July 7, 2014

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Nitrogen in the Winery

 

Winemaking begins in the vineyard, and so does nitrogen. Nitrogen is one of the most common elements in the universe. On Earth, in its elemental form, it exists as a gas that forms 80% of our atmosphere. However, it is also a chemical constituent of many important components essential to life. Nitrogen makes up the building blocks of DNA, and it is also an important element in the composition of amino acids. When linked together, amino acids form the enzymes that drive all of life’s biochemical reactions. They are the building blocks to all proteins, hormones, and some plant metabolites that are responsible for wine flavor. Plants draw mineral nitrogen from the soil and convert it to amino acids and other compounds. Animals who consume plants in turn ingest the nitrogen that the plants have drawn from the soil. Even single-cell organisms, such as yeast, need nitrogen for survival.

 

Many of us are well aware of the effects of nitrogen on the growth of plants. Nitrogen is the most important nutrient involved in regulating vine growth, morphology, and tissue composition. Soils that are high in nitrogen cause an increase in vigor, which can lead to shaded canopies and high yields of unripe fruit in vineyards. However, it is also important to understand how the nitrogen that is in fruit at harvest can have an effect on fermentation.

 

What’s your YAN, man? When grapes or other fruits are harvested, they contain nitrogen in many different chemical forms. The most important nitrogen-containing compounds for fermentation are free amino acids (FAN), ammonium ions (NH3), and small peptides. These compounds can, for the most part, be consumed by yeast during fermentation and are collectively called yeast assimilable nitrogen, or YAN.

 

YANThe free amino acid content (FAN) of the grape juice can be measured by a variety of different methods, but the most commonly accepted way to measure it is the NOPA assay. I won’t detail the procedure here as there are plenty of resources available, but it is worth noting that a spectrophotometer is needed in order to interpret the results. For wineries looking to upgrade their lab, I’d highly recommend investing in this piece of equipment.

 

The ammonia (NH3) content of juice (which is 83% nitrogen) is measured enzymatically, and the results are also determined by a spectrophotometer. The sum of the FAN and the NH3 collectively give us the amount of YAN in the juice.

 

Another method for measuring YAN is called the Formol titration method. While it is a simpler method, involving only a titration, it does involve using a Formaldehyde solution. In order to mitigate health and safety risks with this method, the titration must be performed under a fume hood – which is a much greater investment for a winery than the cost of a spectrophotometer. Newer methods of measuring YAN are also available, but require highly specialized lab equipment.

 

Nitrogen and fermentation. After sugar, nitrogen is the most important macronutrient for yeast. When juice is lacking in nitrogen, the yeast can exhibit sluggish fermentations, create off-odors, and eventually expire before consuming all the sugar resulting in stuck fermentations. Yet, while every winemaker I know carefully tracks the ºBrix (sugar) in their fruit, many winemakers don’t always measure the nitrogen content of the juice. Why? Well, many simply add a set amount of nitrogen (in the form of commercial yeast nutrients) as part of their regular fermentation protocol. Or, perhaps they don’t add a standard addition at the start of fermentation, but as soon as the wine starts smelling “stinky” (sulfide aromas like cooked cabbage or rotten eggs), they add nitrogen in the form of salts such as diammonium phosphate (DAP). When yeasts lack amino acids in their diet, they start to synthesize their own. Unfortunately, yeasts’ recipe for amino acids includes adding a bit of sulfur to create cysteine and methionene. When they then metabolize these amino acids, hydrogen sulfide is a byproduct.

 

Nonetheless, although a minimum amount nitrogen is important in preventing fermentation difficulties, it is possible to have too much of a good thing. When the nitrogen concentration in the grape must is too high (>450-500 mg/L YAN), it can stimulate the yeast to start overproducing undesirable aroma compounds such as ethyl acetate – an acetate ester with a nail polish aroma. Acetic acid production is also increased, as well as other aroma compounds that can be both beneficial and/or detrimental to a wine’s character. Even more disconcerting is the fact that wines made from high nitrogen juice contain greater amounts of the possibly carcinogenic compound ethyl carbamate. Bacteria can transform any excess amino acids following fermentation into biogenic amines like histamine and phenylethylamine – compounds which can cause headaches, nausea, or extreme reactions such as heart palpitations and shortness of breath in those who are sensitive. Thus, knowing the quantity of nitrogen at the start of fermentation can help prevent some of the undesirable consequences of adding more nitrogen than necessary (not to mention the added cost of using these nutrients!).

 

How much YAN do I need? The minimum amount of YAN needed for fermentation depends on a variety of factors such as the initial sugar concentration of the must, the fermentation temperature, and the strain of yeast used to ferment the wine. Nonetheless, it is generally accepted that juice with YAN less than 140-160 mg/L should be supplemented. Recommendations for initial YAN based on Brix levels have also been reported and used with success (table 1). Winemakers wishing for a fruitier style wine may wish to adjust their YAN to 300-350 mg/L, as at this level the maximum production of fruity ester aromas is obtained.[1] YAN levels above 450-500 mg/L can lead to the production of off-aromas and flavors.

