Natural wines have been gaining popularity over the past several years as consumers increasingly seek food products and beverages that are sustainable and minimally processed.
But what exactly is the deal with wild yeast & spontaneous fermentation?
Natural yeast strains are everywhere. Yeast is simply a single cell fungal microorganism that can be found on sugar rich surfaces such as ripe fruits and flowers. Prior to using cultivated yeast strains in winemaking, the naturally occurring yeast found on grape clusters was responsible for fermentation.
During fermentation, yeast consumes sugar and converts it to alcohol and carbon dioxide. In traditional (modern) wine fermentation, winemakers inoculate the grape must with a cultured yeast strain (typically Saccharomyces cerevisiae). This inoculated dose is large enough to start fermentation quickly, overwhelm any wild yeast strains that may be present, and ensure that a single strain dominates the entire fermentation. These cultured strains provide more consistent results in regard to flavor, aroma, alcohol level, & mouthfeel.
How does wild fermentation work?
Inoculation isn’t strictly necessary for fermentation. If left alone, the native yeasts found on the fruit & wine making equipment will eventually ferment the grape must. During this process, there are often multiple strains of yeast that are responsible for fermentation. Different yeast strains have different ABV tolerance levels, so as alcohol level increases, some yeast strains die off and are replaced with new ones.
The lack of control in wild fermentation potentially makes it a riskier process. Because wild yeasts have a lower ABV tolerances than cultivated strains, the yeasts can die off before fermentation is complete. It’s typically a longer fermentation process and the grape must is left open to bacteria, spoilage yeast, and oxidation, making it potentially unpalatable or unsafe to drink.
Natural wine making requires full commitment in both the vineyard & winemaking. The use of chemicals in the vineyard will kill off naturally occurring yeasts. The use of cultivated yeasts in winemaking means that even if you try to use spontaneous fermentation, any residual cultivated yeasts in the winemaking area may overpower wild yeasts during the fermentation process.
Why wild ferment?
Knowing the difficulties and risks involved, it may seem fruitless to even attempt wild fermentation but there are benefits! The longer fermentation process means more skin contact leading to deeper color, greater depth of & fuller flavor, and a certain intensity & complexity that can’t easily be achieved with cultivated yeast.
For those who value authenticity, fermenting this way allows the terroir (the environmental factors that affect everything from flavor to aroma to tannins) to fully express itself. The same wine grapes grown in different regions produce vastly different wines depending on climate, soil, weather, geography, etc. Once native yeast strains are thrown into the mix, the differences only continue to grow.
So why not both?
Both is an option and one that more and more wineries are experimenting with! Winemakers can keep an eye on spontaneous fermentation and choose to inoculate with a cultivated yeast strain when the wine reaches 3-4% ABV. This gives the wine the complexity from spontaneous fermentation while still allowing the winemaker more control over the body, flavor, and aromas of the finished product.
Alternatively, winemakers can choose to use a pied de cuve – it literally translates to “foot of the tank” and is essentially a sourdough starter for winemaking. A small amount of grapes are picked and crushed 5-7 days prior to harvest and allowed to undergo fermentation. These grapes are then added to the rest of the grape must after the main harvest. This allows the winemaker to test the starter culture for quality control prior to inoculating the rest of the grape must and gives a jumpstart on fermentation – providing a measure of predictability that wild yeast cultures don’t always have.
Conclusion
Spontaneous fermentation provides a unique way for winemakers to experiment and really let the terroir of their vineyard shine. By extension, it provides an equally unique experience for wine drinkers and may not be for everyone, however, it is worth trying at least once!
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