One of the questions winemakers in northern climates ask most often is what yeast strains are recommended for fermenting various cold-hardy grape cultivars. While I understand why this question is asked – most catalogs selling yeast don’t list ‘Marquette’ or ‘Frontenac’ as recommended cultivars for a particular strain – it is also difficult to give a recommendation based on grape cultivar alone. Variables such as growing conditions of the grapes, winemaking conditions in the cellar, and stylistic goals are all important factors in determining what yeast should be used for making a certain wine. Vintage variation (especially in northern climates) can mean that a certain outcome with a commercial yeast strain one year doesn’t necessarily mean that we will have the same outcome the following year. Yeast can’t enhance the spicy character of Marquette, for example, if the aroma compound(s) responsible for that character aren’t in the grapes when they are harvested. Complicating matters is the fact that we are just beginning to learn what aromatic compounds might be involved in varietal aroma for these grapes!

Development of new yeasts.  Before a new commercial yeast strain is released, it undergoes extensive fermentation trials, from lab-scale to commercial scale and with various grape cultivars, to understand its impact on the wine. These trials require a great deal of costly research in order to be certain that the yeast activity will be fully understood once it is released. Unfortunately, the costs of this research guarantees that more obscure grape cultivars are not typically used in these trials. You are about as likely to see yeast recommendations for Picpoul or Vermentino as you are Marquette or Frontenac Gris. Fortunately, with the assistance of the Northern Grapes Project, researchers in the Midwest and Eastern US will be able to perform small-scale yeast trials this year for our cold-hardy grape cultivars.

Yeast trials.  While we may already have some ideas of how certain yeasts behave with cold-hardy varieties, we have yet to perform a study that includes statistical analysis of sensory data in replicated wine trials. This will allow us to evaluate whether a certain aroma or flavor can be attributed to a difference in yeast, grape cultivar or to the growing conditions. Although we do not fully understand the key aromatic compounds involved in the varietal aroma of cold hardy wines, we are able to build on knowledge gained from studies of of wine aroma and yeast metabolism to make educated yeast and cultivar matches. After several years of trials, we will be able to give confident recommendations for yeast strains to winemakers desiring a certain style wine from their cold-hardy grapes.

Grape aroma vs. Wine aroma.  Wine primary aroma compounds, which are also described as the varietal aroma of grapes, are the key aroma compounds that are used to distinguish wines made from one grape cultivar over another. In the grape berry, they are present in both volatile and non-volatile forms. The term ‘volatile’ simply refers to the fact that these compounds can be found in the headspace above the wine in a glass. In other words, this is what you smell when you stick your nose in a wine glass. Some volatile or ‘free’ aromas that are present in the grape berry are also present in the wines. When this occurs, drinking a wine made from that cultivar may remind you of how the grapes tasted when you picked them ripe off the vine.

However, the grape berries are also full of bound aroma compounds that can’t be tasted when you eat a grape, but are transformed into their free form by the action of yeasts, bacteria, and enzymes over the course of vinification. These bound compounds are often present in much higher quantities than the free volatile compounds, and are also considered an important component of the varietal aroma of wine. This is one of the reasons why the aroma and flavor of a wine is much more complex than the juice from which it was made. Yeast can play a key role in liberating these bound aromatic compounds so that they can contribute to the overall bouquet of a finished wine.[i] Thus, using a compatible yeast when vinifying a certain grape cultivar can help to enhance the varietal aroma of the wine. Some of the most important primary aromas that scientists have identified in grapes, and which yeast play a role in releasing during winemaking, are thiols and monoterpenes.