°Brix of must or juice

Target YAN concentration (mg/L)

21

200

23

250

25

300

27

350

Table 1 – Recommended YAN concentrations as a function of sugar concentration[2]

 

YAN and Cold-Hardy Hybrids. In general, University of Minnesota-developed hybrids contain high quantities of YAN, though variations in total YAN concentration can be seen depending on the geographic area of the vineyard. A recent survey of YAN in cold-hardy grape cultivars across the Eastern US conducted by Amanda Stewart as part of her phD dissertation at Purdue University found that 19 of the 20 highest reported YAN values were for University of Minnesota-developed cultivars. In fact, the highest ever reported YAN value for grapes (938 mg/L) was recorded in Frontenac Gris grown in Iowa.[3] Her study also confirmed that YAN is highly variable and dependant not only on grape cultivar, but also by geographic location and vintage. This is confirmed by YAN data compiled at the Horticulture Research Center in Excelsior, MN. We have found YAN to be highly variable in Minnesota grapes. Because it is impossible to predict YAN concentrations, even from fruit grown in the same vineyard, it is recommended that winemakers always have their YAN quantified by a reputable lab prior to addition of any yeast nutrients.



[1] Ugliano, M., P. Henschke, M. Herderich, I.A. Pretorius. 2007. Nitrogen Management is critical for wine flavor and style. Australian Wine Research Institute. Wine Industry Journal. (22)6: 24-30.

[2] Bisson, L.F., C.E. Butzke. 2000. Diagnosis and rectification of stuck and sluggish fermentations. Am. J. Enol. Vitic. 51:168-177.

[3] Stewart, Amanda. 2013. Nitrogen composition of interspecific hybrid and Vitis vinifera wine grapes from the Eastern United States. Doctoral Dissertation. Retrieved from Proquest Dissertations and Theses (Accession order No. AAI3592130)

“What Yeast Should I Use?”

The title of this post is one of the most common questions asked by winemakers working with cold-hardy grape cultivars. It is a simple question, but one that doesn’t have an easy answer. I have written on this topic in the past, so let me just throw out something that you probably haven’t heard yet: your yeast choice probably isn’t going to make or break your finished wine. There. I said it. I diminished the importance of yeast choice. To be fair, yeast selection does have an impact on the characteristics of your wine. Poor-quality fruit can be enhanced by choosing the correct yeast, and high-quality fruit can lose some of its potential by choosing the “wrong” yeast. The argument being made here is that your yeast choice isn’t going to make the difference between a wine that is worthy of a gold-medal, and one that is worthy of being poured down the drain.

Frontenac Gris lined up for sensory evaluation

Frontenac Gris in Wine Preference Study

When yeast choice REALLY matters, it’s when the environment in which the yeast will live (the  juice, and eventually fermenting wine) is inhospitable. Very acidic (pH < 3.2) or very high sugar juice are stressful to yeast, as are very hot or very cold temperatures. Certain strains of yeast are more tolerant than others of these harsh conditions. If for example, you harvest Marquette at 25.5 °Brix and hope to make a dry wine, you’d better make sure that the yeast is tolerant to alcohol levels greater than 15%. Making a late harvest or ice wine? You need a yeast with high osmo-tolerence to handle the high sugar environment.  If you plan on using bacteria to convert the malic acid to lactic acid, you’d better make sure that the yeast is compatible with Malolactic Fermentation (MLF). Do you have a cooling system in your winery? If not, then you probably should pick yeast that can tolerate hotter temperatures. If you plan on cold-fermenting the wine (to guard fruity aromas), the yeast should be tolerant of cold temperatures. All of these planning questions help to eliminate the outright poor yeast choices, then you can get into some of the nitty-gritty details.

Sensory effect of yeast choice. After eliminating yeast strains that won’t work with your juice chemistry and fermentation goals, the main concern is the sensory effect of the yeast strain. In general, cultivated yeast strains will produce low amounts of off-aromas (H2S and VA) when given sufficient nutrients. Some yeast can affect the mouthfeel of a wine by producing higher amounts of glycerol. There are yeast strains that produce high amounts of tutti-frutti ester aromas – great for young wines, but for high-end wines that are going to age a year or more before release, there isn’t much of a point in using these strains. Esters are extremely volatile, and are the first aromas to disappear – sometimes within a few hours of opening the bottle! Other yeasts will enhance the aroma by releasing some of the aroma precursors found in the grapes at harvest. This is all well-and-good, but in the end the yeast can’t do much unless the precursors for these aromas are in the grapes themselves. This is where the big question lies with cold-hardy grapes. For the most part, we know very little about the nature of their inherent aromas. We know that La Crescent is related to Muscat, and has some of the same floral and perfume aromas that are found in all Muscats. We know that it does contain high quantities of monoterpenes, the class of aroma compounds that have these flowery characteristics. However, we also know that Marquette contains significant quantities of monoterpenes, although it is rare to see floral descriptors used when tasting Marquette wines.  Frontenac contains  methoxypyrazines when unripe (similar to the green pepper aroma in Cabernet Sauvignon) and minty aromas (methyl salicylate and menthol).[1] As we learn more about the impact aromas of these grape cultivars, it may affect our decisions for yeast selection. You can read about why these particular yeast strains were chosen for this trial in a previous post.

Yeast trial with cold-hardy grapes. Last year, we decided to ferment the four University of Minnesota grape cultivars with various commercial yeast strains. This was a trial that was sponsored by the Northern Grapes Project, and was replicated at Cornell University with fruit from Vermont and New York. Over the past few weeks, I asked a group of 27 people who all have experience tasting regional wine to participate in a wine sensory panel. The panel consisted of 16 men and 11 women, whose ages ranged from 26 to 74 with a median age of 50. They were served three wines from each of the four grape varieties and asked to rank them from their most preferred to their least preferred. The only difference in the three wines was the type of yeast that was used for fermentation, which is highlighted in the chart below.

