Wine Yeast Selection Criteria
One of the most intriguing issues facing a winemaker is the choice of a yeast strain to convert the sugars in sweet grape juice into wine. Simplistically, there are two basic choices – natural or native yeast that is present on the grapes themselves when harvested, or a designed commercial yeast strain. Both types of yeast will convert sugar to alcohol, but the choice of yeast can, and usually does, impact the aroma, flavor, and texture of a wine, as well as the conditions & rate of fermentation.
Natural or native yeasts often present more risk to a winemaker. The rate of fermentation may not be well controlled, and the capability to convert sugars to alcohol and other flavor components (good or bad) may not be predictable. Sluggish or even stuck fermentations can be a major issue with native yeasts. However, many winemakers carry forward with native yeast fermentations in an effort to create more “natural” wines. Through experience with a particular grape variety and vineyard source, winemakers may become comfortable with the native yeasts available and choose this option.
Commercial yeast strains typically provide greater predictability in the rate and overall conditions of fermenting sugar to alcohol, as well as producing a range of side-products that create desired aromas and flavors in the wine. Some key considerations in making a yeast selection are outlined below.
Grape variety is a significant consideration as many yeast producers have tailored strains to match various grape types. A yeast designed to ferment Viognier is probably not the best choice for Mourvedre. And a yeast that will produce that rich, cherry-flavored Tempranillo is an unlikely first choice for Albarino. Matching a yeast to the type of wine desired - dry vs. sweet, red vs. white, simple vs. complex, still vs. sparkling, etc., is also important. The condition of harvested grapes, such as their acidity, sugar content, and amount of mildew or rotten berries, etc., will also impact yeast performance, and thus influence yeast selection. For example, one very important consideration is grape sugar content which will ultimately be converted into alcohol in the finished wine. Some yeasts are designed to function in higher sugar and alcohol environments, whereas other yeasts will simply stop working if alcohol concentration rises above a certain level.
Another consideration is the types of aromatic and flavor components that can be produced by a particular yeast strain that will enhance the finished wine. For example, some yeasts are developed to produce citrusy aromas in white wines, whereas others will produce apple, pear, or peach notes. Yeasts can produce a very wide range of aromas and flavor compounds, so winemakers choose yeasts to produce preferred organoleptic components in their wines.
Some factors to consider when choosing a yeast are sort of “hidden” and not so obvious. For example, Some yeasts produce a lot more foam than others, and the amount of foam that can be produced during fermentation may be a problem if an inappropriately sized (too small) vessel is not available. Difficulty in settling flocculant material, including dead yeast cells, may create difficulty in producing a clear, uncloudy finished wine. The choice of yeast strain may also impact secondary steps, like malolactic transformation that most reds and some white wines undergo. The production of “off-odors”, typically from sulfur compounds, can become a problem if the proper fermentation conditions and nutritional requirements for the chosen yeast strain are not optimized. Yeasts are living organisms while active in the fermentation process, and often certain nutritional additives are required in order for yeasts to remain active and healthy throughout the process.
As you may surmise, wine yeasts can be a complex subject and selection of an appropriate yeast strain can be a complicated process that requires not only careful study, but a measure of experience with grape variety and vineyard source. Wine makers often turn to selection criteria provided by major yeast producers to help make these important decisions on yeast performance (see below). This is just another reason to thank our winemakers for their dedicated efforts to produce the delicious wines we enjoy.
More detailed information on wine yeast selection can be found in the following references:
Choosing a Wine Yeast Strain by Daniel Pambianchi, June, 2019 https://winemakermag.com/technique/1073-choosing-a-yeast-strain-techniques,
https://winemakermag.com/resource/yeast-strains-chart
Websites for major yeast producers, such as
www.lallemandwine.com,
www.wyeastlab.com,
scottlab.com/fermentation-cellar/yeast
Home Winemaking Step by Step by Jon Iverson, p 39-43