One of the most important ingredients in winemaking is yeast. Without yeast, the wine must will not undergo fermentation. During fermentation, wine yeast functions to convert the sugar in the wine must into alcohol and carbon dioxide. The interaction of the yeast with the wine must creates aromatic byproducts which, depending upon which yeast strain you use, will express different aromatic characteristics from wine to wine.
Most wines are made with the yeast species Saccharomyces cerevisiae, though the exact strain used varies widely. Depending upon what type of wine you’re creating, and what aromatic profile you’d like to achieve, you must select a specific strain of wine yeast in order to create your desired style of wine.
Once the yeast cells die, they sink to the bottom of the fermentation vessel. The dead yeast cells in combination with skin, seed, and pulp fragments form what is known as “lees”. The longer the wine is in contact with the lees, the more they contribute to the overall complexity and quality of the wine. This practice of leaving wine “on the lees” has been documented for generations, dating as far back as ancient Roman times. Presently, this practice of leaving wine on the lees is employed often in red wine making as well as Champagne or Sparkling wine making and occasionally white wine making.
One type of yeast that is undesirable in wine is Brettanomyces bruxellensis (a.k.a. “Brett”). Brett is responsible for economic hardships for many wineries due to the ability of this yeast to decrease wine quality. The presence of Brett in wines is somewhat controversial, as there are some individuals that believe Brett is beneficial (particularly with some dessert wines). In general, higher levels of Brett in wine results in poor wine quality, due to the yeast’s ability to produce ethyl phenol compounds that significantly contribute to “off” aromas in wine, including “barnyard” and “Band-Aid” aromas.
In order to avoid the wine yeast B. bruxellensis from infecting your homemade wine, a sterile working environment is key. Generally, Brett infections can be well controlled in a home winemaking environment through the simple use of sanitizers. Sodium metabisulfite and water being a common one. B. bruxellensis is naturally present everywhere (particularly in cellars) one must always take precautions and maintain a clean environment.
In general, if you’re following a wine recipe, pay close attention to which type of wine yeast the instructions call for. If you’re creating one of your own wine recipes, it’s very important that you do some research in order to determine exactly which strain of yeast will be optimal for the style of wine you’re trying to create. Finally, maintaining a sterile environment will help ensure that you don’t have any rogue infections from any other undesired yeasts in your wine.
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Ed Kraus is a 3rd generation home brewer/winemaker and has been an owner of E. C. Kraus since 1999. He has been helping individuals make better wine and beer for over 25 years.
I make 6 gal. of Alicante and mix it with 6 gal. of White Zinfandel.
After natural fermentation slows down should I kill it and add a different kind of yeast to start it up again?
Frank, I would not recommend doing what you suggest. Your best option would be to kill the natural yeast up front. Then add a domesticated wine yeast from the very beginning. But if you don’t want to do that, I would then recommend that you keep doing what your are doing and not change a thing.