Frontenac Frontenac Gris Marquette La Crescent
ICV – GRE Lalvin – DV10 ICV – GRE Lalvin – DV10
Lalvin – Rhône 4600® Anchor – Vin13 ICV – D254® Vitilevure – Elixir
ICV – Opale® Anchor – NT 116 Levuline – BRG Cross Evolution®

The panelists were also asked to write comments on each of the wines. Not surprisingly, many of the tasters noted differences between the wines. On several occasions, it was noted that one of the wines was “far superior” to the two others in the flight, with notes such as “most complex” and “most interesting” written in the comments section. I even had one panelist who stated afterwards (when he found out what the trial had entailed) how he is always surprised by how much yeast choice can “make or break” a wine. In the end, we were testing whether there was a difference in preference for these different wines in order to give recommendations to winemakers. So which of the three yeasts for each grape cultivar were preferred by our tasting panel?

Drum roll please….

For each wine flight, the judges scored the wines in order of preference, with 1=most preferred, and 3=least preferred in the flight. We tallied the total points for each wine and the results are in the charts below. A lower score indicates a higher overall preference (more #1 ranks) by the judges. Statistical analysis was done using the Basker Critical Values for Rank Sum.

Sensory Panel

The small letter next to the sum indicated whether the difference seen is statistically significant (p < 0.05). If there is the same letter next to the sum, then there is no statistical difference in the observed count. As you can see, for every single yeast trial, no clear difference in preference was shown for one yeast over another yeast in this particular trial.  We may be able to say that for La Crescent, there is a trend towards a preference for yeasts that release monoterpenes (both Cross Evolution® and Elixir enhance floral characters in aromatic whites), but we would need to recruit a larger panel to see if this holds true.  However, at this point, there isn’t a clear preference for those yeasts over a more neutral yeast (DV10).

We chose the yeasts for this trial based on their ability to work well within the chemistry limitations of our varieties.  The subtle differences in these wines that may have been observed by individual panelists didn’t translate into a difference in preference for one wine over another for the group as a whole. This is just to highlight why yeast choice probably isn’t as critical as one might think. In the end, it’s a decision that a winemaker makes based on his or her own personal preference and wine-style goals. This is part of the art of making wine. In the  future, we hope to also do descriptive analysis of these wines, to see if these differences can be appreciated by a panel of consumers. Descriptive analysis will also help guide winemakers towards understanding how yeast choice may affect the sensory characters of their wine.

Grape Cultivar – Yeast Used in Trial

Rank Sum*

Frontenac – ICV GRE

49 a

Frontenac – ICV OPALE®

50 a

Frontenac – Rhône 4600®

56 a

*For Frontenac we could only used the scores from 26 panelists due to an error on one score card

Grape Cultivar – Yeast Used in Trial

Rank Sum

Marquette – ICV GRE

54 a

Marquette – D254®

54 a

Marquette – ICV BRG

54 a

 

Grape Cultivar – Yeast Used in Trial

Rank Sum

La Crescent – DV10

63 a

La Crescent – Elixir

52 a

La Crescent – Cross Evolution®

47 a

 

Grape Cultivar – Yeast Used in Trial

Rank Sum

Frontenac Gris – DV10

55 a

Frontenac Gris – NT 116

52 a

Frontenac Gris – Vin 13

55 a

 


[1] Pedneault, K. (November, 2012). Canada: Maturity and Quality of Some Hardy Grape Varieties Grown in Quebec. International Conference Neubrandenburg and Vitinord. Neubrandenburg/Szczecin.

 

 

Keys to Successful Fermentation: Part 1

facebook_32Fermentation is a natural process by which yeast consume sugar and convert it to ethanol.  A successful fermentation is one in which the winemaker ensures that the conditions are met to enable a population of yeast to live and thrive until the winemaker wishes – generally until all the sugars have been depleted. All this needs to be done while minimizing the production of volatile acidity and sulfur off-aromas, and maximizing the desirable aromas and flavors produced during fermentation. It sounds easy enough, but for anybody who’s been around the industry can attest, stuck and sluggish fermentations happen more often than you might wish.  So, I present, the key points to a successful fermentation in four parts: yeast hydration and addition, the first quarter of fermentation, mid-fermentation, the last quarter of fermentation.

Yeast Population Kinetics

There are four main stages that a population of yeast will go through in a typical wine fermentation as illustrated in figure 1 below.

1) Lag phase – this is a very short period of time in which the yeast become acclimated to the juice or must. The duration of the lag phase is less than a few hours, until the yeast realize that they are in a sugar and nutrient-rich environment and they begin to multiply by budding (yeast division).

2) Exponential growth phase – yeast multiply rapidly. The yeast population can double every 4 hours until a maximum population density is achieved. There is an increased demand for oxygen as yeast cells replicate.

3) Stationary phase – The yeast population has reached a critical mass. This is the longest phase of fermentation in which the yeast are actively converting sugar to alcohol through anaerobic fermentation. At this point oxygen isn’t necessary for yeast survival, but a winemaker may choose to aerate a wine for other reasons (reduction aromas, color stability, etc.)

4) Yeast Death – Over time, the yeast will slowly deplete the nutrients available in the juice (sugar), and will also be producing waste products that are toxic (ethanol). Dead yeast cells will break apart (lyse) as they fall to the bottom of the tank and release more toxins that will kill surviving yeast. Thus, the decline of the yeast population is a rapid, exponential decline.

By understanding the important steps that  winemaker needs to take during each of these phases of fermentation, one can be assured that the risk of a stuck or sluggish fermentation is minimized. The first part begins with hydrating active dry yeast, and adding the yeast to juice or must.

 

yeast growth

Part 1: Yeast Hydration and Addition

 

1) Choose the correct yeast (Account for Osmotic Shock).

