July 26, 2017

Taking the Mystery Out of Wine Yeast Selection

Nick Smith, Enologist, University of Minnesota

I get asked a lot of questions from winemakers. I like questions; it means that winemakers are thinking about their wine. Questions also improve and enhance my knowledge and understanding of winemaking, so please keep them coming.

One question, however, gets asked more than any. It is not about grapes, analysis, or equipment. Nope. It is all about the yeast.

For whatever reason, yeast selection causes untold anxiety in many winemakers. Asking me for help probably does not bring much relief because no magic bullet yeast strain exists. I cannot tell a winemaker to use XYZ yeast and wait for a 90 point rating from a wine magazine.

However, I understand the concern about yeast. Yeast companies and their distributors print beautiful catalogs loaded with yeast strains. Testimonials from highly regarded winemakers tout the nuances of Yeast A over Yeast B and how they owe their 98 point wine to the strain.

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It’s true, yeast has a significant role in winemaking. But before we get lost in descriptions and details, let’s take a step back. Success with a strain of yeast is more than just the yeast. The yeast must be paired with the proper conditions that match its strengths with the desired style of wine.

Microsoft Word - vinemountsad.docx

When asked about yeast strains, my first response is commonly, “What style of wine are you trying to achieve?”  Next,  I ask winemakers a set of follow up questions about winery capabilities and analytical expertise.   A thorough understanding is necessary because yeast selection requires an assessment of wine style, winemaker knowledge, and winery technology.  Specific areas of importance include:

• Temperature Control

• Analytical expertise and equipment

• Filtration

Yeasts come in a wide arrangement of metabolic abilities and tolerances. This array of abilities has many advantages as it allows for production of many different wine styles. Important yeast attributes include:

• Alcohol tolerance

• Temperature tolerance

• Sugar (osmotic) tolerance

• Nutritional needs

• Fermentation speed

• Glycosidic activity

• Thiol and other flavor production

• Glycerin production

• Cooperation with other organisms

• Acid metabolism

There are also style and processing considerations:

• Dry or sweet

• Malolactic fermentation

• Amount of skin time

• Lees Contact

• Fruit quality and contamination

All of these factors, and perhaps more that I forgot to list, need to be considered when choosing a yeast strain.

When it comes to yeast, it is vital to remember that yeast success is directly related to the yeast’s environment. Bad must equals bad wine regardless of yeast strain. Clean fruit, appropriate nutrition, and temperature are among the most important aspects of fermentation.

Yeasts are small creatures. What seem like minute changes in temperature can have a huge impact on yeast metabolism. Some of the advertised properties relating to enhanced aromatics and mouthfeel for certain strains of yeast depend on specific temperature and nutrition management. If your winery has limited resources to manage temperature and nutrition, it does not pay to spend much time considering such complex strains.

This long list of considerations is not intended to overwhelm. However, forethought is required to eliminate strains and narrow down the yeast candidates to a more manageable number. I can not simply recommend a certain strain of yeast for La Crescent or any other varietal. Also, what works for one winery may not work well for another.

I recommend first determining the style of wine you want to create, then matching yeast options to fit the style. From there, I suggest speaking with yeast suppliers and distributors to narrow yeast selection further. Scott Laboratories is a great company that has amazing customer service. They know their yeast better than anyone.

Let’s work through a scenario:

Grape: Marquette

Harvest Chemistry: 26 Brix, 11.0 g/L TA, 3.10 pH (common for Minnesota)

Desired style: Dry red with malolactic fermentation (MLF), 7 to 10 days skin, barrel aging possible.

Winery limitations: Small non-jacketed tanks, no ability to measure available nitrogen

The TA and pH numbers will not affect yeast fermentation greatly. They will have impact on MLF however. At the low pH, mind your sulfur use. Over sulfuring can impact yeast, but will further impact MLF later. If you add SO2 at crush, and the fruit is clean, use 35 ppm or less SO2.

The elevated brix is the main concern. Once brix levels surpass 24, things get complicated quickly. Since this is meant to be a dry wine, the strain needs to be able to handle both the initial osmotic stress and the final alcohol concentration. Eliminate strains with alcohol tolerances below 14%. Further, higher brix increases the nutritional demand of the yeast strain. Limited nutrients will result in stuck fermentations and potential off aromas. In this example, the winery does not have a reliable way of measuring nutrition. Eliminate any strain that has high nutritional demand. Focus on strains with low H2S output and minimal nutritional complexities.

The wine will be treated with malolactic bacteria (MLB), so ensure that the strain has a high compatibility with MLB. Some strains of yeast can produce SO2 and other substrates that inhibit the bacteria. Northern cultivars can have a difficult combination of high alcohol, low pH, and high malic acid. Those conditions alone stress MLB, try to avoid any further stress by choosing a compatible yeast strain. A good reference for compatibility can be found:

practicalwinery.com

No glycol cooling here, so in order to manage temperature the fermentations will happen in small containers. Therefore, the yeast needs to be vigorous enough to finish fermentation by the time of pressing. Since temperature control is lacking, avoid highly vigorous strains that will finish fermentation in a few days. Consequently, avoid those that will take longer than desired skin contact time. Ideally, the wine will be finished with fermentation at pressing. If you want a longer skin time, choose a strain with slower fermentation kinetics.

Looking through the Scott Laboratories catalog, we find that the following strains might be good candidates for this style of Marquette:

VRB and D254

Why consider these two strains of yeast? First, they are moderate fermentors with high MLF compatibility and medium to low nutritional needs. In addition, these yeasts also have appropriate ethanol tolerance.

Indeed, these are the two strains I use the most with Marquette. VRB tends to be a bit more neutral with some nice fruit expression. D254 tends to highlight the black pepper aromas of Marquette. I have found D254 to kick out some sulfur compounds, so don’t neglect nutrition all together.

There are over 30 strains listed for red wines at Scott Laboratories alone. By examining the style of wine, chemistry of the grape, and the technical capacity of the winery, we were able to greatly narrow the range of possible strains and find a couple that have the best combination of attributes. I still recommend contacting the yeast supplier to get the latest information. There may be new strains or existing strains that have been discovered that may work well for your desired scenario.

Yeast photo courtesy of Scott Laboratories

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Comments

  1. Just wanted to throw in my two cents. Yes, the metabolic reactions of the yeast you use is the most important reaction occurring in the must. But their are also molecules already present in the wine must, molecules from grape solids during fermentation, those derived from metabolisms, even from wood. The temperature and amount of oxygen dissolved can have a significant on these “other reactions” and therefore the quality of your wine.