April 15, 2014

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Monitoring sulfur dioxide in the winery

Wine, all on its own, is a fairly good antiseptic. The tartaric acid in wines made from grapes is a relatively strong organic acid that helps keep the pH of the wines low, which in itself is a good way to inhibit microbes. Add to that the antimicrobial properties of alcohol, and you have a beverage that could help you survive through a plague. However, that’s not to say that nothing will survive in wine. Acetic Acid bacteria, Brettanomyces, and other spoilage organisms can literally turn a wine sour and make it generally unpleasant to drink. This is where the use of sulfur dioxide (SO2) in the winery is imperative. In addition to antimicrobial properties, SO2 is also an antioxidant and antioxidasic. It is arguably the most important additive in wines and, except for alcohol, is the only component in wine that requires a warning statement on the label. Thus, it is important that wineries not only ensure that they are correctly dosing and monitoring their wines with SO2, they also need to ensure that they are properly measuring it as well.

Using SO2 in the winery. Sulfur dioxide is a pretty noxious gas, and is not typically used in its pure form in small wineries. Most often, wineries employ it by adding a potassium salt of sulfurous acid, known as potassium metabisulfite (KMBS). It is important to understand that by weight, KMBS is about 57% sulfur dioxide. Thus, for every 100 grams of KMBS added to a wine, 57 grams of SO2 is added. To complicate matters, the majority of that 57 grams of SO2 becomes chemically bound to certain compounds in wine when it is first added, rendering it useless to protect the wine!

SO2 Binding in wine and why we want it free! Any compound in wine with a carbonyl function will bind sulfur dioxide. While many of you have no idea what that means, just know that there are many compounds in wine that have a carbonyl group. Glucose, acetoin, diacetyl, galacturonic, α-ketoglutaric and pyruvic acids, and acetaldehyde are all compounds that will bind SO2. This effectively does exactly what it sounds like: it “ties the hands” of sulfur so that it is unable to do its job! The good thing to know is that once all the binding sites are filled with sulfur, the remaining sulfur is floating around in the wine, free to do its job!

The term “free sulfur” is used to describe the unbound or “working” portion of sulfur in wine. In reality, only a small percentage of your free sulfur is actually “working” against microbes, and this working portion depends on a wine’s pH.  At a lower pH, more of the free sulfur is in the SO2 form, while at a higher pH, more of it is in the form of bisulfite (H2SO3-), which is essentially ineffective in wine (see table 1).

Adding and monitoring Sulfur in wine. To follow good winemaking practices, there are three critical times when a winemaker should think about sulfur addition: at crush, following the completion of alcoholic fermentation or malolactic fermentation, and any time a wine is moved (exposed to oxygen). In unfermented juice or must, a small amount of added sulfur will help kill spoilage bacteria and provide some protection from oxidation. Generally, 30 mg/L of total sulfur is sufficient to halt bacterial problems without hindering fermentation in low pH wines from quality fruit (absence of rot). Following fermentation, the quantity of sulfur to add is not quite as formulaic.

pH

% of Free Sulfur as molecular SO2

Free sulfur concentration needed to ensure 0.8 ppm molecular SO2

3.0

6.06

14 mg/L

3.1

4.88

18 mg/L

3.2

3.91

22 mg/L

3.3

3.13

28 mg/L

3.4

2.51

35 mg/L

3.5

2.00

44 mg/L

3.6

1.60

55 mg/L

3.7

1.27

69 mg/L

3.8

1.01

87 mg/L

3.9

0.81

109 mg/L

A dry wine should contain enough free sulfur to ensure that the molecular SO2 concentration is at least 0.8 mg/L, while sweet wines should be maintained with higher free sulfur concentration (1.5 mg/L molecular SO2). Of course, these recommended levels can also vary depending on which book you read. The initial sulfur addition following fermentation needs to be higher to account for the fact that much of what is added at this point will become bound to sugar, acetaldehyde, and other compounds in the wine. Often adding more sulfur during the initial dose following fermentation will help keep the overall additions lower. Once all the binding sites have been filled, any added sulfur will increase the free sulfur concentration proportionate to the amount added.