Grapes are naturally high in sugar. When yeast encounter this high sugar environment, there is a certain amount of osmotic pressure placed on the outside of the yeast cell wall.  Since the cell wall is permeable, the yeast expend energy to ensure that they maintain an equilibrium between the the pressure on the inside and the outside of the cell. To do this, they tend to produce more glycerol inside the cell, but they also will produce acetic acid to try to decrease the viscosity of the fluid outside of the cell (the grape juice). This phenomenon is well known in ice wine production, and is why these wines tend to have higher levels of volatile acidity than table wines. In this type of environment, the yeast need an array of micronutrients and amino acids to form the  fatty acids and sterols that will strengthen their cell membrane. A winemaker can also minimize damage to the yeast by making sure it isn’t exposed to further stress such as cold temperatures and excess SO2.

The initial osmotic pressure placed on the yeast will impact the physiology of the cell for the duration of its life, that is, until the end of fermentation. The resistance of yeast to alcohol in the final stages of fermentation depends on the initial osmotic pressure placed on the yeast and its ability to resist this stress. If a winemaker knows that the potential alcohol of the juice is greater than 13%, it is important to choose a yeast that has the ability to resist higher alcohol levels. Late harvest or ice wine styles should be fermented with a yeast that is intended for high sugar musts in order to minimize the potential problems with volatile acidity, and to ensure that the fermentation begins in a timely fashion.

 

2) Proper yeast re-hydration practices (resistance to other shock factors).

As mentioned above, sterols and polyunsaturated fatty acids are important factors that the yeast need to create a strong cell membrane. When one rehydrates the yeast in water (along with yeast nutrient), the yeast metabolism is in a respiratory state (consumes oxygen) which allows it to more easily synthesize these resistance factors in its cell wall. If yeast is rehydrated in juice, the yeast are more inclined to have a fermentative metabolism from the get-go, which makes it difficult to synthesize the products necessary to strengthen its cell wall to provide protection from stress during fermentation. The initial content of these resistance factors will become diluted with each generation during the multiplication phase.

The yeast multiplication phase corresponds to the consumption of the first 30-40 grams of sugar. Once the initial population of yeast cells reaches 100 million cells/mL of must, the juice is considered completely colonized. This level of colonization does not depend on the initial population of the yeast. So, in order to arrive at 100 million cells/mL, the greater the initial population of yeast, the less they need to replicate to reach their maximum population. Thus, their resistance to stress becomes less diluted, and the yeast are more able to survive in the high alcohol environment near the end of fermentation. This isn’t to say that you should double or triple the recommended dose for yeast in your fermentation. This dilution of stress factors is only seen if the initial amount of dried yeast used is less than 300 mg/L. Thus, the recommended quantity of 400 mg/L on the package of active dry yeast accounts for this.

 

3) Yeast Nutrition.

During the multiplication phase, yeast need amino acids/nitrogen, fatty acids, and micro-nutrients (vitamins and minerals). Some of these elements aren’t bioavailable in the juice at this critical moment when the yeast need them the most. By adding nutrients that make these  elements immediately bioavailable to the yeast, it diminishes the risk of added stress to the yeast due to a nutritional deficit. Adding yeast nutrients during rehydration and at the moment of yeast addition to the must allows the yeast to multiply in the best conditions. However, the different enological yeasts all have different needs when it comes to nutrition. The dose necessary during yeast addition depends on which yeast you use, along with other factors: potential alcohol, maximum fermentation temperature, oxygenation, and the initial temperature of the must during addition.

 

4) Accounting for cold shock in low temperature juice.coldshock

Have you ever jumped into water that is just above the freezing point?  You know then, how yeast might feel if they are immediately dumped into a cold tank of juice – something that is common in white and rosé fermentations. It is easy to evaluate the potential for cold shock to the yeast: the greater the temperature difference between the water at the end of yeast hydration and the juice in the tank, the greater the stress to the yeast. If the temperature difference is greater than 10ºC, the stress on the yeast caused by the cold shock will have physiological consequences to the yeast that will affect it throughout the fermentation. When it is known that there is a high potential for this cold shock during yeast addition, it is important to take some steps to compensate for these risks. The most important is to slowly acclimate the yeast to the juice temperature by adding some of the juice to the hydration water to bring down the temperature. The temperature decrease should not be more than 10ºC over a 20 minute period. When the yeast is added to the tank, the temperature difference should not be greater than 10ºC. Other ways to compensate for this stress are by adding a higher dose of active dry yeast, and ensuring adequate nutrition.

 

5) Compensation for the elimination of fatty acid sources (white and rosé wines). 

In all white and rosé fermentations the juice is racked 24-48 hours after pressing to eliminate suspended solids. The degree of clarification can be enhanced by using fining agents and enzymes in the juice – an important step if the grapes arrived in poor sanitary state. Ideally the turbidity of a juice following the first racking falls between 100 and 250 NTU. Nonetheless, while eliminating pectin particles and insoluble solids, you are also removing poly unsaturated fatty acids that are important for yeast survival. If the juice clarification is less than 200 NTU, it is important to take steps to reduce stress on the yeast. Adding yeast nutrients rich in fatty acids, or increasing the initial yeast population are ways to ensure yeast survival through the end of fermentation.

To Be Continued with Part 2: The first quarter of fermentation….

