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Vinifying Grapes From Difficult Climates
How to deal with fruit that has green flavors and acid/pH issues
When grapegrowers and winemakers hear the words “challenging environment,” most think of regions that have extreme temperatures or precipitation. Those conditions lead to further consequences once grapes travel from the vineyard to the winery. Winemaking for Challenging Environments, a symposium held this summer during the national conference of the American Society for Enology and Viticulture in Austin, Texas, addressed both the viticultural impact of difficult growing conditions as well as the winemaking problems that can result. For a review of the vineyard management tactics for stressed locations, see “Surviving Challenging Environments” in the August 2014 issue of Wines & Vines.
Grapes harvested from vineyards that have experienced less than optimal growing conditions can present a range of problems for the winemaker, including grapes with green, herbaceous flavors; high pH; high acidity; and color and phenolic issues.
In the winery
Vinification problems in challenging regions can come from a variety of factors, according to Misha Kwasniewski, assistant research professor and enology program leader at the University of Missouri. Some problems begin in the vineyard as a result of disease pressure or weather-related issues such as spring frosts or too much rain. In other cases, the cultivars being grown can be challenging in and of themselves. Kwasniewski cited Norton as an example of a varietal having “abysmal fruit chemistry” with both high pH and high acid levels. Growers and winemakers—and, importantly, researchers—often don’t know much about the basic chemistries and other important aspects of the new varieties of grapes now being grown in difficult regions, and frequently these grapes don’t act like vinifera varieties.
There are different solutions or interventions to improve problematic wines. Some are analysis driven; others are based on equipment choices; some can be solved by additives or with processing solutions. Winemakers can know more now about their wines through either in-house analyses or from laboratory services, and knowing what went right in one year can help the winery figure out what went wrong in a bad situation. In these environments, wineries need to know what occurred in the vineyard from its establishment to how the grapes were handled for each season.
The next question is what type of wine the winemaker is trying to make. These are far more critical questions in such regions, because the grapes probably are not going to hit the “optimum” or “textbook” numbers. As Kwasniewski noted, we need to start to understand the entire system of what is going on with grapes and the wines made from them.
“We have a lot of Band-Aids,” he stated. “There are many ways—using equipment, biological means or through additives—to start our wine at a good place. The question is, where will this take the wine? Worst-case scenario, you’ve got some terrible grapes. What can you do with it? First step, remove the worst stuff that you can. Sort it, clarify it (if it’s a white wine), and get it as close to what you want as possible. Maybe some of the enzymatic interventions can help. If it’s already bad, you might as well take a heavy-handed approach. Activated charcoal is not a way to make premium wine, but is a way to make drinkable wine. Then you’ve (got) some options of adding something back that the consumer may want, whether it’s adding fruity character or floral notes through esters, everybody likes sugar; oak will mask quite a bit. Then ultimately, you don’t know where the wine is going on its trajectory, so sell it fast.”
Green flavors in wine
Herbaceous or green flavors, especially in red wines, are often associated with unripe grapes or fruit of lesser quality, and these flavors can have a masking effect on more preferable fruity flavors. One class of odorant compounds, the methoxypyrazines (MPs), is responsible for green flavors sometimes found in Bordeaux cultivars such as Cabernet Sauvignon, Cabernet Franc and Merlot as well as in hybrid grapes with some vinifera parentage. These compounds can be detected at concentrations as low as 1 part per trillion.
According to Justin Scheiner, viticulturist for the Texas A&M AgriLife Extension Service in College Station, Texas, the best way to manage green flavors caused by MPs starts in the vineyard. MPs are predominately located in the skins of the grapes, with significant concentrations in the stems or the rachis of the berries. Research in New York has shown that light exposure of clusters affected the level of concentration of MPs in the grapes, with more light resulting in lower MPs. High-vigor vines also had higher levels of MPs, but cluster thinning to reduce the crop load and improve ripening may not result in lowering the MPs in the crop that was left. Control of vigor is important, as excessive vigor can also cause shading of clusters. In addition, green harvest after véraison can help with asynchronous ripening problems, and by increasing maturity of the fruit, also reduce the levels of MP.
One factor that has proved to be a good predictor of MP levels is temperature. Higher levels of MPs occurred in warmer years, as warm climates stimulate vigor and therefore higher MPs. In cooler climates, warmer temperatures (especially early in the gr owing season) have been shown to result in higher MP levels.
Once the grapes arrive at the winery, what can be done to reduce or remove those methyoxypyrazines? Because the MPs are found in the grape skins, good destemming practices will help to minimize the green flavors in white wines and, in some cases, settling will also reduce the green aromas and flavors. However, during red wine fermentation, these compounds are extracted from the skins within the first two or three days and will remain a problem.
Most winemaking interventions are either ineffective in terms of actually pulling out the MPs or they are not selective. Yeast selection can help, as yeast strains known to produce strong, estery or fruity types of aromas may have the potential to mask some of those green flavors, but it is unlikely that the yeasts will actually remove the MPs. Oak exposure and micro-oxygenation don’t remove the MPs, but both treatments can mask their effects. Use of activated charcoal can remove the compounds, but it is not selective and may be deleterious, especially for premium red wines. Thermo-vinification, some packaging systems (such as Tetra Pak), and closures (such as synthetic corks) can either bind or pull out the MPs, but they may also cause other—not necessarily positive—changes in the wine.