How much sulfur should I add? Free and total SO2 measurements can be tricky and time consuming, so it is often tempting to simply come up with a standard addition and rely on guesses to ensure that the proper amount was added to the wine. Without measuring, however, you have no idea if you’ve added too little, too much, or just the right amount. Ideally, the free and total sulfur should be measured before and after any addition, and adjustments should be made to make sure that the free sulfur follows the guidelines in table 1. After fermentation and racking wine off lees, wine becomes susceptible to oxygen exposure. The oxidation of ethanol to acetaldehyde is the most noticeable result of oxygen exposure. This compound gives wines an apple or nutty aroma, and will mask fruity and floral aromas. Any time a wine is racked or pumped, there is some oxygen exposure that can result in the formation of acetaldehyde that will bind a portion of the free sulfur. One can expect to lose 10-20 ppm of free sulfur any time a wine is moved. Winemakers should be measuring the free sulfur before and after moving a wine, and accounting for this loss in SO2. This becomes trickier during bottling, as it impossible to adjust the free sulfur once it is bottled! Measuring the free sulfur on wines before and after bottling can help winemakers predict the expected loss of free sulfur, and ensure that additional sulfur is added prior to bottling to make up for this expected loss.

Wine storage post-fermentation. Containers that are used to store wine need to be topped up to prevent oxygen exposure and the formation of Acetaldehyde. Plastic (polypropylene and polyethylene) tanks are somewhat permeable to oxygen, as are variable height tanks (along the inflatable rubber gasket). Some of the fermentation locks used by winemakers may not be suitable for post-alcoholic fermentation storage, either. Any lock which does not create a tight seal (e.g., spring loaded rubber seal with over-pressure protection) or barrier (e.g., traditional liquid filled fermentation lock) will allow oxygen exposure. Thus, over long-term storage, it is important to measure free sulfur on a monthly or quarterly basis, depending on the type of storage container used.

Legal Limits for SO2. Unfortunately, SO2 is an irritant and can have some serious side effects for consumers who are sensitive. The U.S. Food and Drug Administration estimates that one out of 100 people has an increased sensitivity to sulfites, which can cause an array of symptoms of varying severity from skin reactions to gastroenterological problems and pulmonary distress. For this reason, wines with added sulfites need to be labeled as such, and the FDA limits the maximum amounts that can be added to wine. These are limits for total sulfur, or the bound and unbound portion of sulfur in the wine. It is easy to see how high pH and sweet wines might easily exceed this limit if a winemaker is not carefully monitoring and adding sulfur.

Country

Wine Type (Residual Sugar)

Total SO2 Limits

USA

All Wines

350 mg/L
Australia

< 35 g/L sugar

250 mg/L

> 35 g/L sugar

300 mg/L
European Union

White/Rosé (< 5g/L sugar)

200 mg/L

Red (< 5g/L sugar)

150 mg/L

Specific Wines (e.g. German Spatlese, Auslese, Beerenauslese, Eiswein; Sauternes)

300-400 mg/L

 

 This article was originally published for Notes from the North, a quarterly newsletter for members of the Minnesota Grape Growers Association.

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.

 

 

Predicting Harvest Dates

Yesterday, I took a few pictures in the vineyards at the Horticulture Research Center near Victoria, MN. We are well behind schedule for fruit ripening – which is expected with the late spring we experienced this year. There is some hope that we could catch-up somewhat if the rest of the summer remains warm and sunny.

However, years like this highlight the fact that selecting the right grape variety is crucial if you want to plant a vineyard at the extremes of where they will ripen. The following three pictures were all taken on July 1st.  All three of these pictures come from different grape cultivars, although they are planted within steps of each other. In a year as cool as this year is turning out to be, it is easy to see that simply choosing the right grape variety to plant will have a huge impact on whether or not one will see those grapes ripen before the leaves fall off the vine!

The first two photos are from Vitis vinifera cultivars: Cabernet Sauvignon and Pinot Gris. Even though they are the same species of grapevine, one can see that the Cabernet Sauvignon is about 10 days behind the Pinot Gris in berry development. This is one point in the vine growth where this lag can be appreciated, thus these early stages of berry development (flowering and fruit set) are typically noted by winemakers in order to plan for harvest. For growers, it’s important to note these phenological stages in order to plan for disease management.

In V. vinifera, harvest occurs, on average, 120 days from flowering. This means that we should expect to harvest the Cabernet Sauvignon around Halloween this year. This may all be well and good if we were in California, but in Minnesota it could mean harvesting in snowshoes if the vines somehow make it through a frost by then.  The Pinot Gris could possibly be ripe by mid-October. This is still not news to bet the farm on (literally speaking), but there is some hope. The average first frost in some areas of southern Minnesota is between October 11 -20th. If the weather stays warm and the leaves don’t start to drop, we might see some ripe fruit. Most of Minnesota will see a frost by the first week in October.