 

Biological Reduction of Total Acidity

A balanced wine should be the goal of every winemaker – not only in the wine’s chemistry, but in the wine’s aroma and flavor. While the latter is often up to interpretation (heavy-handed oak treatment is an example), much is known about how taste components such as acidity, sweetness, and alcohol can work together in harmony or discord on the palate. Cold-hardy wine grapes developed at the University of Minnesota are rarely harvested with a total acidity (TA) under 10 g/L. It is not uncommon to see total acidity at harvest of 15-18 g/L in Frontenac, and even the newest cultivar, Marquette, sees total acidity ranging from 9-13 g/L.

In dry wine production, wine balance can be a trickier dance, as sweetness can help soften both acidity and alcohol. In technical terms, any wine with less than 5 g/L (0.5%) of residual sugar may be considered dry if the yeast population dies. The perception of dryness, on the other hand, can vary based on other aspects of the wine, such as acidity, dry extract, and aroma. A wine that is dry and acidic can taste harsh, astringent, and un-balanced to the consumer. Because tannin and alcohol can accentuate the sensation of acidity, winemakers using cold-hardy cultivars to make dry red wines must consider ways to mitigate this high acidity.

There are three general methods one can use to lower high acidity dry wine production: physical methods (blending and amelioration), chemical methods (bicarbonates), and biological methods (yeast and bacteria). For the acid levels seen in Northern vineyards, the best approach is most likely a combination of all three of these methods. The Northern Grapes Project will be exploring these methods individually, so that winemakers can have a host of different tools in their arsenal for reducing acidity in their own wines.

Biological Deacidification. The most important thing to remember about biological deacidification is that it only affects the malic acid portion of your wine’s total acidity. The most common method of biological deacidification is through malolactic fermentation. Although not a true fermentation, bacteria that exist naturally in the environment have the ability to consume the malic acid in grapes and convert it to lactic acid, softening the wine’s acidity. Nearly all red wines around the world undergo MLF and some white wines also benefit from acid reduction of this practice. Traditionally, red wines are stored in barrels following alcoholic fermentation, where MLF will naturally occur as long as the wines are left unprotected from microbial spoilage. Wineries choosing to allow “spontaneous” MLF to occur often have to wait months for the malic acid to be consumed. The risks involved with leaving the wine un-sulfured, as well as the development of reliable bacteria starter cultures have pushed many wineries to inoculate their wines rather than waiting for MLF to occur naturally.

Yeast also have the capability to consume malic acid (malate), though they convert it to ethanol rather than lactic acid. It has long been known that certain yeasts (Schizosaccharomyces pombe, Hanseniaspora occidentalis, Issatchenkia orientalis) are especially efficient at consuming malic acid. However, because these yeasts have poor alcohol tolerance, they must always be used in conjunction with Saccharomyces yeasts in order to complete fermentation in wine. While  S. pombe has been available commercially for some time for use in wine production, the development of other non-Saccharomyces yeasts for commercial use is a hot topic at the moment. We will likely see more of these yeasts available in an active-dry form to use in sequential yeast inoculations for wine.

Until then, we decided to look at some of the commercially available Saccharomyces yeast strains that have a reported ability to reduce malic acid, and trialed them with cold-hardy grape cultivars. After consulting with several enological product suppliers, we came up with a list of several different yeast strains: Lalvin C (Lallemand), Exotics (Anchor), Lalvin ICV OPALE (Lallemand), and Uvaferm VRB (Lallemand). We also trialed a non-Saccharomyces yeast that Lallemand has made available in an active dry form for sequential inoculations: Torulaspora Delbrueckii (sold commercially as Level 2TD). Although its malate-consumption hadn’t been verified, a technician at Lallemand had recommended it because they had observed some softening of the acidity in wines that had been fermented using it.

Yeast deacidification trial. We did a small trial with these yeasts in which we used juice from the 2012 vintage that had been previously frozen. For each MN cultivar, we trialed three different yeast strains, and used a fourth yeast strain as a control. One lot of juice was divided into 20 micro-vinification lots of 500 mL each. Thus each yeast was replicated in 5 fermentation lots. For this initial trial, we were concerned with monitoring mainly the chemistry change using each yeast. For white wines we used Lalvin DV10 as control, and for red wines we used ICV GRE as a control yeast. Both are considered reliable fermenters with no reported malate degradation.  The unusually hot weather in 2012 caused initial brix levels to be extremely elevated, so initial malate numbers reflect juice that had been diluted to bring the sugar concentration down to 25° Brix.

NanoVinification

Results: With Frontenac Gris, we started with an ameliorated juice that had a total acidity of 9.92 g/L, pH of 3.00, and 5.1 g/L of malic acid. Although all the added yeast strains showed some reduction from the initial malate levels in the juice, the acid reduction seen in the Lalvin C, Exotics, and the combination of Torulaspora delbrueckii with Exotics all were significantly lower than the control yeast (p <0.05). We used Lalvin C in a larger lot following this trial in order to evaluate the sensory impacts of this yeast. It’s worth noting that in all 10 micro-vinifications in which Exotics was used, the wines exhibited some stuck fermentations. Thus, some care may be needed when using this yeast in order to complete fermentation in low pH juices.

 

microvin FGRIS

 The La Crescent juice that we divided up for the micro-vinification trials was ameliorated to 25 Brix, which left the starting malate levels at 5.3 g/L. The decrease in malic acid during fermentation was less pronounced than what we saw with the Frontenac Gris fermentation. In fact, only the vinification lots in which Exotics was used showed a statistically significant drop in malic acid (p< 0.05). ICV Opale is advertised to lower malate levels by 0.1 to 0.4 g/L. Our trials show that it exceeded this level in high malate juice, however, this decrease was not significantly lower than our control yeast which has no reported malate reducing properties.

