With every grape harvest or juice purchase, the home winemaker faces the choice of what yeast to use for the fermentation. We may develop a preference for a few reliable strains we go back to again and again. Other times, we may factor in analytical results of the juice with winemaking objectives. Based on sugar (Brix) content, we can check a yeast producer’s chart for alcohol tolerance. Knowing our cellar conditions, we can look at the temperature range that will work for the yeast and the wine style.
Sometimes, though, we have some very specific condition or fermentation objective that is out of our usual routine. Fortunately, yeast production companies work constantly to provide yeast strains that can do whatever a winemaker wants. While they are developed for commercial winemaking, home winemakers can use them just as well. For this column, I have selected some categories of special purpose yeasts to illustrate choices and give a few examples.
High-Vigor Yeasts
Yeast strains in this group share certain characteristics. They are highly tolerant of alcohol, often rated up to 18% ABV or higher. They also have good resistance to high sugar levels in must that might retard the start of a less vigorous yeast. Some of the yeast strains also show a wide range of temperature tolerance and relatively low nutrient need.
Consider these yeasts with blockbuster reds coming in at more than 25 °Brix or a must with added sugar for making an aperitif or dessert wine. In most cases, these same vigorous strains can be used to restart a fermentation that has stalled out using a weaker fermenting strain. Traditional method sparkling wine, refermented under pressure in a sealed bottle and in the presence of alcohol, presents another special demand for such strains.
Yeasts employed for these purposes include EnartisFerm D20, Red Star Premier Blanc, Uvaferm 43, Fermentis SafOeno BC S103, and Lalvin EC1118.
Malic Acid Management
Winemakers encounter multiple situations that call for metabolizing malic acid. The goal may be completion of malolactic (ML) fermentation in a traditional red wine to achieve a desired flavor and aroma. In Chardonnay, there may be a specific preference for the introduction of a buttery note. In some musts and juices, the titratable acidity (TA) is high and a reduction in malic acid is desired without added aromas.
There are yeast strains available that can assist. The first group here is strains that are considered favorable to subsequent or simultaneous growth of added ML bacteria. If you are particularly interested in using a yeast that will support ML fermentation, choices include strains like Red Star’s Premier Rouge or Premier Blanc, Enoferm AMH (Assmannshausen), and Lalvin 71B. Using strains like these for alcoholic fermentation sets the stage for selecting a strain of ML bacteria that supports your wine characteristics and stylistic goals. If your preference is to reduce the malic acid concentration without an ML bacterial fermentation, you can select from yeast strains that directly convert some of the malic acid to ethanol during primary fermentation. This situation might arise if you are making a fresh-tasting, clean-profile white or pink wine and you do not want the added complexity of aromas that will often accompany bacterial ML fermentation. While the Saccharomyces yeast strains that consume malic acid do not achieve near 100% removal like selected bacteria strains, a partial removal may be enough to meet your needs. The same Lalvin 71B noted earlier for supporting bacterial ML metabolism is itself capable of metabolizing 20–40% of the malic acid content without adding a bacterial inoculation. Lalvin C metabolizes up to 45% of the malic acid and Uvaferm SVG can remove about 25%.
Sweetness
Sweeter wines can be made by fermenting dry and backsweetening, but another option is selecting a yeast that can help achieve the sweet style you want to make. Some yeast strains are easier to stop during active fermentation. While most wine yeasts are glucophilic, selectively metabolizing glucose over fructose, most will still go entirely or almost entirely dry. I consider a wine with less than 4 g/L (0.4%) residual sugar to be effectively dry. Since fructose tastes sweeter than glucose, a yeast strain that leaves even a small portion of the fructose behind can give a wine an off-dry impression. These characteristics are not often listed in producers’ yeast application charts, so you may need to talk with other knowledgeable winemakers to find one you like.
My homegrown Chardonnay some years comes in at slightly higher acid than would be ideal for a balanced wine. When that happens, I ferment with a strain I have found to leave a trace amount of fructose after fermentation, Enoferm M2, that balances the high acid with a touch of sweetness. For a bit more residual sweetness, or to stop a wine by chilling and sulfiting during fermentation, you might choose Uvaferm CEG (aka Epernay II). This strain often slows or stops under stressed conditions, leaving some sugar behind.
