Ah that is a wonderful and complex question. When a winemaker chooses to inoculate for fermentation (which I generally recommend) there are many factors to take into account when making that choice. To follow is a list in order of what I believe the importance of each factor to be. A complete article (or more) could be written on each of these parameters as each carries its own subtle effects. Many yeast suppliers have charts addressing many or most of these factors to help guide you, or visit winemakermag.com/yeast-strains-chart.
• Suitability for varietal and application: Zinfandel yeasts typically need to be able to ferment higher potential alcohols under adverse nutrient conditions whereas a yeast to ferment a 21 °Brix delicate Riesling could be a completely different animal. To make high-Brix, high-residual sugar dessert wines, you’ll need a strong strain that produces low VA (volatile acidity). Always match your goals with your yeast.
• Alcohol tolerance: This is my second most important criteria. As alcohol levels increase in a fermentation, the yeast cell walls become increasingly permeable and grow unable to perform their basic metabolic functions, eventually dying off. Because of this, it’s critical to take a guess at what your final alcohol level will be and choose the right yeast strain for the job. The actual sugar to alcohol conversion rate can be different from one year to the next and from one variety to the next but my general rule of thumb is to multiply your initial Brix reading by 0.56. For example, a 25 °Brix must will yield around 14% alcohol.
• Temperature range: Like to ferment your whites cold? Then be sure to choose a yeast strain that can operate well at 55 °F (13 °C). Some strains have very specific operational ranges, so be sure to check with your supplier.
• Nutrient needs: Some yeast strains are what I call nutrient hogs. They just need more nitrogen, amino acids, and micronutrients than others. For example, RC 212, which is a strain I like for rosé wine and Pinot Noir, has a high nutrient requirement whereas Prise de Mousse is classified as “normal.” For nutrient hogs, they just won’t perform as well unless fed adequately.
• Flavor/mouthfeel/aroma contribution: For example, in Syrah production, D254 is known for producing more fruit-forward, balanced wines whereas D80, another excellent choice for Syrah, is known for its licorice spice notes. It’s important to give these effects at least a passing glance when making a yeast choice. You might be surprised that I list flavor and taste as less important than all the criteria earlier. It turns out that making sure your yeast can accomplish the job is more critical than worrying about flavor impact. The sensory contribution of one yeast strain vs another is minor and often undetectable in wines.
• Malolactic compatibility: Some yeast strains enable malolactic fermentation (MLF) better than others. For example, Prise de Mousse isn’t the best choice if you know you’ll have difficult conditions for MLF (low temperature, low residual nutrients, high alcohol, high acidity) whereas a strain like RC212 is considered highly compatible with enabling a later ML fermentation. I never let the ML compatibility overrule any of the above criteria; I just treat it as something to be aware of and to give my subsequent ML fermentation (if I’m doing one) as many favorable environmental conditions as I can.
The below yeast characteristics are those that I consider much less important than any of the previous. They remain factors that you should simply be aware of as they are minor and in no way are “deal breakers.”
• Rate of fermentation: This is simply one parameter to keep in mind; adjust your daily Brix testing accordingly and as needed.
• H2S Production: I would shy away from known hydrogen sulfide (H2S) producers if I were making a variety that is a natural “stinker” like Syrah.
• VA production: You may want to go with a low VA producer if you know that you’ll make your yeast perform in challenging conditions like high Brix where they’ll be liable to produce elevated VA levels already.
• Foam production: In white winemaking or when tank space is at a premium, a low-foam producer is preferred. Most of us can deal with higher levels of foaming just fine.
• SO2 sensitivity: All yeast will be sensitive to SO2 to some degree, but some are less so than others. Give this factor more weight if you intend to leave residual sugar in your wine and are relying on sulfur dioxide to help ward off a re-ferment.
• Application format: Live culture on a slant? Dried cells to hydrate? It’s your choice. All give great results.
• Cost: In the grand scheme of things, yeast is cheap. This is one of the reasons I almost always inoculate with a known culture. Call me old-fashioned, but I love reliable and predictable results, especially if I’ve paid over $2,000 for a ton of grapes and over $1,000 for a French oak barrel. Spending $20 on yeast (and much less than that on smaller home wine batches) is a no-brainer. Take these factors into consideration with your next wine and you’ll be on the right track . . . making sure your yeast can accomplish the job is more critical than worrying about flavor impact.