 

microvin LC

Our Frontenac was pressed and fermented as a rosé. Again, it was necessary to ameliorate to reduce the high sugars that we achieved in 2012, however, the initial malate concentration of the juice was still relatively high at 4.6 g/L. All yeast used for this trial caused a decrease in the final malic acid concentration of the wine. All observed differences in malate reduction were statistically significant (p<0.05), except for the two lots that were fermented with Lalvin C. There is no statistical difference between the observed malate reduction when using Lalvin C in conjunction with T. delbrueckii yeast. This (along with the other results seen when using T. delbrueckii) suggests that any impact on the perception of acidity due to this yeast is likely not related to malate degradation. All the Frontenac fermentations finished dry with no stuck or sluggish characters.

microvin frontenac

 Marquette was also pressed immediately and fermented as a rosé. The ameliorated juice had an initial malic acid concentration of 4.1 g/L. Exotics and VRB showed identical malate reduction capabilities, and even though the difference between these two yeasts and the control (ICV GRE) was only slight, the difference is statistically significant (p=0.046). Once again, Lalvin C proved to have the greatest potential for malate reduction, with a 1.10 g/L decrease in malic acid concentration from the juice.

microvin Marq

It is important to keep in mind that there are many different tools available to a winemaker to manage high acidity in their wines. The selection of yeasts that we looked at here are only a small example of what is available on the market. It is important to talk with technicians who supply your winery in order to get a better idea of what products might help with managing your acidity.

 

 

 

Yeast Selection Trials for Cold-Hardy Grapes*

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.

*This article was published in the Nothern Grapes Project newsletter on August 17th, 2012


[i] A. Zalacain, J. Marín, G.L. Alonso, M.R. Salinas. 15 March 2007. Analysis of wine primary aroma compounds by stir bar sorptive extraction, Talanta 71:4, 1610-1615

[ii] Logan, Gerard. University of Auckland, New Zealand. 5 August 2012. Personal communication

[iii] Moio, L. and Etievant, P.X. (1995) Ethyl anthranilate, ethyl cinnamate, 2,3-dihydrocinnamate, and methyl anthranilate – 4 important odorants identified in Pinot Noir wines of Burgundy. American Journal of Enology and Viticulture. 46, 392-398


 

Lees, Glorious Lees

Winemaking decisions regarding lees are as important to the final composition of a wine as yeast selection, temperature control, and cap management. Following fermentation, winemakers need to decide how they are going to age and mature their wines. Any decision that involves wine maturation should also consider lees management, and should be planned-out in the same manner you plan any other step in vinification. It is essential that winemakers understand what constitutes “lees” in both grape must and wine, and how the lees should be managed throughout fermentation and aging.

First, what are they?

Lees are simply a deposit that forms in your wine after allowing it to settle.  While that seems like a simple enough definition, there are many different types of particles that make up the lees, and while some can be beneficial to a wine, others can produce bitter or herbaceous compounds. In French, “lees” refers to a very specific kind of deposit – mainly dead yeast and bacteria cells. Thus, there is less confusion as to what type of particles are involved when using lees as an enological tool. In English speaking countries, where lees can contain a number of different deposits from the wine, winemakers often use the term “light” lees when talking about lees containing yeast and bacteria (the good lees), and “heavy” lees when referring to the lees that should be discarded during racking. From now on, when using the word “lees,” I’ll be referring to “light lees.” I’ll refer to “heavy lees” as dregs. This will hopefully help avoid any confusion.

So lets talk about the positives of using lees as part of your aging program. A good rule to remember is that the lees are light, fluffy particles that generally remain in suspension 24 hours after moving your wine (e.g. racking, stirring, or pumping), as long as you didn’t use pectins. They are typically very small in size (from 1 to a few dozen micron). In contrast, dregs are particles that will settle at the bottom of your vessel after 24 hours, and can be very large (100 microns to several millimeters). Because lees are essentially dead yeast and bacteria, they contain different types of molecules such as proteins and polysaccharides that can benefit the taste and aroma of your wine when correctly managed.

Polysaccharides are involved in the sensation of roundness and fullness in a wine. They also create chemical bonds with tannins, color pigments, and aromatic molecules, thus making them unable to precipitate from your wine. This chemical bond is what makes barrel-aged wines less susceptible to protein instability. The binding of polysaccharides with aromatic molecules is what causes a wine to have a good “length” on the palate, as the aromatics will be released more slowly when you taste the wine. Wines aged in oak need lees contact to help integrate the flavors of the oak with the flavors of the wine.

Other products released by the yeast cells include amino acids and nucleic acids. Both are used as flavor enhancers in the food industry, and work the same way in your wine. Yeast cells will also release esters as they start to break down. Many of the yeast esters have floral and spicy aromas that will contribute to the overall bouquet of a wine.

Lees also have the ability to remove certain undesirable compounds from wines. It is well-known that they help bind diacetyl, the buttery-smelling aroma produced during malolactic fermentation, but they have also shown some ability to remove the volatile phenols produced by Brettanomyces. Not only do they remove unpleasant odors, but they also help remove potentially harmful compounds such as residual pesticides and fungicides that may appear in wines, as well as a class of compounds known as biogenic amines (e.g. histamine, tyramine, phenylethylamine). These compounds are known to cause headaches, nausea, hypotension or hypertension, and cardiac palpitations in people who are sensitive.