Minimize Faults
When judging homemade wines in competitions, there are two main categories of faults that come up most often. The first of these is the browning and loss of fruity aromas that occur when wine becomes oxidized. Oxidation can appear at any stage of winemaking, beginning with harvest of the grapes and carrying all the way through to bottle aging. The other largest category of faults is stinky aromas reflecting volatile reduced sulfur (VRS) compounds. These manifest primarily as similar to rotten eggs or burned rubber, but include other unpleasant smells (in wine) like cabbage, onions, and garlic. Most VRS problems develop during primary fermentation and arise from hydrogen sulfide produced by the yeast.
Fortunately, there are special yeast strains that can help avoid both kinds of problems. Freshly pressed juice for white or rosé wine production is particularly vulnerable to oxidation. To address this, we can turn to the first non-Saccharomyces yeast strain in today’s column: A particular isolate of Metschnikowia pulcherrima that is available from numerous yeast labs, such as Lallemand’s Initia. This yeast strain has very low alcohol tolerance and is not suitable for full wine fermentation. However, added directly to juice as early as while that juice is still in the press pan, Initia grows rapidly and consumes oxygen (as well as copper, if present, which can otherwise accelerate oxidation). Once the juice has been racked to the fermentation vessel, it is time to go ahead and inoculate with the Saccharomyces yeast you have selected for the alcoholic fermentation.
There are two modes of attack on the development of VRS stink. One way these compounds develop is due to stressed yeast in a difficult fermentation, most often when there is an inadequate supply of yeast assimable nitrogen (YAN). In addition to supplementing your fermentation with diammonium phosphate (DAP) or a balanced yeast nutrient like Fermaid K or Superfood, you can select a yeast listed as having low YAN requirements. Strains mentioned earlier, like Red Star Premier Blanc, Uvaferm 43, and Lalvin 71B, are in this category. Other choices include Red Star Premier Cuvée, EnartisFerm EZFerm 44, and EnartisFerm SB. That last strain introduces the other approach to avoiding sulfur stink: Choosing a yeast that is low in VRS production to begin with. Besides the SB strain from Enartis, the Renaissance yeast company of Canada has developed a series of non-GMO strains that do not produce hydrogen sulfide during fermentation. Strains include Allegro (AL-48) for aromatic whites, Andante (ADT-36) for fruity reds, and several others. Lalvin Sensy also offers low production of sulfur off-odors while also carrying low nutrient requirements.
Many of these yeast strains (and others in this column) are not packaged in small quantities for carboy-size home fermentations. Some home winemaking shops are re-packaging them in smaller quantities, or going in on a group-buy with other home winemakers is another option.
Stylistic Improvement
Sometimes, a winemaker just wants a neutral, efficient yeast to get a fermentation done. Other times, a yeast that will enhance varietal and fermentation aromas is desired. Most producer yeast charts include a category like this. From Enartis, for instance, there is EnartisFerm Q9 that produces intense aromas of esters and release of thiol compounds — desirable sulfur-based aromas — during fermentation. From Lallemand, strains like IVC D47 and QA23 — as well as the Sensy mentioned earlier — are recommended for aromatic enhancement, and Fermentis has their SafOeno SH 12 for similar purposes.
Some winemakers like to seek out additional complexity in their wines by allowing indigenous or feral yeasts to grow in their must. Since taking chances on unknown yeasts can be risky, some yeast producers are now emulating that process with yeast blends that include non-Saccharomyces strains. Chr. Hansen produces Melody, a blend of Kluyveromyces thermotolerans and Torulaspora delbrueckii for complexity with Saccharomyces to reliably finish the alcoholic fermentation. In other examples, producers are blending two or more Saccharomyces strains for added complexity and aromatic enhancement, as with the Alchemy series from Anchor Yeast. One more stylistic enhancement from non-Saccharomyces yeast comes from the Laktia strain from Lallemand. This is a strain of Lachancea thermotolerans that converts glucose to lactic acid during fermentation, raising the TA and lowering the pH for a fresher, more complex profile in the finished wine. Laktia has a low alcohol tolerance, so a Saccharomyces strain should be added 24–72 hours after the Laktia inoculation to finish fermentation.
Winemakers should still account for the traditional variables when selecting yeast: Sugar level, fermentation temperature, desired wine style, and so on. But also keep in mind the range of special purpose yeasts for novel applications.
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