in acidity. Pressing followed completion of primary fermentation, and wines were immediately inoculated with the Lallemand malolactic bacteria strain, Alpha. Malolactic fermentation (MLF) was tracked via paper chromatography.
At completion of MLF, wines were moved to cold storage and treated with potassium metabisulfite with a dosage rate based on wine pH. Wines were later racked and final additions of potassium metabisulfite were made prior to bottling. Wines were left unfinished to emphasize the effect of the juice tartaric acid treatments.
The tables show the differences in pH and TA for each pre-fermentation tartaric acid addition treatment following primary fermentation and MLF for our Merlot and Cabernet Sauvignon wines in the 2015 vintage year.
Bottled wines and treatments were evaluated at various extension programs throughout 2016, though sensory analysis was not professionally quantified or analyzed. For example, the first difference tasters noted in the Merlot was the color variation among the three treatments. The Merlot wines treated with 4 g/L and 6 g/L tartaric acid had the most vibrant and red-hued color, which was visually noticeable. The Merlot with a 2 g/L tartaric acid addition had a stronger purple-blue hue and appeared hazy. We did not quantify these differences analytically.
A number of the winemakers’ tasting notes commented on the enhanced perception of acidity in the Merlot that received a 6 g/L tartaric acid addition treatment. Many of these winemakers agreed that the perception of acidity could be manipulated with some deacidification trials, and a few were willing to try the larger acid additions to the must/juice commercially in an effort to retain color post-malolactic fermentation. In the Merlot with a 2 g/L tartaric acid addition treatment, the wine tasted noticeably flat and flabby, had burnt rubber-like flavors and was relatively unappealing. A few commented that it did not represent a typical flavor profile associated with Merlot.
An article by Shea Comfort, published in WineMaker Magazine in 2010, mentioned previous research by the Australian Wine Research Institute indicating high-pH musts did not retain as much “berry, fruit and spice characters” if the acid was adjusted post-primary fermentation compared to adjusting acid pre-fermentation. We noticed similar results in our wine trials: The 4 g/L and 6 g/L tartaric acid addition treatments pre-fermentation were a stark contrast to the 2 g/L addition treatment, as they had more noticeable red fruit flavors and less earthy characters associated with the flavor profile.
As mentioned in Schneider (2012), the increased sourness also affected mouthfeel of the red wines, and many of them tasted thin or angular. This approach to dealing with high-potassium problems will obviously have an effect on the final wine style. However, several of these wines were later used for deacidification trials, and the results did provide some validation for commercial winemakers to deacidify post-MLF to improve a wine’s final quality and style.
Several commercial winemakers utilized pre-fermentation acid adjustments for the 2016 vintage with confirmed high potassium concentrations in their fruit. While the results of these adjusted wines have yet to be tasted, many winemakers expect dealing with high-potassium fruit may be a continued challenge for future vintages.
Denise M. Gardner is the extension enology associate at the Pennsylvania State University. She is currently based at the Montgomery County extension office in Colle geville, Pa.
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