At this point you may be wondering why you haven’t been using lees all along! However, even the “light” or “fine” lees can cause some problems with your wine. While the chemical bonds created by the polysaccharides help make a wine more protein stable, it also makes them more stable against tartrate precipitation. While this is a good thing in most areas of the world, in Minnesota most wines benefit from the drop in acidity that results from tartrate precipitation. Thus, cold stabilization may want to be carried out prior to leaving your wine age on lees.

Also, while the amino acids liberated by the dead yeast can contribute positively to the wine, often these same amino acids can contain a sulfur-group that can lead to reduction odors and sulfur-off odors in your wine. Regularly stirring your lees will help minimize the risks by ensuring that the bottom of your tank or barrel doesn’t become a concentrated zone of reduction. Stirring is also essential to breaking up the cell walls of the yeast to ensure that all of the polysaccharides are liberated. Thus, it is essential that wines are stirred regularly while being aged on lees.

What about the dregs?

Heavy lees essentially have no benefit to a winemaker. They are, in fact, detrimental to wine quality.  They are made up of large particles and agglomerations of different particles in wine. They can be vegetal particles, crystals, large tannins, coloring matter, and any combination of the above along with yeast and bacteria cells! Wines that are left in contact with the dregs can become bitter and herbaceous. It should be noted that dregs will continually form in your wine through various reactions that occur over the course of maturation. Thus, wines should be evaluated over the course of their maturation. Each wine will vary somewhat in the frequency that it should be racked. Generally, once wines are initially racked following alcoholic and malolactic fermentations there is no reason to rack before a 3 month aging period.

 

Wines prior to racking

Further Reading

Delteil, Dominique. 2002. Working with lees: key elements to wine maturing. Australian Grapegrower & Winemaker, 30th
Technical Issue

J.A. Pérez-Serradilla,  M.D. Luque de Castro. 2008. Role of Lees in Wine Production: A Review. Food Chemistry. Vol 111:2

What yeast should I use?

One question I am asked again and again are my recommendations for which commercial yeast strain I prefer for a certain grape variety. There has been some work on this by the U of MN in conjunction with Scott Labs and Fieldstone Vineyards. The chart that they came up with for yeast recommendations is posted here. (click on the photo to see it enlarged). However, there are new yeast varieties that are released on the market every year, and we can’t possibly test all of them at a time. We can have an idea as to which yeast selections might work well based on what we know about certain varieties, and this is how we choose which varieties to trial. I’m hoping to give insight as to how I might choose a yeast to trial with a particular cultivar, so perhaps more wineries can think about trying different yeast, too.
The first question I usually have when someone asks my recommendation is “what style are you shooting for?” Though yeast in itself won’t help you achieve a certain style, it can be an important tool. Often it is the quality of the grapes coming in that will determine what type of wine you will make, and using a yeast that promises to enhance certain aromas such as “spiciness” will be of no use if the grapes themselves lack this character. In areas and/or vintages where grapes are affected by rot, it may be wise to choose a yeast that will ferment your red grapes quickly, so you can press the fruit, and get it off the skins and filtered as quickly as possible. Are you going to cold-ferment the wine, or barrel-ferment? What characters do you want to enhance or diminish?  Add the fact that in Minnesota we are working mainly with hybrids that haven’t been given complete chemical and sensory analysis, and one can see how quickly the answer to the question “What yeast should I use?” becomes increasingly complicated. The advancements made in commercial yeast strains make it an important tool for winemakers, and knowing how to select the yeast you use will have a postive effect on your final wine.
History

Even though alcoholic beverages have been made since antiquity, it wasn’t until 1863 when Louis Pasteur first described yeast as being responsible for the process of alcoholic fermentation. With that knowledge, we had a better understanding of winemaking.  Muller-Thurgau was the first to introduce the concept of inoculating wine fermentations with pure yeast cultures in 1890. Of the 100 different genera that represent over 700 different species of yeasts, only 15 are associated with wine: Brettanomyces/Dekkera, Candida, Cryptococcus, Debaryomyces, Hanseniaspora/Kloeckera, Kluyveromyces, Metschnikowia, Pichia, Rhodotorula, Saccharomyces, Saccharomycodes, Schizosaccharomyces, and Zygosaccharomyces (Pretorius, 2000). While all these yeast genera have been found in the wine industry and are capable of fermentation, only Saccharomyces is able to ferment wine to dryness. Nearly all commercial yeast strains intended for fermenting wine are genetic variants of Saccharomyces cerevisiae. While hundreds of different commercial yeasts exist, they all have been selected for certain properties to ensure good fermentation, while adding positive aromatic and gustatory properties.

When looking through a catalog of commercial yeast strains, one can quickly become overwhelmed by the choices of yeasts available, especially when we aren’t working with the recommended grape varieties. Although some of them have been proven to work well with certain hybrids, others may work well too, but we just haven’t proven it.

Fermentation properties

Ethanol Tolerance – All commercial yeast strains have good tolerance to alcohol, though some can tolerate higher levels than others. This is an important factor to consider for several reasons:

  • Many hybrid grapes are high in sugar at harvest – on par with sugar levels in warm climates (26-28 brix). If you plan on fermenting a wine to dryness, make sure it will tolerate high ethanol levels. Ethanol weakens the cell walls of yeast, but so does an acidic environment. High acid wines coupled with high alcohol are not an ideal environment for yeast ot survive. You may find that a yeast tolerant to 14% has trouble finishing a wine to 14% alcohol when the wine also is high in acid.
  • If you are not planning on fermenting a wine to dryness, you want to make sure the yeast isn’t TOO hardy. This is especially important when making a port-style wine in which you plan on muting fermentation with an alcohol addition. If you have a yeast that is known to be tolerant to 17% alcohol, you may find that it adapts to 18% in your port. Also, if you are making a sweet, late-harvest style with a potential alcohol of 20% at harvest, the wine may be out of balance if you allow the yeast to ferment most of the sugars.