Thiols.  Volatile thiols are one of the most potent groups of compounds found in wine. Some can impart a negative aroma, while others contribute positively to a wine’s bouquet. They are almost non-existent in grape juice, and tend to only develop during fermentation. In Sauvignon Blanc, they are responsible for the box tree, passion fruit, grapefruit, and guava aromas that give the wine its varietal character. However, they have also been identified in wines made from other grape varieties such as Colombard, Merlot, Riesling, Semillon, and Cabernet Sauvignon. In grape juice, researchers have been able to identify the thiols in their bound form, also called an aroma precursor. Because of this, they have been able to understand the biochemical processes that yeast use to break apart the glycoside bonds with the thiols. Although all yeasts are capable of cleaving these bonds, certain strains of yeast have been shown to be better at it than others. Just as human metabolism varies according to a person’s genetics, so does yeast’s. Those that can efficiently release thiols are typically marketed as yeast that will enhance the varietal aroma of Sauvignon Blanc. This year we will be trialing two strains of yeast that are known thiol-releasers to see how they effect the overall aroma wines made from Frontenac gris. We suspect that perhaps some of the tropical fruit aromas found in wines made from this cultivar could be due to thiols.

Monoterpenes.  The second class of primary aroma compounds released by yeast are monoterpenes. Often simply referred to as terpenes, they are potent aromatic compounds found throughout the plant world. In grapes, they are found in large quantities in aromatic varieties like Gewurztraminer, Muscat and Riesling. Monoterpenes such as geraniol and linalool are often used as a fragrance in everything from soaps to air fresheners due to their rose or rose-like aromas. Other terpenes (cintronellol and nerol) can smell like citrus or lemongrass. Unlike thiols, monoterpenes often exist in a free, or volatile, form that can be detected in the grapes themselves. Nonetheless, a significant portion of monoterpenes found in grapes exist in a non-volatile, bound form. Yeast, bacteria, and enzymes in the grapes themselves are all capable of cleaving glycoside bonds and enhancing the varietal aroma of a wine. Knowing that La Crescent heady floral aromas are similar to a Muscat or Gewurztraminer, one can suspect that monoterpenes play a role in its varietal aroma. Using a commercial yeast strain that is a good terpene releaser can help intensify the primary aromas found in the grape. For our trials with La Crescent this year, we are using two yeast strains intended for aromatic white wine production, but are especially interested in yeast that will help with terpene expression. Vitilevure Elixir and Cross Evolution are two yeasts that we hope will show off the varietal attributes of La Crescent.

Enhancing spicy aromas. Spicy aromas exist in many different grape cultivars, however the chemical basis of these aromas isn’t completely understood. Although the compound responsible for black pepper aromas in Syrah and other cultivars has recently been discovered, researchers are still trying to identify if there is a biological method (yeast) of expressing it in wines.[ii] Nontheless, through sensory analysis of wines fermented with different yeast strains, we know that some are better able to enhance spicy characters than others.We aren’t certain what aromatic compound(s) is(are) involved in that spicy character, but we know that it exists. We know that sometimes Marquette wines can have a spicy character, even though we don’t know what causes it Thus,we will be trialing two yeasts that are known to enhance spice in two different cultivars. The strain D254 has been used in Rhone varietals, whereas the strain BRG has been used successfully in Burgundian varietals to enhance spicy characters. We are hoping that both can be used with success to enhance the varietal aroma of Marquette.

Yeast-derived aroma and flavors.  While we are looking for certain yeasts that may help to express the varietal aroma of cold-climate grapes, yeast also produce a number of aromatic compounds as a by-product of fermentation that will affect overall wine bouquet. Of course, the most important job of yeast is the production of alcohol from sugar. The presence of ethanol is essential to enhance the other sensory attributes of a wine. However, excessive ethanol can mask the aroma and flavors in a wine and give the wine an overall impression of “hotness” that is undesirable. While there are many important yeast by-products that contribute to the overall aroma and flavor of wines (fusel alcohols, glycerol, sulfides, volatile phenols, succinic acid, acetic acid…), perhaps the most important aromatic compound to consider when selecting a commercial yeast strain is its ability to synthesize esters. The esters produced by yeast will contribute to the fruity and floral aroma of a wine. These compounds can have aromas ranging from pear drops to flowers, honey, and bananas. Often they are used in the food industry to give artificial fruit flavors to candies.