Now take a peek at the third photo. Marquette was bred as a cultivar that flowers early and ripens quickly. The cluster at the bottom of the page shows fruit that is past flowering, with many of the bunches starting to point downward on the vine and showing peppercorn-sized berries. Marquette seems to ripen more quickly than European grapevines, so we are likely looking at harvest sometime between mid-September and the beginning of October. However, even if we look at harvesting at the beginning of October it could still be a safe bet for most vineyards planted in Minnesota if we see typical weather for the rest of the year.

 

CabernetSauvignon 7.1.13 flowering

Cabernet Sauvignon – beginning Flowering July 1st, 2013

PinotGris 7.1.13 Fruit Set

Pinot Gris – end of flowering/fruit set – July 1st, 2013

Marquette 7.1.13 peppercorn

Marquette – Fruit set to Peppercorn-sized berries – July 1st, 2013. The rain and cold weather over the past few weeks during flowering means many vineyards in the state will likely see poor fruit set like we do here.

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.

 

 

 

Winemaker Roundtables

I’m excited to announce that in 2012, a series of regional Winemaker Roundtables will be held at several host wineries around the state. A review of common wine faults will be given at each roundtable, along with methods to prevent and fix these problems. Winemakers who attend will have the opportunity to have several of their wines evaluated blind by a panel of their peers. The enology lab at the University of Minnesota will also present wines from research trials to winemakers in the state who would like to have some examples with new techniques and practices with cold-hardy grapes.

Winemakers from bonded wineries in Minnesota are eligible to attend these sessions. Space will be limited to 10 winemakers at each session.

 

 

 

High Total Acidity AND high pH?! How to handle it…

One of the reasons that grapes have been used to make wine for thousands of years is that they are one of the few fruits in the world that contain large concentrations of tartaric acid. The strength of acids is measured by their ability to shed protons – or more specifically, hydrogen ions (H+). Without going too deep into a chemistry lecture (which I’m sure will lose most of you in a few sentences), when you measure the pH of your wine, you are measuring the concentration of these ions – that’s what the big ‘H’ in pH stands for. The tricky thing to remember is that while pH is a measurement of H+, the formula for its calculation causes the pH to be inversely proportional to the H+ concentration. Thus, as the H+ concentration increases, your pH decreases.

So what is the big deal about pH? Because tartaric acid is relatively strong, it works to keep a wine’s pH near 3.0, which in turn keeps the wine stable against microbes. This is one of the reasons why wine made from grapes has flourished around the world: it doesn’t spoil easily, and acts as an antiseptic. The combination of ethanol and the acidic environment are extremely inhospitable to most microbes. In an indigenous yeast fermentation, after the wine hits 5-6% alcohol, one yeast will dominate the fermentation: Saccharomyces cerevisiae or S. bayanus. After the sugar is depleted, there isn’t much left in the wine to act as a food source for microbes that are capable of surviving in those harsh conditions. Lactic Acid bacteria, if present, will begin to consume the malic acid (transforming it to lactic acid), while Acetobacter species are capable of turning ethanol into acetic acid (vinegar). However, Acetobacter needs oxygen in order to do this, so as long as you keep your containers full, you don’t need to worry much about them.

This year, like in 2010, we saw problems with high pH in many of our wines, but we saw it especially in Marquette. The most likely explanation is that Marquette grown under certain conditions has an excess of potassium, which can drive up the pH. Malic acid concentration likely also plays a role in increasing the pH, since it is a weaker acid that in turn is converted to an even weaker acid (lactic acid) in red wine vinification. In any case, the high pH is worrisome and steps need to be taken to ensure that the wine remains stable.

Sulfur Dioxide Addition. While it is still possible to limit microbes with sulfur addition when the pH creeps up to 3.8, you need to use substantially more SO2 as your pH increases. Most of the sulfur you add to wine becomes bound to sugars and other compounds in your wine. The rest of the sulfur exists as “free” or unbound SO2. At a pH of 3.4, you should aim for 35 mg/L of free sulfur in your wine in order to be sure that it’s protecting your wine against microbial spoilage. However, at a pH of 3.8, you’d need nearly 90 mg/L of free sulfur to get the same protection. Considering that the legal limit for TOTAL sulfur in your wine cannot exceed 400 ppm, one can see how maintaining a high free SO2 rate can quickly make it possible to exceed that limit. Though it’s possible to keep your wine clean with a high pH, it isn’t easy. One should consider a pH greater than 3.8 the breaking point where acidification becomes necessary.

