Natural vs Cultured Yeasts
4.0 Cellars – Carl’s Corner
Natural vs Cultured Yeasts
Jeff Chorniak recently posted an article in WineMaker Magazine, Virtual Edition (Sep-2020) entitled Wild Yeast: The Pros and Cons of Spontaneous Fermentation. Because many winemakers today are talking about and using natural or indigenous yeasts for fermentation of their grapes, this topic seemed appropriate for a Carl’s Corner discussion. Making wine requires yeast, and there are two sources available to the winemaker, commercially developed cultured yeasts from various supply companies, and natural, wild, or native yeasts that cling to grape and equipment surfaces, or just fly through the air in and around the vineyard and winery.
Native or wild yeasts just naturally exist everywhere and will end up in the winery with harvested grapes. If left alone, these yeasts will start fermentation, converting grape sugars to alcohol and carbon dioxide, plus a myriad of trace compounds that have a significant impact on the aromas and flavors of the wine. This can be good or bad, and it requires the winemaker to learn how and what these native yeasts do to grapes from particular vineyards in his/her winery. It is important to recognize that native yeasts on grapes from different sources and in different wineries are very likely to be different.
Some winemakers really like native yeast fermentation, harkening to the age-old traditions of winemaking that have existed for centuries. This has become a key rallying point for those interested in producing “natural” wines. Because native yeasts can give unpredictable results, at least until some history is developed and understood, many other winemakers choose to inoculate their fermentations with more proven and predictable yeast strains purchased from commercial sources. Typically, commercial yeasts will be more active, and will take over fermentations from any native yeasts.
Commercially available, cultured yeasts have only been around in recent times. The companies (laboratories) that produce and sell these yeasts have conducted extensive studies on how the yeasts perform and what types of compounds and esters are produced that contribute to the aroma and flavor of wine. Yeasts also have an impact on the amount of color extracted from grape skins. So, yeast selection is an important winemaking consideration. Perhaps the following description of a very common and popular yeast strain from a Scott Labs catalog will help illustrate the info a winemaker considers when selecting a cultured yeast.
ICV D254 Yeast – Recommended for Cabernet Sauvignon, Syrah, Zinfandel, Sangiovese, Chardonnay. Isolated from a Rhône Valley Syrah fermentation. Known to have an alcohol tolerance of up to 16% (v/v) when the fermentation is aerated and the temperature is maintained below 28°C (82°F). In red wines, D254 develops ripe fruit, jam and cedar aromas together with mild spiciness. On the palate it contributes high fore-mouth volume, big mid-palate mouthfeel, and intense fruit concentration. When used for white wines (particularly Chardonnay), sensory descriptors include butterscotch, hazelnut and almond aromas.
It would be great if similar descriptions were available for native or wild yeasts that can spontaneously ferment grapes, but learning how these yeasts work usually requires trial and error in the winery over several vintages. In addition to aroma and flavor characteristics to consider, the actual performance of the native yeast during fermentation is important to understand. Native yeasts typically have slower rates of sugar consumption. This can be a good thing in terms of a gentler, cooler fermentation with greater retention of aromatics and more skin contact time with red wines. On the other hand, slower fermentation can provide the opportunity for “bad” indigenous yeasts or bacteria to gain a foothold and cause off-flavors to develop. And, it is common for native yeasts to really slow down as sugar levels drop and alcohol levels increase, sometimes resulting in “stuck” fermentations that may be difficult to resolve. Wild yeast fermentations always involve a bit of risk, but many winemakers believe that the risk is worthwhile in order to produce as natural a product as possible.
All types of yeast require some measure of nutrients to perform fermentations. And, contact with air (oxygen) at some level is usually required to minimize various off-odors and flavors. Most commercial yeasts come with recommendations on what types of nutrients, and how much, are needed. Plus, there are recommendations on how much air contact may be required to maximize yeast performance. Because natural nutrient levels in native yeasts are typically not well-known, and are usually lower than in commercial yeasts, the winemaker is required to really pay attention to both nutrient needs and air contact to keep fermentation performance under control.
Another factor to consider when making a choice on yeast is the consistency that consumers may expect from a particular wine blend or varietal bottling. The native yeast that fermented grapes from XYZ vineyard last season may not be exactly the same strain that shows up this vintage. Some winemakers may relish such differences, but consumers may be less tolerant of new and different aromas and flavors in the wine they really enjoyed in the past. The use of commercial yeasts tends to level out potential vintage variations and help provide a more consistent product year to year.
Much more could be written on this subject, but the above should illustrate some key issues involved in choice of yeast for fermentation. I often enjoy discussing this “nerdy” and somewhat controversial subject with winemakers while comparing wines that were produced with either native or commercial yeasts. Throughout Texas wine regions you will find examples of both types of fermentation, and the available wines should provide you an opportunity to explore this subject yourself – just ask questions, drink a lot of wine, and enjoy the talents of and risks taken by our Texas winemakers.