Esters characterize young wine aroma.  While ester producing yeast strains aren’t typically associated with enhancing the varietal aroma of a wine, it has been shown that their production can be influenced by grape variety. For example, in Pinot Noir wines, the characteristic fruity aromas of plum, cherry, strawberry, raspberry, blackcurrant and blackberry characters were shown to be influenced by esters. These esters are synthesized by the yeast, but from aroma precursors found in the grape berry.[iii] Nonetheless, these compounds are some of the first to disappear during wine aging. The fruity and banana aromas that you smell in the winery during fermentation are typically associated with esters which disappear quickly in finished wine.. Mixed yeast cultures containing non-Saccharomyces yeast can also have a positive impact on the production of esters in wine. If a winemaker wishes to guard these aromas in a wine, they should be sure to ferment the wine cold and limit oxygen uptake. Ester-producing yeast strains should typically be used only if the wines are meant to be bottled and consumed while they are still young. In years where poor growing conditions (rot or botrytis) make it difficult to get fruity aromas from the grapes themselves, esters from yeasts may help make up for lack of varietal character. There is also some market demand for wines with this fruity aromatic profile. We will be using two high-ester producing yeasts in trials with Frontenac this year: Rhone 4600 and ICV Opale.

Selecting a yeast.  Think of yeast as one tool in your toolkit to help direct a wine to what you want it to be. The first step a winemaker needs to take when deciding what yeast to use is to determine the stylistic goal he or she has in mind for a wine. Is it going to be fresh and fruity with some residual sugar, or will the wine undergo a significant aging period in new oak and made into a dry wine? Perhaps you are making wines in both those styles. You probably wouldn’t want to use the same yeast for both of those wines. A wine that is meant to be fresh, young, and fruity should probably be fermented with yeast that will add some fruity esters to the wine. However, if you put that wine into a barrel, those ester aromas will quickly disappear due to their high volatility. You are better off trying to get the most fruit flavor out of the grapes themselves by using yeast that enhances varietal character.

Vineyard environment.  Sometimes the stylistic goal the winemaker has in mind may not even be possible depending on growing conditions of the grapes. In a warm year, if the Brix is greater than 25, yeast that only tolerate 14% alcohol should not be used (assuming you want a dry wine). We battle with high acidity in all our wines, but growers in the most extreme growing regions of the north may have to face the fact that their grapes may have too much acid to ever turn them into a palatable dry wine. This may also be true in short growing seasons where it is difficult to get the acid numbers down prior to harvest. Sometimes trying to force a wine to be something that it is not is a sure way to end up with a mediocre wine. It is important to remain realistic and understand that no matter how hard you try, you probably will never be able to make a “big” Bordeaux-style wine from Marquette or Frontenac.

Winemaking environment.  Winemaking conditions are also important. While Saccharomyces yeast tolerate  the harsh conditions in grape juice and wine, each strain has their own special range of ideal conditions for growth. The yeast cell wall is made up of fatty acids in a lipid bilayer. Think of it as a layer of oil. Just as some fats react differently to extreme temperature changes, so does this lipid bilayer surrounding the yeast cell. Really cold temperatures can make it stiff and hard to move, while really hot temperatures make it thin and runny. The yeast cell wall  is also sensitive to alcohol and osmotic pressure. The cell wall needs to transport nutrients into the cell and export waste products out of the cell, and both can make it difficult for the yeast to do so. The sugar concentration of the  juice ormust can make it difficult for the cell to get rid of waste, as it’s pushing against the osmotic pressure of the solution against its cell wall. A buildup of waste inside the cell will lead to cell death. Also, each strain of yeast varies in how efficiently it uses nutrients. Although all winemakers should be checking the YAN levels of their juice or must, this becomes even more important when using a yeast strain that has higher nutrient needs.

In the end, selection of a commercial yeast strain can have a significant impact on your finished wine.  Yeast can play an important role in ensuring that a fermentation finishes clean and dry with a predictable outcome to a wine, which is crucial to successfully marketing cold-hardy cultivars.