Wine Sensory. The pH has a huge effect on the color of red wine, as it affects the colored pigments. If you start to keep track of your wine color and corresponding pH, it becomes almost possible to predict your wine’s pH based on color alone. A high pH wine will lose the vibrant red tones, and become more of an eggplant purple color. Low pH wines will have a bright pink rim and vibrant red hue. Differences occur between grape cultivar, of course, but generally if you observe the rim of color at the edge of the wine when you tilt your glass, if it’s purple then the pH is high. High pH wines also have a tendency to be described as “flabby” or “flat,” however it is difficult to say whether or not that holds true when the wine has a corresponding high total acidity, like we often see in Marquette. In Riesling, wines with equal sugar/acid ratios can taste sweeter at a higher pH.

Cold Stabilization. Wines with a pH greater than 3.65 should not be cold stabilized. When wines are cold-stabilized, the goal is to precipitate potassium bitartrate crystals so that they don’t fall out of solution in the bottle. Above pH 3.65, this salt acts like an acid. So, by removing an acid from the solution, it causes your pH to increase. However, if the wine’s pH is LESS THAN 3.65, cold stabilization will help to LOWER your pH. Below this point, potassium bitartrate acts as a base, so removing from solution causes the solution to become more acidic. Pretty cool, huh?

What we were faced with this year. The Marquette grapes that were harvested this year arrived at the winery with a pH of 3.6, but also had a total acidity of almost 1.0%! Knowing that the pH would increase during skin maceration (potassium is extracted from the skins), and again during malolactic fermentation, I acidified the must at harvest with tartaric acid at a rate of 0.2%. This brought the pH below 3.5. During Malolactic fermentation, we saw the pH creep up again to 4.0, so we were forced to once more acidify the wine to make it stable.

So here’s where a decision needed to be made: how much tartaric acid should we add? The total acidity was around 0.65%, which is pretty good for a red wine. Adding too much tartaric acid would make the wine tart and unpalatable. If I was working in a commercial winery, these are the options I’d see:

1) Acidify with Tartaric Acid. Aim to get the pH to 3.8, and hope that the tartaric acid additions didn’t make the wine too tart, then avoid cold-stabilization. A rule of thumb to use when acidifying:  1.0 g/L of tartaric acid will generally lower the pH by 0.1 (this is a guideline, of course… to be accurate, always perform bench trials before making a large addition).

2) Acidify with Tartaric Acid. Aim to get the pH below 3.65 and KNOW the wine was going to be very tart, but then cold-stabilize. With this option, the cold-stabilization will further lower the pH another 0.1 to 0.2 points (depending on the potassium bitartrate concentration). Then, working at a pH of 3.4-3.5, we will have room to remove the tartaric acid using chemical deacidification methods. Chemical deacidification comes with the worry of losing some of the aromatics, so bench trials should be performed to determine the amount of additive works best for the individual wine.

3) Blend the wine with a lower pH wine (of course do bench trials to see if you like the blend). This of course is still an option if you choose option 1 or 2, especially if you find the wine is still too tart. Blending is one of the the real arts in winemaking.

4) Use an anion exchanger. However, while an ion exchanger is available on the commercial scale for wineries, the cost of the equipment isn’t practical unless your last name is Mondavi.

We went with option #2. Since we are an experimental winery, blending is not an option. If I went with the first option, the amount of tartaric acid needed to get the wine under a pH of 3.8 made the wine too tart.  The wines were acidified with 4 g/L of tartaric acid, which brought the pH down below 3.6 (and the TA above 1.0%), and they are now chilling  at 28°F. I’m hoping that cold stabilization removes 1-2 g/L of total acidity, and we can use potassium bicarbonate to remove an additional 1-2 g/L.  In the end, I’m hoping that nearly all of the added tartaric acid that was added to the wine can be removed, and we’ll be left with a wine that has a healthy pH between 3.6-3.8, with a palatable TA around 0.6%.

 

Edelweiss Kabinett

Today we harvested some Edelweiss. I’ve struggled a bit on what we can do for vinification trials with this grape. For those who are unfamiliar with Edelweiss, it was originally developed as a table grape by Elmer Swenson back when he was working for the University of Minnesota. Although it is not seedless, which is a problem for the table grape market; it has some aromas and flavors similar to Concord grapes and can be used to make a nice aromatic white wine. We tend to simply refer to the grape as having a labrusca character (grapes from the species Vitis labrusca  have this distinct aroma), though in most wine circles this aroma is called “foxy” – an unfortunate term that really does nothing to describe the flavor to most people.