Herbaceous aromas and flavors can come from other sources than grapes. Multicolored Asian lady beetles (MALBs) also produce isopropyl methoxypyrazines and, when MALB get into grape must, they release the MP compounds that can give wines flavors that are described as asparagus, peanut butter or bell pepper. Green June beetles can create a taint problem for winemakers in places such as Texas. Green leaf volatiles are another class of compounds that may be a source of green flavors; they can be responsible for aromas of freshly cut grass. Fortunately, these compounds lose most of their potency during fermentation.
Acid-pH balanceAs with much of winemaking, managing acidity and pH in wine begins in the vineyard. Tartaric acid is the dominant acid in grape juice and wine, with malic acid being the second major acid. During malolactic fermentation, the malic acid is degraded into lactic acid. pH is a “resultant variable” in grapes, according to Roger Boulton, professor of enology at the University of California, Davis. The level of pH is an outcome of the conditions of grapegrowing, and consequently it can be manipulated—at least to a certain extent.
In his speech “Managing Acidity and pH in Wine,” Boulton noted that measuring potassium might help explain more about pH in wines. He said that people who grow corn in the Midwest understand the relationship between pH and potassium, because if you plant corn in the same place for four years, it will pick up potassium through the roots and result in acidification of the soil. The same thing occurs with grapes: As the vines take up potassium from the soil, the potassium causes titratable acidity to decline and pH to get higher.
According to Boulton, “Whenever you pick up a potassium, you lose a proton. The only question is: What are the conditions of berry size, tartrate and malate content as the consequence of that potassium uptake? And that will be different depending on the cultivars and different sites.”
For young vines with a smaller crop growing on soils with a high potassium level, the amount of soil moisture and/or late harvesting can lead to higher pH levels in grapes. If the root system is pulling more potassium into the vine, increasing the crop level will cause there to be less potassium per berry, and the pH will be lower and the acidity higher. For older vines that have deeper roots with less growing tips, these are the things that have modified the potassium available over time. Higher crop levels, early harvesting and dry soils can lead to higher titratable acidity and lower pH levels. Soil moisture content at the growing tips, not evaporation, is probably a critical factor to consider in terms of the availability and the time of potassium uptake. If a vine is overcropped, there will be less potassium per berry, resulting in a lower pH and titratable acidity. When the crop is reduced, there will be more potassium per berry, resulting in higher pH and lower titratable acidity.
Grapes grown in challenging environments can present problems of high acidity and low pH, low acidity and high pH, or, most difficult of all, high acidity and high pH. The amount of potassium uptake in challenging environments is critical in determining the titratable acidity and pH in the berries. Most important is the ratio of potassium to the amount of acid that you have.
High in both acid and pH
When a wine has high TA and low pH, the remedy is to de-acidify using calcium carbonate. For low acid and high pH, the solution is simply to add tartaric acid. In situations where grapes are high in acid and high in pH, they contain lots of tartaric and malic acids. Because there is also a lot of potassium, the pH is high as well. In de-acidification, the winemaker needs to shrink the tartaric and malic pool, which will pull potassium out and bring a proton back in.
In this case, the winemaker has to do two things: a carbonate de-acidification to address tartrate and malate concentration, and then go back and add tartaric acid. There are two effects of adding tartaric acid; one is instantaneous, while the other takes quite a while. The winemaker adds tartaric acid before fermentation; some of it disassociates, and some will begin to push the pH down. During aging, sometimes in fermentation and cold stabilization, there will be significant precipitation of potassium bitartrate. As that happens, the pH will drop. However, this is only true if the pH starts off below 3.8. If the pH is above 3.8 and you precipitate tartrate, the pH will rise. Therefore, it is important to get the juice below 3.8 prior to fermentation by adding tartaric acid, so that afterwards the pH will be in a place that when the tartrate precipitates, the pH will actually fall.
Tartaric additions will drive the TA up, but during fermentation the amount of TA isn’t important. After the pH has been lowered initially, the wine can be de-acidified by adding calcium carbonate. However, to prevent potential calcium tartrate crystallization, Boulton recommended using the double-salt method, in which a smaller fraction of the tank being de-acidified has the full amount of carbonate that would have been added to the entire tank. The goal is to precipitate out both the calcium tartrate and malate. In such a case the p H rises up to 6 or 7. The de-acidification releases a CO2 blanket over the wine. When the bubbles stop, the wine is filtered and back blended into the lot. This method will reduce tartrate, malate and calcium so that there will not be a calcium tartrate problem when the small lot is blended back.
Color and tannin
Two of the important indicators of red wine quality are color and tannin structure, and in cool and cold-climate growing regions, color instability and low tannins are major concerns. Anna Katharine Mansfield, assistant professor of enology at Cornell’s New York State Agricultural Experiment Station, who spoke about these topics at the symposium, will be writing an article on the subject for a future issue of Wines & Vines.
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