Osmotolerance – the high sugar environment of late-harvest wines is difficult for survival of some yeasts. If you have a very high-sugar must, make sure you are innoculating with a yeast that is designed for it. This is especially important when making ice wine.

Temperature tolerance – Again, this goes back to the wine style you desire. Often white and rose wines are cold-fermented to enhance fruity characters in the finished product. If you are planning on cold-fermenting, make sure the yeast is tolerant to colder temperatures. If your plan is to cold-ferment your wine, you want to be sure that the yeast is tolerant of cold temperatures. On the other hand, if chilling the tank is how you wish to stop fermentation, you might want to choose a less cold-tolerant yeast.

Fermentation efficiency – Another thing to think about is the efficiency of fermentation. On average, it takes 16.8 g/L of sugar to make 1% alcohol. Some yeasts have been selected for being less efficient at fermenting - they require more sugar (18g/L) to make 1% alcohol. In places where high potential alcohol is a problem, these yeasts may help make the final wine lower in alcohol, and thus more balanced.

Flavor Characteristics

Perhaps the most important factor in deciding which yeast to use is the flavor characteristics of that yeast. This is also one of the most complicated factors to discuss without giving a detailed lesson in biochemistry, but I’ll try my best.

Low sulfide/DMS

These compounds are responsible for cooked vegetable and onion/garlic aromas in wine.  All commercial yeasts are designed to be low-producers of these compounds.

Thiol Production

While certain thiols smell like burnt rubber or even skunk, other thiols are responsible for desirable odors such as grapefruit and passionfruit. These are the main compounds that give Sauvignon Blanc wines their distinctive aroma. Often yeasts are selected to be “thiol producers” for varieties which contain the positive thiol precursors. Because it is unknown what type of volatile precursors are present are not present in grape juice as free thiols, but are released from non-volatile precursors during alcoholic fermentation. The characteristic boxtree and grapefruit aromas of Sauvignon Blanc is due to thiols, and it  precursers have been identified in the juice.

Other aromatic grape varieties like gewurztraminer, muscat, and viognier contain another type of aroma precursor that produces terpenes during fermentation. Using a yeast that cleaves the bound terpenes from sugar will help improve the aroma of the finished wine from these aromatic varieties. Because we know that La Crescent is a relative of muscat, and has similar heady aromatics, one might consider using a yeast designed for these aromatic varieties.

Yeast not only helps to release aroma precursors already found in the grapes themselves, but it also produces aromas as a byproduct of metabolism. The most important positive aromas it produces are esters. They typically have fruity characters like banana and pineapple. If the goal is to make a young, fruity wine then you should choose a yeast that is a high ester producer. Typically rose is made in this style, as well as wines meant to be sold en primeur like Beaujolais Nouveau.

Besides aromatic contributions made by yeasts, they can also affect the mouthfeel of wine by glycerol production. Some yeast cells may also autolyse (break down) more readily after cell death to provide proteins that can enhance the palate. The glycerol producing property might be important in a white or rose wine that won’t undergo aging before bottling, while the enhanced autolysis may be important if you are aging the wine on lees (in barrel aged whites).

Other Yeast Properties

More than just affecting the taste and aroma of the wine, yeast can also be selected for technological properties. In wineries with limited tank capacity, a low-foaming yeast allows for optimization of tank space. This would also be an important property with wineries that do long pump-overs on their red wines. Low tolerance to sulphites, and low sulphite-binding properties can also be important for certain wines. People who develop yeast also want to be certain that they resist dessication, and are genetically stable. Health concerns have led to laboratories looking for yeast strains that are low sulphite and biogenic amine producers. In Minnesota, our high acid levels make us look toward yeast strains that can partially degrade malic acid. 

Yeast Recommendations in Minnesota?

We can quickly start to see how recommending yeast for a certain variety begins to be difficult. A single variety can be grown in a different manner, and harvested with different sugar and acid levels. The grapes from a single can have such different properties depending on where and how they are grown that using one yeast across the board can give quite different results. Today yeast catalogs contain a ton of information regarding the sensory and chemical impacts of certain yeast strains on various grape varieties. Unfortunately, not a lot of proper trials with trained sensory analysis panels have been carried out with our hybrid grapes. Nonetheless, it is easy to compare our varieties to wine “types” to get an idea of what yeasts may work well. La Crescent, for example, is an aromatic variety similar to a muscat or gewurztraminer. It would make sense, then, to use yeast varieties that are meant for aromatic whites. When making a late-harvest or ice wine, look for a yeast that has a high sugar tolerance and is designed for making dessert wines. Marquette has some nice spicy black pepper notes when fermented with yeast intended for Rhone varietals. It’s Pinot Noir background might lead us to look for yeast strains that are intended for this varietal. Also, the lack of tannin in our red varieties might make looking toward yeasts that will enhance mid-palate structure. Vitis labrusca based hybrids might benefit from a high-ester producing variety. The foxy aroma may be enhanced by other fruity notes. People producing fruit wines might also want to look for high ester producing varieties, as well as yeast that will enhance the mid-palate.

In the end, the possibilities are really endless for yeast trials in wineries. With new strains being released every year, we really have limitless options. It is up to individuals to decide what style they are shooting for, and do trials in their winery to see what works best for them.

Further Reading:

Scott Labs Yeast

yeast selection charts

Pretorius, 2000. Tailoring wine Yeasts for a New Millenium