September 6, 2014

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“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.

 

 

Winemaking Classes in MINNESOTA!!!

Summer school for adults?! YES!

I’ll be teaching winemaking classes this summer at the Minnesota Landscape Arboretum.

Understanding Wine Faults (200-07-14-11)
Thursday, July 14, 7-9 p.m.
$70 member/$75 non-member, All Levels, Lecture / Demo, Limit 30, Learning Center.
Introduction to Winemaking (200-07-23-11)
Saturday, July 23, 9:30 a.m.-Noon
$90 member/$95 non-member, All Levels, Lecture / Demo, Limit 30, Learning Center.
The Chemistry of Wine (200-08-06-11)
Saturday, August 6, 9:30 a.m.-Noon
$90 member/$95 non-member, All Levels, Lecture / Demo, Limit 30, Learning Center. 
Advanced Winemaking (200-08-20-11)
Saturday, August 20, 9:30 a.m.-Noon
$90 member/$95 non-member, Intermediate-Advanced, Lecture / Demo, Limit 30, Learning Center.
Wine Treatments (200-10-15-11)
Saturday, October 15, 9:30 a.m.-Noon
$90 member/$95 non-member, Intermediate-Advanced, Lecture / Demo, Limit 30, Learning Center.

The first course, “Understanding Wine Faults,” on July 14th will be a good class for both professionals and enthusiasts. We will go over the nature of the main organoleptic defaults in wine, how they happen, and what to do to prevent them. Wine samples with faults will be given as demonstration.

Saturday, July 23, will be an introduction to winemaking. We will cover basic red and white winemaking, and also discuss sanitation, proper use of sulfur, basic wine chemistry, and oxidation/reduction reactions. We will also learn about fermentation management, including how to properly innoculate must with yeast, some basic yeast nutrition, and malolactic fermentation.

Saturday, August 6th, will be a wine chemistry course. This is designed for all levels – for those who haven’t had chemistry since high school, to those who have a better grasp of chemistry. The purpose of this course is to give examples of how an understanding of chemistry will help you become a better winemaker. The importance of pH and TA and how it relates to proper SO2 management will be discussed in depth. We will also address acid reduction, tannin-anthocyanin reactions, oak alternatives, and oxidation/reduction reactions.

August 20th will be an advanced winemaking course. Building on the fundamentals that you have already learned, this course will assume a basic understanding of winemaking and wine chemistry. We will discuss tannin structure, ripeness, fermentation management, and microbiology. An overview of how and why you may want to use various enological products (fining agents, enological tannins, sorbic acid, gum arabic, lysozyme…) will also be given.

For those wishing to have a more in-depth discussion of wine treatments, a course on October 15th will be offered. As most wines will have finished fermentation at this point, we will focus on treating wines that have just finished fermentation. A demonstration of different fining agents will be given, and we will discuss how to properly set up a bench trial. We will look at other finishing agents (tannin, copper…), and discuss how to properly stabilize wine prior to bottling.

To register for the classes, go the Arboretum learning page, and follow the instructions for registration.