Early European settlers, upon eating the wild grapes that grew along the riverbanks in the Eastern US, decided they had an “animal-den” aroma and nick-named them fox grapes.  Perhaps our early ancestors were more familiar with fox aroma than most Americans are today, but apparently they were onto something. Methyl anthranilate, the compound that is most often cited as the compound responsible for the characteristic aroma of V. labrusca grapes such as Concord, is used as a flavor additive in candy to give it a “grape” aroma. However, researchers also point to another compound present in Concord and other V. labrusca grapes that has a similar ‘foxy’ or ‘grapy’ aroma:  ortho-amino acetophenone (OAP).[1] Athough present in grapes in much smaller quantities than Methyl anthranilate, humans are able to detect it at a lower threshold, thus it is believed that it may play a greater role in the distinctive foxy aroma of V. labrusca grapes.[2]  Coincidentally, OAP is also found in the scent glands of certain weasels,[3] so perhaps our early ancestors weren’t so far off in relating the aroma to an animal-den. However, perhaps due to our undying love of peanut butter and jelly sandwiches (or perhaps our failure to adopt fox-hunting as enthusiastically as our Aristocratic ancestors), Americans will almost always describe the aroma of OAP as grape-like or candied.

While the grapey aroma of Edelweiss and other V. labrusca hybrids isn’t necessarily off-putting to most people, it is also not an aroma that wine-drinkers associate with high-quality wine. It’s a candied-fruit aroma that is more reminiscent of candied strawberries, Jolly Rancher candy, or Welch’s White Grape Juice – not exactly flavors that go well with that roasted chicken dinner. However, on the patio on a hot summer day, Edelweiss wine can be quite refreshing. In fully-ripe Edelweiss, the candied fruit aromas can make for a tasty grape, but it can overpower the flavor of the wine. Thus, most people growing Edelweiss for wine production will harvest it before it reaches full ripeness to keep the wine aromas more subdued.

Here is a look at some of the harvest numbers we’ve had for Edelweiss over the past few years:

Harvest Date

Brix

Total Acidity (g/L)

pH

9/9/2002

17.8

8.68

3.16

8/30/2005

18.8

10.57

3.05

9/5/2007

17.0

7.02

3.23

9/4/2008

17.2

12.03

2.97

9/10/2009

17.4

9.62

3.1

8/25/2010

15.4

7.43

3.29

9/8/2011

18.1

8.25

3.2

Although Edelweiss isn’t a high-sugar grape to begin with (remember, it was developed as a table grape), one can see that we’ve never harvested it much higher than 18°Brix, so the potential alcohol of the wine will likely not be greater than 10% in any given year. Thus, many winemakers will add sugar to the juice in order to make a wine with a more “acceptable” table-wine level of 12-14%. In my opinion, the higher alcohol level tends to overpower much of the delicate aroma and flavor of the wine. There’s a disconnect between the fresh acidity and light flavors of a grape harvested underripe with the alcohol level of a grape that was left to soak up the sun a bit longer. There is precedent in the world for harvesting grapes early for winemaking. In German-speaking countries, wines made from early-picked grapes are given the designation ‘Kabinett.’ I’m a huge fan of Kabinett Rieslings. Often they are made in a semi-sweet fashion by stopping fermentation early – at say 7-8% alcohol. They are wonderfully delicate, easy-to-drink, and refreshing – something I admire in a well-made Edelweiss. By law, Germans harvest grapes for Kabinett wines between 17-19 brix, and they are not allowed to add any sugar. This year I intend to make a wine in that style. Our numbers this year will work perfectly: 17.7 brix, 7.8 g/L total acidity. We’ll try cool-fermenting it with high terpene releasing yeast (Laffort VL1) to see if we can enhance some of the delicate floral aromas, then we will arrest fermentation with about 1% residual sugar to keep it slightly sweet. A perfect wine for summer.

Happy Harvest!



[1] Shure, K.B. and T.E. Acree. 1995. In vivo and in vitro flavor studies of Vitis labruscana Cv. Concord. ACS Symposium Series 596, American Chemical Society, Washington. pp. 127-133

[2] Acree, T.E., E.H. Lavin, R. Nishida, and S. Wantanabe. 1990. The serendipitous discovery of ortho-amino acetophenone as the ‘foxy’ smelling component of Labruscana grapes. Chem. And Eng. News 9:80

[3] Brinck, C., S. Erlinge and M. Sandell. 1983. Anal sac secretion in mustelids: a comparison. Journal of Chemical Ecology. 9(6): 727-745

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