Successful fermentation is largely dependent on the yeast strain used. Modern winemakers are fortunate to have a myriad of yeast strains commercially available from several different companies including Lalvin and Anchor. These companies isolate yeasts from great wine regions and also create new strains via hybridization. This provides winemakers with options that can provide more stylistic control over fermentation. Strains have widely varying fermentation kinetics and nutrient requirements, and impact wine aromas and flavors differently.
Historically, winemakers relied solely on indigenous yeast strains, often referred to as wild yeast. Basically, fermentation was allowed to begin naturally. This technique is still widely utilized today and many argue wild yeast provides more unique flavors and mouthfeel unobtainable with commercial strains. The major issue with wild yeast is the increased risk of a problem fermentation as these strains are more prone to problems such as off-flavor production and poor sugar:alcohol conversion. Nevertheless, wild ferments are often used to produce blending components. Indeed, wild yeast has become so popular that commercial companies have begun producing "wild yeast" products, hybrid strains that are meant to provide the positive benefits of wild yeast without the risk factors associated with them.
Many winemakers ferment small batches of juice/must separately using several different yeast strains. This is primarily to add complexity to a final blend, but has the added benefit of acting as a precautionary measure to ensure one bad strain or ferment doesn't ruin the entire batch of wine. It is not uncommon for a finished bottle of wine to be a mix of five or six lots fermented with different strains.
Below are a few yeast strain parameters to keep in mind when selecting what is desired for your fermentation:
- Low volatile sulphur and acetaldehyde producer
- Oxygen and nitrogen demand
- TA production or reduction
- Malolactic fermentation compatibility
- Color extraction/stability
- SO2 production and sensitivity
- Weak vs. competitive fermentation capabilities
- Glycerol and polysaccharide production
- Alcohol tolerance & occurrence of stuck fermentation
- Slow vs. rapid fermentation rate
- Wine to be produced and the desired aroma/flavour
- Foaming and flocculation