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

Sulfur Dioxide as an Antimicrobial

Sulfur Dioxide (SO2) has many benefits in winemaking. It acts not only as an antioxidant and antioxidasic (inhibits oxidative activity by enzymes), but also as an antiseptic. It is extremely important to accurately measure the sulfur levels in your wine, as it’s effectiveness against microbes changes in function with your free sulfur concentration and the wine’s composition. Excessive SO2 not only is a health concern, but it will also inhibit the bouquet of your wine. It first neutralizes some aromatic compounds, but as your concentration increases a noticeable burning sensation will be felt on both the nose and the palate. Here, I will focus on how to determine what quantity of SO2 to use in your wine to make sure that it is stable against microbes while remaining undetectable.
First, some chemistry. When you add SO2 to wine (most wineries either add it as a compressed liquid or in the salt form: Potassium Metabisulfite), not all of it will be actively working to protect the wine. Most of it gets bound to other elements in the wine: proteins, aldehydes, anthocyanins, and sugars (we’ll discuss this a bit later). It’s in it’s unbound, or “free” form where it goes to work.
Free Sulfur exists as two forms: molecular SO2 and bisulfite. As the pH decreases (the wine becomes more acidic), a greater percentage of the free sulfur exists as molecular SO2. This is the part that inhibits microbes (yeast and bacteria). In other words, SO2 is more effective at lower pH, so you need to add less. As the chart below shows, at a pH of 3.0, 6% of the free sulfur in your wine is molecular SO2, as the wine pH increases, less than 1% of your free sulfur is actually working to protect your wine against microbes!
The guideline to follow when adding sulfur is that you need at least 0.8 mg/L of molecular SO2 to inhibit the development of bacteria and yeast in wine. So, at a pH of 3.0, your free sulfur should measure at least 14 mg/L, while at a pH of 3.9 your free SO2 should be 109 mg/L. The above chart is extremely important for winemakers. I would suggest copying it and posting it to the wall in your lab or winery. Now, knowing that the legal limit of total SO2 (free and bound) in US wine is 350 ppm, one can see how it may be difficult to adequately protect a high pH wine from spoilage microbes during aging. Another important number to be aware of is the sensory threshold of SO2 (in other words, at what level the majority of people will be able to smell it). Literature states this value as 2.0 ppm molecular SO2. Many winemakers will add more than 0.8 molecular SO2 in order to be on the safe side, especially when a wine contains residual sugar. If you are planning on doing this, make sure that you are not passing this threshold value – especially as you approach bottling. Immediately after fermentation this is less important, as much of your first SO2 addition will end up bound. You can also expect your free sulfur levels to drop over time, so an initial big dose after fermentation usually isn’t as worrisome as a large dose later on. Also keep in mind  that dry red wines that have completed malolactic fermentation need less protection against microbes than wines that still contain sugar and malic acid. Even though 350 ppm is the maximum dose allowed, think of this value as the maximum for sweet wines. Dry red wines are often safe with less than 100 ppm of total sulfur.
Remember that most of the sulfur you add to your wine will NOT end up as free sulfur. Most of it ends up bound to other substrates in the wine, notably acetaldehyde, anthocyanins, and sugars. This is another reason why a larger initial dose of sulfur is recommended, especially in sweet wines. Studies also show that a larger initial dose will end up with you using less sulfur over time. Some of the bound sulfur is stable – meaning that the binding reaction is irreversible. Much of the bound sulfur is unstable, which means that it can revert to free sulfur again.
Because each individual wine will go through various equilibrium reactions between the bound and free forms, it is important that you measure your free and total sulfur levels a few days after you make an addition in order to make any readjustments. The various interconvertible states of sulfur dioxide are difficult to predict. Therefore, you should  continue to measure and readjust levels on a regular basis. Only when all the free carbonyl* compounds in your wine have combined with SO2 will your free sulfur content show a direct linear relationship with what is added. I would recommend checking the sulfur on a monthly basis in wines containing residual sugar, and on a quarterly basis in dry wines.
Finally, while SO2 is one of the best antimicrobial agents available to add to wine, it is important to understand that it works by inhibiting the metabolism of yeast and bacteria. It does not control certain spoilage yeasts, and does not directly kill yeast and bacteria. It is therefore not effective as a means to stop fermentation or spoilage by itself. Spoilage yeasts such as Brettanomyces spp. and Zygosaccharomyces bailii have been shown to have high tolerance to SO2. The best protection is prevention. Once spoilage bacteria and yeast start to develop in the wine, SO2 doesn’t work well at eliminating them. Thus, good sanitation and maintaining proper levels of SO2 throughout the winemaking process are important in keeping wines free from spoilage. Also be aware that even wine with no added sulfur contains small amounts of SO2, as yeast produce it naturally as a byproduct of fermentation.
One more final point: when adding sulfur in the form of Potassium Metabisulfite (PMS), don’t forget that it contains 57% sulfur by weight. Therefore, 100 mg of PMS contains 57 mg of sulfur. 
So, the calculation for sulfur addition with PMS would be as follows:
[Desired free SO2 concentration (ppm) - Actual Free SO2 concentration (ppm)] x 1.75 x [volume of wine in L] ÷ 1000 = grams PMS to add
If you want to know how to convert this equation to grams PMS/gallon of wine, I’ll refer you here to learn more about using metric and English units together.
* a carbonyl compound contains a carbonyl functional group – a Carbon atom double-bonded to an oxygen atom: C=O. Examples in wine include acetaldehyde, sugars, esters, anthocyanins…
For further reading