A balanced wine should be the goal of every winemaker – not only in the wine’s chemistry, but in the wine’s aroma and flavor. While the latter is often up to interpretation (heavy-handed oak treatment is an example), much is known about how taste components such as acidity, sweetness, and alcohol can work together in harmony or discord on the palate. Cold-hardy wine grapes developed at the University of Minnesota are rarely harvested with a total acidity (TA) under 10 g/L. It is not uncommon to see total acidity at harvest of 15-18 g/L in Frontenac, and even the newest cultivar, Marquette, sees total acidity ranging from 9-13 g/L.
In dry wine production, wine balance can be a trickier dance, as sweetness can help soften both acidity and alcohol. In technical terms, any wine with less than 5 g/L (0.5%) of residual sugar may be considered dry if the yeast population dies. The perception of dryness, on the other hand, can vary based on other aspects of the wine, such as acidity, dry extract, and aroma. A wine that is dry and acidic can taste harsh, astringent, and un-balanced to the consumer. Because tannin and alcohol can accentuate the sensation of acidity, winemakers using cold-hardy cultivars to make dry red wines must consider ways to mitigate this high acidity.
There are three general methods one can use to lower high acidity dry wine production: physical methods (blending and amelioration), chemical methods (bicarbonates), and biological methods (yeast and bacteria). For the acid levels seen in Northern vineyards, the best approach is most likely a combination of all three of these methods. The Northern Grapes Project will be exploring these methods individually, so that winemakers can have a host of different tools in their arsenal for reducing acidity in their own wines.
Biological Deacidification. The most important thing to remember about biological deacidification is that it only affects the malic acid portion of your wine’s total acidity. The most common method of biological deacidification is through malolactic fermentation. Although not a true fermentation, bacteria that exist naturally in the environment have the ability to consume the malic acid in grapes and convert it to lactic acid, softening the wine’s acidity. Nearly all red wines around the world undergo MLF and some white wines also benefit from acid reduction of this practice. Traditionally, red wines are stored in barrels following alcoholic fermentation, where MLF will naturally occur as long as the wines are left unprotected from microbial spoilage. Wineries choosing to allow “spontaneous” MLF to occur often have to wait months for the malic acid to be consumed. The risks involved with leaving the wine un-sulfured, as well as the development of reliable bacteria starter cultures have pushed many wineries to inoculate their wines rather than waiting for MLF to occur naturally.
Yeast also have the capability to consume malic acid (malate), though they convert it to ethanol rather than lactic acid. It has long been known that certain yeasts (Schizosaccharomyces pombe, Hanseniaspora occidentalis, Issatchenkia orientalis) are especially efficient at consuming malic acid. However, because these yeasts have poor alcohol tolerance, they must always be used in conjunction with Saccharomyces yeasts in order to complete fermentation in wine. While S. pombe has been available commercially for some time for use in wine production, the development of other non-Saccharomyces yeasts for commercial use is a hot topic at the moment. We will likely see more of these yeasts available in an active-dry form to use in sequential yeast inoculations for wine.
Until then, we decided to look at some of the commercially available Saccharomyces yeast strains that have a reported ability to reduce malic acid, and trialed them with cold-hardy grape cultivars. After consulting with several enological product suppliers, we came up with a list of several different yeast strains: Lalvin C (Lallemand), Exotics (Anchor), Lalvin ICV OPALE (Lallemand), and Uvaferm VRB (Lallemand). We also trialed a non-Saccharomyces yeast that Lallemand has made available in an active dry form for sequential inoculations: Torulaspora Delbrueckii (sold commercially as Level 2TD). Although its malate-consumption hadn’t been verified, a technician at Lallemand had recommended it because they had observed some softening of the acidity in wines that had been fermented using it.
Yeast deacidification trial. We did a small trial with these yeasts in which we used juice from the 2012 vintage that had been previously frozen. For each MN cultivar, we trialed three different yeast strains, and used a fourth yeast strain as a control. One lot of juice was divided into 20 micro-vinification lots of 500 mL each. Thus each yeast was replicated in 5 fermentation lots. For this initial trial, we were concerned with monitoring mainly the chemistry change using each yeast. For white wines we used Lalvin DV10 as control, and for red wines we used ICV GRE as a control yeast. Both are considered reliable fermenters with no reported malate degradation. The unusually hot weather in 2012 caused initial brix levels to be extremely elevated, so initial malate numbers reflect juice that had been diluted to bring the sugar concentration down to 25° Brix.
Results: With Frontenac Gris, we started with an ameliorated juice that had a total acidity of 9.92 g/L, pH of 3.00, and 5.1 g/L of malic acid. Although all the added yeast strains showed some reduction from the initial malate levels in the juice, the acid reduction seen in the Lalvin C, Exotics, and the combination of Torulaspora delbrueckii with Exotics all were significantly lower than the control yeast (p <0.05). We used Lalvin C in a larger lot following this trial in order to evaluate the sensory impacts of this yeast. It’s worth noting that in all 10 micro-vinifications in which Exotics was used, the wines exhibited some stuck fermentations. Thus, some care may be needed when using this yeast in order to complete fermentation in low pH juices.
The La Crescent juice that we divided up for the micro-vinification trials was ameliorated to 25 Brix, which left the starting malate levels at 5.3 g/L. The decrease in malic acid during fermentation was less pronounced than what we saw with the Frontenac Gris fermentation. In fact, only the vinification lots in which Exotics was used showed a statistically significant drop in malic acid (p< 0.05). ICV Opale is advertised to lower malate levels by 0.1 to 0.4 g/L. Our trials show that it exceeded this level in high malate juice, however, this decrease was not significantly lower than our control yeast which has no reported malate reducing properties.
Our Frontenac was pressed and fermented as a rosé. Again, it was necessary to ameliorate to reduce the high sugars that we achieved in 2012, however, the initial malate concentration of the juice was still relatively high at 4.6 g/L. All yeast used for this trial caused a decrease in the final malic acid concentration of the wine. All observed differences in malate reduction were statistically significant (p<0.05), except for the two lots that were fermented with Lalvin C. There is no statistical difference between the observed malate reduction when using Lalvin C in conjunction with T. delbrueckii yeast. This (along with the other results seen when using T. delbrueckii) suggests that any impact on the perception of acidity due to this yeast is likely not related to malate degradation. All the Frontenac fermentations finished dry with no stuck or sluggish characters.
Marquette was also pressed immediately and fermented as a rosé. The ameliorated juice had an initial malic acid concentration of 4.1 g/L. Exotics and VRB showed identical malate reduction capabilities, and even though the difference between these two yeasts and the control (ICV GRE) was only slight, the difference is statistically significant (p=0.046). Once again, Lalvin C proved to have the greatest potential for malate reduction, with a 1.10 g/L decrease in malic acid concentration from the juice.
It is important to keep in mind that there are many different tools available to a winemaker to manage high acidity in their wines. The selection of yeasts that we looked at here are only a small example of what is available on the market. It is important to talk with technicians who supply your winery in order to get a better idea of what products might help with managing your acidity.